Need help developing circuit - will pay for help

[ryanm]

Ok, to spare you the boring details, I keep and breed reptiles. At
present, the heat, light, and humidity for each of my numerous enclosures
are controlled manually and monitored using old-fashioned, analog gauges.
There is nothing wrong with the system I have, except that it's a lot of
work. What I would like to do is develop a set of computer controls to
automate much of this. I've have the majority of it worked out, thanks to a
small company I found that sells prefabricated, chainable circuits
(including USB interfaces) that all contain a CPU with an easy to use API
for all the major programming languages (www.makingthings.com). They take
care of the vast majority of the parts I need to build the system I want,
but there is one part that they don't make (and they want an arm-and-a-leg
to make it for me), and I need to figure out how to build it.

Basically what I need is an electronically controlled dimmer circuit to
make a proportional thermostat. It is essentially a standard light dimmer
circuit, but instead of a pot to adjust the voltage sent to the heat source,
there needs to be some circuit that returns a resistance directly
proportional to the input voltage. In case I said that wrong, let me say it
like a 4 year old: I have this other circuit with a PWM output that is
capable of sending from -5v to +5v in 1024 discrete steps, and what I want
to do is make that output control a rheostat (dimmer) that my enclosure's
heat source will be connected to.

I can follow a wiring diagram and even a schematic if it's simple. The
reason I'm using these prefabricated circuits is because I am, obviously,
not an EE and have no training in electronics. Everything I know about
electronics I learned by jacking around in the back of old guitar amps,
which contained no ICs or PCBs (I can also only work on cars that have a
carburetor). I'm a software developer by trade, so these prebuilt circuits
make it simple for me to wire a temp sensor to an input on the little module
with a CPU on it, plug the USB port into my computer, and then use the
provided API to sample it at pretty much whatever rate I want. So all that's
left is to be able to adjust the voltage going to a 110v/20a/<200w heat
source. There are a number of reasons why I don't simply put a relay on the
heat source (like the HVAC in your house), but they would require
long-winded descriptions of reptile husbandry requirements that I'm sure you
don't care about. There are also a number of good reasons why I don't simply
buy a proportional thermostat that is intended for reptile enclosures, not
the least of which is that I also want to control light and humidity, and I
want to have logging of average temps and humidity, alerts in case temps
rise or drop to dangerous levels, etc.

I would be willing to pay (a reasonable price) if someone could provide
me with a schematic and a parts list (and a wiring diagram if possible), but
a schematic I paid for I would expect to be over-engineered for fire safety,
etc (since I'm only building a few of these, I can afford to over-engineer
it to ensure that it doesn't start any fires or kill my animals). I feel
pretty confident that this is a simple circuit, since it's effectively a
modified light dimmer, and that it should be about $5 worth of parts and
less than an hour to solder together. I may be wrong about that, because
like I said, I don't know exactly what's involved, but I'm sure there's an
IC that can replace the pot in a standard light dimmer circuit so that I can
adjust the resistance by varying the input voltage, I just don't have any
idea how to find out what, if any, IC would do the job.

Any help, pointers to urls, or sanity checks are appreciated. Let me
know if you need more detail or if I left out anything major. Oh, and the
standard disclaimer for these groups: I'm not a student trying to get you to
do my homework, I'm not trying to get someone else to do my job, this is a
hobby project and I have hit the limits of my experience. I myself hang out
in programming groups and email lists and play the geek with all the answers
for the newbs, so I figured there must be someone similar in here who can
either point me in the right direction or who would be willing to do it for
a reasonable price. Thanks.

ryanm
ryanmAThorsefishDOTnet (do the obvious to email me)

 

[Rich Grise]

Basically what I need is an electronically controlled dimmer circuit to
make a proportional thermostat. It is essentially a standard light dimmer
circuit, but instead of a pot to adjust the voltage sent to the heat source,
there needs to be some circuit that returns a resistance directly
proportional to the input voltage. In case I said that wrong, let me say it
like a 4 year old: I have this other circuit with a PWM output that is
capable of sending from -5v to +5v in 1024 discrete steps, and what I want
to do is make that output control a rheostat (dimmer) that my enclosure's
heat source will be connected to.

I'm wondering how you get bipolar outputs from PWM.

