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Thermal control question

R

Robert Wolcott

This is more of a design question, but there are some pretty sharp people on
this board so I'll post it here. I am designing a closed loop thermal
control unit that would like to be very accurate (+/- .2 degrees C). I plan
to use a PID controller with an RTD sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power the
heating element. If I had a PID controller providing a low voltage signal
(mV), what sort of intermediate circuit would I need to power a 10-50 watt
heating element? My main goal is not necessarily accuracy, but stability.

Thanks,
Bob
 
N

NSM

| This is more of a design question, but there are some pretty sharp people
on
| this board so I'll post it here. I am designing a closed loop thermal
| control unit that would like to be very accurate (+/- .2 degrees C). I
plan
| to use a PID controller with an RTD sensor and a small tape style heater.
|
| Most PID controllers only provide a signal out and do not directly power
the
| heating element. If I had a PID controller providing a low voltage signal
| (mV), what sort of intermediate circuit would I need to power a 10-50 watt
| heating element? My main goal is not necessarily accuracy, but stability.

I'd be inclined to make a closed loop controller which is controlled by the
PID output. What volts/amps for the heater?

If you can find an old GE SCR manual you should find some good starting
points.

N
 
R

Robert Wolcott

I would guess that the heater puts out a few watts (12V 1A or so).
 
J

Jerry G.

I am not going to design this for you.

I am assuming that you want to run an AC operated heater. I am taking it
that you know how AC light dimmers and speed controllers work that are Triac
driven.

What you have to do is make a VCA (Voltage Control Amplifier), which is used
to drive the an optical isolator that is in a circuit to be used to do the
replacement of the pot (sort of speaking) to control the phase drive of a
Triac, that is driving the heater unit. It is important that you maintain
proper electrical isolation between the VCA output, and the power driver
that is on the AC side (Triac circuits).

If your heating element is a DC voltage type, then you can use the VCA to
drive a DC power amplifier circuit that would be able to drive your heating
element.

Normal heating systems that have a window of about 2 degrees F are able to
use a power relay on the output of the thermostat system, and thus
eliminating the complexity of a VCA, and an output stage of some type. The
power relay would be a lot easier to drive, and would require less
precision.

There are commercially available thermostats that can do what you want to
do. I am sure that if you research this out, you will find some. They are
very precise, and well engineered. Going by the fact that you have to ask
this question, you are going to have a bit of a complicated job to do, to
make this work accurately. There are a lot of design aspects to be
considered.


--

Jerry G.
=====

This is more of a design question, but there are some pretty sharp people on
this board so I'll post it here. I am designing a closed loop thermal
control unit that would like to be very accurate (+/- .2 degrees C). I plan
to use a PID controller with an RTD sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power the
heating element. If I had a PID controller providing a low voltage signal
(mV), what sort of intermediate circuit would I need to power a 10-50 watt
heating element? My main goal is not necessarily accuracy, but stability.

Thanks,
Bob
 
N

NSM

| I would guess that the heater puts out a few watts (12V 1A or so).

DC power or can you run it from AC? ISTR that the GE manual had a very
precise temperature control circuit that ran on half wave AC.

N
 
S

Sam Goldwasser

Robert Wolcott said:
This is more of a design question, but there are some pretty sharp people on
this board so I'll post it here. I am designing a closed loop thermal
control unit that would like to be very accurate (+/- .2 degrees C). I plan
to use a PID controller with an RTD sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power the
heating element. If I had a PID controller providing a low voltage signal
(mV), what sort of intermediate circuit would I need to power a 10-50 watt
heating element? My main goal is not necessarily accuracy, but stability.
Click to expand...

Do you have a specific PID controller in mind? Are you designing from
scratch or will be using a commercial controller (Omega, etc.?)?

As others have said, if your controller only provides a voltage or current
output (not pulse width modulation or phase control), you need to provide
a circuit to convert it. Some controllers provide a pulse width output
of sorts to drive a SS relay or logic input. If this is a one-off and
you don't mind using a commercial controller, they seem to be going for
about $30 on eBay.

Converting to pulse width modulation from a DC voltage is nothing more
than an amplifier and a single IC like those used in SMPS controllers,
or a 555 ramp generator and comparator.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
..
 
G

GS

Howdy Robert.....Based on my own experience designing a similar
situation I'd want to know what the thermal mass is that you're trying
to heat to get a handle on the stability issue. Based on my own
experience, if you can get away with something simple like a relay to
switch your heater, use that (could be a solid state relay etc).
Depending on the anticipated repairability of this circuit, you could
design your own switching based on everything from bipolar, Power FET to
the SCR's or TRIAC's as others have alluded to.

Because you mentioned stability, the amount of times you'll have to
switch on versus heat loss will be the basic issue assuming you have
sufficient heat input to overwhelm a quickly cooling load. If the heat
load is small and the heater has a lot of output in relation, then
you'll be playing with overshoot a lot more.