If you already have PWM, just use a switch. If you're controlling AC,
you'd have to put your switch inside a bridge, if it's just a transistor
or FET. Or, if you're programming at the level where you can trigger a
PWM on the power line, then just fire a Triac at the appropriate time;
that's all that dimmers do.

If you really simply have a control voltage, then somebody else will have
to field this one, but I'm sure there's something there.

If you're a programmer and USENET denizen, you should know by now how
to operate http://www.google.com. Just put in as many keywords as you
want, like: "voltage-controlled dimmer":
http://www.google.com/search?hl=en&ie=UTF-8&q="voltage-controlled+dimmer"&btnG=Google+Search
with quotes, and
http://www.google.com/search?hl=en&lr=&ie=UTF-8&q=voltage-controlled+dimmer&btnG=Search
without.

Have Fun!
Rich

 

[Spehro Pefhany]

On Thu, 10 Jun 2004 01:00:21 -0500, the renowned "ryanm"


Can you run the PWM with a very slow cycle, perhaps a couple of
seconds or less?

If so, then you can just use a zero-voltage-switching SSR to control
the heaters.

There are lots of things that can go wrong with control systems and I
suggest you add an INDEPENDENT and SIMPLE (viz mechanical)
high-temperature cutout and ALARM set to a temperature that may not be
healthy long-term for the animals, but will not hurt them for the time
it might take you to discover the alarm and fix the problem. I suppose
the same might be necessary for low temperature. Also use a modest
amount of heating so that the normal ratio is perhaps 80% on and 20%
off (we typically use 67% in industrial systems). SSRs generally fail
*ON*...

Best regards,
Spehro Pefhany

 

[Rob]

Ok, to spare you the boring details, I keep and breed reptiles. At
present, the heat, light, and humidity for each of my numerous enclosures
are controlled manually and monitored using old-fashioned, analog gauges.
There is nothing wrong with the system I have, except that it's a lot of
work. What I would like to do is develop a set of computer controls to
automate much of this. I've have the majority of it worked out, thanks to a
small company I found that sells prefabricated, chainable circuits
(including USB interfaces) that all contain a CPU with an easy to use API
for all the major programming languages (www.makingthings.com). They take
care of the vast majority of the parts I need to build the system I want,
but there is one part that they don't make (and they want an arm-and-a-leg
to make it for me), and I need to figure out how to build it.

Basically what I need is an electronically controlled dimmer circuit to
make a proportional thermostat. It is essentially a standard light dimmer
circuit, but instead of a pot to adjust the voltage sent to the heat source,
there needs to be some circuit that returns a resistance directly
proportional to the input voltage. In case I said that wrong, let me say it
like a 4 year old: I have this other circuit with a PWM output that is
capable of sending from -5v to +5v in 1024 discrete steps, and what I want
to do is make that output control a rheostat (dimmer) that my enclosure's
heat source will be connected to.

I can follow a wiring diagram and even a schematic if it's simple. The
reason I'm using these prefabricated circuits is because I am, obviously,
not an EE and have no training in electronics. Everything I know about
electronics I learned by jacking around in the back of old guitar amps,
which contained no ICs or PCBs (I can also only work on cars that have a
carburetor). I'm a software developer by trade, so these prebuilt circuits
make it simple for me to wire a temp sensor to an input on the little module
with a CPU on it, plug the USB port into my computer, and then use the
provided API to sample it at pretty much whatever rate I want. So all that's
left is to be able to adjust the voltage going to a 110v/20a/<200w heat
source. There are a number of reasons why I don't simply put a relay on the
heat source (like the HVAC in your house), but they would require
long-winded descriptions of reptile husbandry requirements that I'm sure you
don't care about. There are also a number of good reasons why I don't simply
buy a proportional thermostat that is intended for reptile enclosures, not
the least of which is that I also want to control light and humidity, and I
want to have logging of average temps and humidity, alerts in case temps
rise or drop to dangerous levels, etc.

I would be willing to pay (a reasonable price) if someone could provide
me with a schematic and a parts list (and a wiring diagram if possible), but
a schematic I paid for I would expect to be over-engineered for fire safety,
etc (since I'm only building a few of these, I can afford to over-engineer
it to ensure that it doesn't start any fires or kill my animals). I feel
pretty confident that this is a simple circuit, since it's effectively a
modified light dimmer, and that it should be about $5 worth of parts and
less than an hour to solder together. I may be wrong about that, because
like I said, I don't know exactly what's involved, but I'm sure there's an
IC that can replace the pot in a standard light dimmer circuit so that I can
adjust the resistance by varying the input voltage, I just don't have any
idea how to find out what, if any, IC would do the job.

Any help, pointers to urls, or sanity checks are appreciated. Let me
know if you need more detail or if I left out anything major. Oh, and the
standard disclaimer for these groups: I'm not a student trying to get you to
do my homework, I'm not trying to get someone else to do my job, this is a
hobby project and I have hit the limits of my experience. I myself hang out
in programming groups and email lists and play the geek with all the answers
for the newbs, so I figured there must be someone similar in here who can
either point me in the right direction or who would be willing to do it for
a reasonable price. Thanks.

ryanm
ryanmAThorsefishDOTnet (do the obvious to email me)

Look at kit # K8003 from Velleman. Has 0-10v control voltage so all
you need to do is level shift your -5 to +5v voltage source.
If you don't have any luck, come back to me and I'll have a go at
designing something.

 

[John Fields]

Ok, to spare you the boring details, I keep and breed reptiles. At
present, the heat, light, and humidity for each of my numerous enclosures
are controlled manually and monitored using old-fashioned, analog gauges.
There is nothing wrong with the system I have, except that it's a lot of
work. What I would like to do is develop a set of computer controls to
automate much of this. I've have the majority of it worked out, thanks to a
small company I found that sells prefabricated, chainable circuits
(including USB interfaces) that all contain a CPU with an easy to use API
for all the major programming languages (www.makingthings.com). They take
care of the vast majority of the parts I need to build the system I want,
but there is one part that they don't make (and they want an arm-and-a-leg
to make it for me), and I need to figure out how to build it.

Basically what I need is an electronically controlled dimmer circuit to
make a proportional thermostat. It is essentially a standard light dimmer
circuit, but instead of a pot to adjust the voltage sent to the heat source,
there needs to be some circuit that returns a resistance directly
proportional to the input voltage. In case I said that wrong, let me say it
like a 4 year old: I have this other circuit with a PWM output that is
capable of sending from -5v to +5v in 1024 discrete steps, and what I want
to do is make that output control a rheostat (dimmer) that my enclosure's
heat source will be connected to.

I can follow a wiring diagram and even a schematic if it's simple. The
reason I'm using these prefabricated circuits is because I am, obviously,
not an EE and have no training in electronics. Everything I know about
electronics I learned by jacking around in the back of old guitar amps,
which contained no ICs or PCBs (I can also only work on cars that have a
carburetor). I'm a software developer by trade, so these prebuilt circuits
make it simple for me to wire a temp sensor to an input on the little module
with a CPU on it, plug the USB port into my computer, and then use the
provided API to sample it at pretty much whatever rate I want. So all that's
left is to be able to adjust the voltage going to a 110v/20a/<200w heat
source. There are a number of reasons why I don't simply put a relay on the
heat source (like the HVAC in your house), but they would require
long-winded descriptions of reptile husbandry requirements that I'm sure you
don't care about. There are also a number of good reasons why I don't simply
buy a proportional thermostat that is intended for reptile enclosures, not
the least of which is that I also want to control light and humidity, and I
want to have logging of average temps and humidity, alerts in case temps
rise or drop to dangerous levels, etc.

I would be willing to pay (a reasonable price) if someone could provide
me with a schematic and a parts list (and a wiring diagram if possible), but
a schematic I paid for I would expect to be over-engineered for fire safety,
etc (since I'm only building a few of these, I can afford to over-engineer
it to ensure that it doesn't start any fires or kill my animals). I feel
pretty confident that this is a simple circuit, since it's effectively a
modified light dimmer, and that it should be about $5 worth of parts and
less than an hour to solder together. I may be wrong about that, because
like I said, I don't know exactly what's involved, but I'm sure there's an
IC that can replace the pot in a standard light dimmer circuit so that I can
adjust the resistance by varying the input voltage, I just don't have any
idea how to find out what, if any, IC would do the job.

Any help, pointers to urls, or sanity checks are appreciated. Let me
know if you need more detail or if I left out anything major. Oh, and the
standard disclaimer for these groups: I'm not a student trying to get you to
do my homework, I'm not trying to get someone else to do my job, this is a
hobby project and I have hit the limits of my experience. I myself hang out
in programming groups and email lists and play the geek with all the answers
for the newbs, so I figured there must be someone similar in here who can
either point me in the right direction or who would be willing to do it for
a reasonable price. Thanks.

---
It sounds like your +/- 5V control signal isn't PWM at all, but a
steady, controllable variable voltage coming out of a 10bit DAC,
resulting in the 1024 ~ 9.8mV steps.

IMO, what you'll need to do is to shift that output voltage so it goes
from, say, 0V to 10V and then use that voltage to control the firing
angle of a TRIAC. Since you have 1024 discrete steps available, and
you'll want to turn the TRIAC on somewhere between 0° and 180° in each
mains half-cycle, with 1024 steps you ought to be able to get
180°/1024 ~ 0.2° resolution, which is ridiculously fine, but it'll
work. Basically, what you'll want to do is set it up so that 0V = no
output pulse to the TRIAC gate, 9.8mV = an output pulse at a little
before 180° and 10V = an output pulse just after the mains
zero-crossing.

So, what you'll need is a zero-crossing detector and a pulse generator
with a voltage-variable delay which will put the TRIAC gate-driving
pulse wherever you want it between 0° and 180° after the zero
crossing.

If you like, I can design something for you and supply you with
documentation and a working unit on perfboard which you can copy to
get as many units as you want.

I'm pretty sure you're right about the $5 worth of parts, and if
you're interested, email me with what you have in mind as a
"reasonable" fee and maybe we can work something out.

 

[ryanm]

I'm wondering how you get bipolar outputs from PWM.

It's possible that I misread the specs on the PWM output, it may run
from 0 to 5 or 0 to 10 instead for the PWMs. Right now this is all on paper,
I'm just trying to make sure I can come up with all of the parts before I
start trying to build it.

If you already have PWM, just use a switch. If you're controlling AC,
you'd have to put your switch inside a bridge, if it's just a transistor
or FET. Or, if you're programming at the level where you can trigger a
PWM on the power line, then just fire a Triac at the appropriate time;
that's all that dimmers do.

The PWM is only capable of low voltage DC, and the heat source is
110v/20a AC. I believe that all I have is a control voltage. Of course I
could build the whole system from scratch to whatever specs I wanted, but
that would be decidedly outside of my understanding of electronics. The
premade circuits I can buy take care of the majority of the system,
including the computer interface which, to me, seems like it would be rather
difficult to build on my own, so I'll use their parts unless it's simply
impossible to do what I want with them.

If you're a programmer and USENET denizen, you should know by now how
to operate http://www.google.com. Just put in as many keywords as you
want, like: "voltage-controlled dimmer":

I did search for "electronically controlled dimmer, but all I got was a
bunch of house light switches, which aren't what I need. I'll keep
searching, of course, but I thought this looked like a good place to ask.

ryanm

 

[ryanm]

Can you run the PWM with a very slow cycle, perhaps a couple of
seconds or less?

What I got from the specs is that I can tell the PWM output to put out
anywhere from 0v-5v, and what it does is vary the point in the cycle that it
switches on and off to achieve variable line voltage. It is essentially a
dimmer circuit in itself, the problem is that it's low voltage DC, and I
need to control a high voltage AC device.

If so, then you can just use a zero-voltage-switching SSR to control
the heaters.

So, for example, I could use an AB 700SE SSR (http://tinyurl.com/39pz4),
which has a 5v control voltage and can handle switching a 110v/20a load?
Will that essentially do what I am asking for, which is let me use a 5v
control voltage to adjust the voltage going to the heat source? I'm not
familiar with how a solid state relay works, so if I'm way off, tell me.

There are lots of things that can go wrong with control systems and I
suggest you add an INDEPENDENT and SIMPLE (viz mechanical)
high-temperature cutout and ALARM set to a temperature that may not be
healthy long-term for the animals, but will not hurt them for the time
it might take you to discover the alarm and fix the problem. I suppose
the same might be necessary for low temperature. Also use a modest
amount of heating so that the normal ratio is perhaps 80% on and 20%
off (we typically use 67% in industrial systems). SSRs generally fail
*ON*...

I appreciate the concern. My intention was always to overbuild for
safety. The heat sources would actually run at about 30w-40w, but I want to
build the circuit to handle 200w so that the heat is negligible. The heat
sources, which are radiant heat panels specifically designed for reptile
enclosures, actually have safety measures built in and will not heat to
dangerous levels, even with inappropriate power applied to them. The heating
element is completely embedded in a self-extinguishing polypropylene, and is
UL listed, so the fire hazard from the heat source itself is minimal.

Of course I planned to burn in this system for several weeks to a month
on an empty enclosure before I put it anywhere near my animals. The software
would be logging the whole time so that I could find fluctuations or other
problems before I use the system on an occupied enclosure. The safety of my
household and my animals is top priority, of course, and this is just a
personal project. Like I said in the first post, there is nothing wrong with
the way I'm doing it now, I just think it would be cool to be able to
automate it. Thanks for the help, and any further help is appreciated.

ryanm

 

[ryanm]

Look at kit # K8003 from Velleman. Has 0-10v control voltage so all
you need to do is level shift your -5 to +5v voltage source.

This looks to be *exactly* what I need (and it's only $25). Thanks.

ryanm

 

[nospam]

The heat
sources, which are radiant heat panels specifically designed for reptile
enclosures, actually have safety measures built in and will not heat to
dangerous levels, even with inappropriate power applied to them. The heating
element is completely embedded in a self-extinguishing polypropylene, and is
UL listed, so the fire hazard from the heat source itself is minimal.

What is the thermal time constant of these heat sources - a few seconds?
tens of seconds? minutes?

If it is more than a few seconds just use a simple solid state relay module
and do your own PWM in software. Even a PC running windoze should manage
100ms resolution of the switching point which in a 10 second time frame
would give 1% control resolution. Reliability may be a concern but is your
USB voltage generator much safer?

With an additional time delay relay you could probably incorporate a
hardware watchdog which forces the heaters off should the PWM signal stop
cycling.

Lighting if it is mains powered is a more difficult problem. If it is or
can be dc powered then a voltage controlled dimmer is not so difficult.

 

[Spehro Pefhany]

What I got from the specs is that I can tell the PWM output to put out
anywhere from 0v-5v, and what it does is vary the point in the cycle that it
switches on and off to achieve variable line voltage. It is essentially a
dimmer circuit in itself, the problem is that it's low voltage DC, and I
need to control a high voltage AC device.

So, for example, I could use an AB 700SE SSR (http://tinyurl.com/39pz4),
which has a 5v control voltage and can handle switching a 110v/20a load?
Will that essentially do what I am asking for, which is let me use a 5v
control voltage to adjust the voltage going to the heat source? I'm not
familiar with how a solid state relay works, so if I'm way off, tell me.

Sorry, it wouldn't work very well- this kind of SSR should be fed only
5V for "on" and 0V for "off".

It's possible to adapt the PWM output to an SSR with an LM324 and a
handful of R's and C's. Maybe someone feels like draing it up, or
perhaps you should take JF up on his offer.

Best regards,
Spehro Pefhany

 

[N. Thornton]

Ok, to spare you the boring details, I keep and breed reptiles. At
present, the heat, light, and humidity for each of my numerous enclosures
are controlled manually and monitored using old-fashioned, analog gauges.
There is nothing wrong with the system I have, except that it's a lot of
work. What I would like to do is develop a set of computer controls to
automate much of this. I've have the majority of it worked out, thanks to a
small company I found that sells prefabricated, chainable circuits
(including USB interfaces) that all contain a CPU with an easy to use API
for all the major programming languages (www.makingthings.com). They take
care of the vast majority of the parts I need to build the system I want,
but there is one part that they don't make (and they want an arm-and-a-leg
to make it for me), and I need to figure out how to build it.

Basically what I need is an electronically controlled dimmer circuit to
make a proportional thermostat. It is essentially a standard light dimmer
circuit, but instead of a pot to adjust the voltage sent to the heat source,
there needs to be some circuit that returns a resistance directly
proportional to the input voltage. In case I said that wrong, let me say it
like a 4 year old: I have this other circuit with a PWM output that is
capable of sending from -5v to +5v in 1024 discrete steps, and what I want
to do is make that output control a rheostat (dimmer) that my enclosure's
heat source will be connected to.

I can follow a wiring diagram and even a schematic if it's simple. The
reason I'm using these prefabricated circuits is because I am, obviously,
not an EE and have no training in electronics. Everything I know about
electronics I learned by jacking around in the back of old guitar amps,
which contained no ICs or PCBs (I can also only work on cars that have a
carburetor). I'm a software developer by trade, so these prebuilt circuits
make it simple for me to wire a temp sensor to an input on the little module
with a CPU on it, plug the USB port into my computer, and then use the
provided API to sample it at pretty much whatever rate I want. So all that's
left is to be able to adjust the voltage going to a 110v/20a/<200w heat
source. There are a number of reasons why I don't simply put a relay on the
heat source (like the HVAC in your house), but they would require
long-winded descriptions of reptile husbandry requirements that I'm sure you
don't care about. There are also a number of good reasons why I don't simply
buy a proportional thermostat that is intended for reptile enclosures, not
the least of which is that I also want to control light and humidity, and I
want to have logging of average temps and humidity, alerts in case temps
rise or drop to dangerous levels, etc.

I would be willing to pay (a reasonable price) if someone could provide
me with a schematic and a parts list (and a wiring diagram if possible), but
a schematic I paid for I would expect to be over-engineered for fire safety,
etc (since I'm only building a few of these, I can afford to over-engineer
it to ensure that it doesn't start any fires or kill my animals). I feel
pretty confident that this is a simple circuit, since it's effectively a
modified light dimmer, and that it should be about $5 worth of parts and
less than an hour to solder together. I may be wrong about that, because
like I said, I don't know exactly what's involved, but I'm sure there's an
IC that can replace the pot in a standard light dimmer circuit so that I can
adjust the resistance by varying the input voltage, I just don't have any
idea how to find out what, if any, IC would do the job.

Any help, pointers to urls, or sanity checks are appreciated. Let me
know if you need more detail or if I left out anything major. Oh, and the
standard disclaimer for these groups: I'm not a student trying to get you to
do my homework, I'm not trying to get someone else to do my job, this is a
hobby project and I have hit the limits of my experience. I myself hang out
in programming groups and email lists and play the geek with all the answers
for the newbs, so I figured there must be someone similar in here who can
either point me in the right direction or who would be willing to do it for
a reasonable price. Thanks.

ryanm
ryanmAThorsefishDOTnet (do the obvious to email me)

This isnt so hard to do, although I wouldnt tackle it in precisely the
same way you suggest. These online offers dont usually work out
though, usually theyre just carrots for ideas and discussion. You seem
serious, but:
What are you willing to offer?
How is it going to be set up so you do pay but dont get bitten?
If you can set up one of these offers so it works, well done. These
offers tend not to get taken up by pros because of these problems.

BTW Any potential designer will need to know whether youre on 120v or
240v. What type of lighting you use may also be relevant.


Regards, NT

 

[John Fields]

This isnt so hard to do, although I wouldnt tackle it in precisely the
same way you suggest.

---
Interesting... a pronouncement of knowingingness followed by an
admonition of sorts, but with nothing substantive offered as proof.
---

These online offers dont usually work out
though, usually theyre just carrots for ideas and discussion.

---
An opinion followed by another opinion seemingly introduced to
validate the first as fact...
---

You seem
serious, but:
What are you willing to offer?

 

[Rich Grise]

What I got from the specs is that I can tell the PWM output to put out
anywhere from 0v-5v, and what it does is vary the point in the cycle that it
switches on and off to achieve variable line voltage. It is essentially a
dimmer circuit in itself, the problem is that it's low voltage DC, and I
need to control a high voltage AC device.

I'd give my eye teeth to see that on a scope. This description sounds
tantalizingly like a signal that you could just connect to the gate of
a triac through about a 220 ohm resistor, and have phase control already.

Is that possible? (to get a screenshot)? Or you could even get a small
speaker, or a headphones, and listen to it through about a 100-470
resistor and maybe a small cap. You'll be able to hear if it's pwm,
and at what freq, or if it's 60 hz.

Good Luck!
Rich

 

[Rich Grise]

It's possible that I misread the specs on the PWM output, it may run
from 0 to 5 or 0 to 10 instead for the PWMs. Right now this is all on paper,
I'm just trying to make sure I can come up with all of the parts before I
start trying to build it.

The PWM is only capable of low voltage DC, and the heat source is
110v/20a AC. I believe that all I have is a control voltage. Of course I
could build the whole system from scratch to whatever specs I wanted, but
that would be decidedly outside of my understanding of electronics.

Don't worry - If I can get this one question answered, and it's the answer
I think it is, I'll design the circuit for you for free, and you can have
John Fields write up the contract for going into production or whatever.

What, exactly, is the signal we're dealing with here? Sometimes it
sounds like a 5V PWM, that merely _averages_ to 0-5V, sometimes it
sounds like a plain DC level - which of these is it? You keep referring
to "the PWM output." This is such a tease - it makes it sound like you
don't have to design a freaking thing! If it is a 5V PWM pulse, and
happens to start at the zero-cross of the line freq, then just use
it to gate on a transistor or MOSFET that's wired directly between
the DC terminals of a bridge, and the AC terminals go in series with
the heater. Well, it might have to be optoisolated. But it's incredibly
simple to do, if you really have a "PWM output."

Please, if only to humor me, see if you can find out what the exact
signal is that we are dealing with here.

Thanks!
Rich

 

[ryanm]

What, exactly, is the signal we're dealing with here? Sometimes it
sounds like a 5V PWM, that merely _averages_ to 0-5V, sometimes it
sounds like a plain DC level - which of these is it? You keep referring
to "the PWM output." This is such a tease - it makes it sound like you
don't have to design a freaking thing! If it is a 5V PWM pulse, and
happens to start at the zero-cross of the line freq, then just use
it to gate on a transistor or MOSFET that's wired directly between
the DC terminals of a bridge, and the AC terminals go in series with
the heater. Well, it might have to be optoisolated. But it's incredibly
simple to do, if you really have a "PWM output."

From what I've been able to glean from their documentation, it is really
a PWM output. They say that it is achieved by turning a FET switch on and
off "many times per second", but they don't specify if it starts at the
zero-cross or not. So, from what I understand it is a 5v PWM output.

You can check out their documentaton if you want, in case I missed
something, but I don't think they would be calling it a PWM output if it
wasn't.

Docs: http://tinyurl.com/352zv

ryanm

 

[ryanm]

So everyone knows, Rob pointed out a Velleman kit that will do pretty
much exactly what I want (K8003 or the newer K8064), and it's under $25. I
sent a link to it back to the company that makes those other prefab circits
and they said they might just go ahead and make a few cheap for me, already
tested and matched to the other components I was planning to buy.

Thanks, everyone, for the help. While I realize that you guys don't need
these kinds of things, I think the stuff available at www.makingthings.com
is pretty cool and opens up some hardware to those of us who are usually
software-only types. There are other companies producing kits (like
Velleman) but these guys are really less about cookie-cutter projects than
they are about offering a bunch of parts that can be easily networked and
connected to a computer for control. Their stuff is mostly used for
interactive art projects and stuff like that, but it seemed to be a good fit
for what I'm trying to do. Thanks again.

ryanm

 

[Paul Burke]

I
suggest you add an INDEPENDENT and SIMPLE (viz mechanical)
high-temperature cutout and ALARM

Ah yes, I remember a colleague who "fixed" his new girlfriend's tropical
fish tank thermostat- the relationship finished the following morning,
she didn't like fish soup for breakfast.

Paul Burke

 

[N. Thornton]

---
Interesting... a pronouncement of knowingingness followed by an
admonition of sorts, but with nothing substantive offered as proof.

hmm, hopefully youll get out of bed the other side tomorrow

Regards, NT

 

[N. Thornton]

I'd give my eye teeth to see that on a scope. This description sounds
tantalizingly like a signal that you could just connect to the gate of
a triac through about a 220 ohm resistor, and have phase control already.

no mains isolation.


Regards, NT

 

[John Fields]

Don't worry - If I can get this one question answered, and it's the answer
I think it is, I'll design the circuit for you for free, and you can have
John Fields write up the contract for going into production or whatever.

---
Geez, Rich, when did you you sign on as our business manager???

Actually, since I didn't get the design job :-( and I've still got
the circuit in my head, I'll go ahead and post the design to abse
over the weekend on the chance that it might help somebody out,
altruistic guy that I am!^)

How about if we turn it into a little contest and you post your
circuit too? Just for grins, let's blow off mains isolation...