Gord
 
A

Asimov

"Robert Wolcott" bravely wrote to "All" (14 Jan 05 21:47:29)
--- on the heady topic of "Thermal control question"

RW> From: "Robert Wolcott" <[email protected]>
RW> Xref: aeinews sci.electronics.repair:7183

RW> This is more of a design question, but there are some pretty sharp
RW> people on this board so I'll post it here. I am designing a closed
RW> loop thermal control unit that would like to be very accurate (+/- .2
RW> degrees C). I plan to use a PID controller with an RTD sensor and a
RW> small tape style heater.
RW> Most PID controllers only provide a signal out and do not directly
RW> power the heating element. If I had a PID controller providing a low
RW> voltage signal (mV), what sort of intermediate circuit would I need to
RW> power a 10-50 watt heating element? My main goal is not necessarily
RW> accuracy, but stability.
RW> Thanks,
RW> Bob

If it is AC power then you need to convert the PID voltage level into
a phase angle in order to control a TRIAC. This can be done brute
force with on/off control, slightly better by varying a pulse rate, or
precisely with a countdown circuit driven by a pll sync'd to the AC
phase. It depends on application.

A*s*i*m*o*v

.... Power is obtained by current meeting resistance
 
R

Robert Wolcott

I had planned on using an Omron (E5CK?) controller. They are usually
available on Ebay for $25-$40. Do you know of a source for schematics of
the circuits you mentioned?

Thanks,
Bob
 
R

Robert Wolcott

Gord,

I haven't really sat down and gone through the numbers for heat transfer
yet, but the physical mass would be around 30-50 grams of aluminum. It
would not be exposed to much air flow either. I plan to use a tape style
heating element (http://www.minco.com/products/heaters.aspx) that will be
sandwiched between the two pieces of aluminum.

Thanks,
Bob
 
S

Sam Goldwasser

Robert Wolcott said:
I had planned on using an Omron (E5CK?) controller. They are usually
available on Ebay for $25-$40. Do you know of a source for schematics of
the circuits you mentioned?
Click to expand...

Search on: TL494 or SG2524. I'm sure there are better ones nowadays but
those part numbers come to mind. The datasheets will have enough info
to design a simple PWM controller. They can directly drive common MOSFETs.

In fact, with one of those chips, you can design the entire thing without
any fancy Omron controller. :)

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
 
N

NSM

| I had planned on using an Omron (E5CK?) controller. They are usually
| available on Ebay for $25-$40. Do you know of a source for schematics of
| the circuits you mentioned?

Have you looked at the app notes on http://www.microchip.com ??

N
 
R

Robert Wolcott

I couldn't reply to your email, so I'll post it here.

Gord,

This is very early in the design stage so I'm really
only trying to line up how I will approach the
problem. The temperature I am looking for is 30-33
degrees C so I might be way off on the wattage
estimate. I haven't even looked into the possible
wattage values and I have little experience with
thermal systems. I know it will be a low power system
though. The control voltage and current have not yet
been determined.

Since this is a personal project (for my hobby) cost
is a concern. My approach is:

1. Find out if it can be done.
2. How much will the major components cost
(controller, TC, various circuits)
3. Spend time determining exact requirements
(wattage, etc...).
4. Physically design the system (CAD modeling etc...)
5. Order the parts.
6. Fabricate it.

I'd suppose I'm currently at stage 1 or 2. I have
heard of Peltier elements being used for systems like
this. Do you know anything about these?

Thanks,
Bob
 
S

Sam Goldwasser

NSM said:
| I'd suppose I'm currently at stage 1 or 2. I have
| heard of Peltier elements being used for systems like
| this. Do you know anything about these?

http://www.allelectronics.com/cgi-bin/category.cgi?category=search&item=PJT-1

Needs WELL filtered DC.
Click to expand...

If you know your device will be above ambient, you don't need a Peltier
device (thermo electric cooler, TEC). Yes, they can heat as well but
heating tape will be cheaper.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
 
R

Robert Wolcott

This is for temperature control of an LBO crystal used in doubling a yag
laser (externally). I have heard that there is an optimal temperature for
maximizing efficiency in these and I'm sure there will be some heat
generated by the 1064nm beam within the crystal. My concern is that the
beam's heat will actually drive the crystal's temperature over the optimum
value and cooling might be needed, which would require a means of cooling as
well as heating.

How much does the temperature affect the conversion efficiency anyways? If
it is ~20%, designing a good thermal control system would be worth the
effort, but not if it is only 2-3%. For my purpose, an elaborate thermal
control system might be overkill but it is still interesting to learn about
nonetheless.

Thanks,
Bob
 
S

Sam Goldwasser

Robert Wolcott said:
This is for temperature control of an LBO crystal used in doubling a yag
laser (externally). I have heard that there is an optimal temperature for
maximizing efficiency in these and I'm sure there will be some heat
generated by the 1064nm beam within the crystal. My concern is that the
beam's heat will actually drive the crystal's temperature over the optimum
value and cooling might be needed, which would require a means of cooling as
well as heating.
Click to expand...

In that case, you really should be using a TEC since you don't know
which direction it will need, heat or cool. In addition, that 30-33C may
turn out not to be the optimal temperature.
How much does the temperature affect the conversion efficiency anyways? If
it is ~20%, designing a good thermal control system would be worth the
effort, but not if it is only 2-3%. For my purpose, an elaborate thermal
control system might be overkill but it is still interesting to learn about
nonetheless.
Click to expand...

It could be very significant. For KTP, it could be 50 percent or more.
Don't know specifically for LBO but the principles are similar.

You're going to all the effort, it certainly makes sense to do a proper
job of the temperature control. However, you have to decide if you really
want the learning experience given all the time and effort it will take,
or just get a TEC driver from Wavelength Electronics or elsewhere
for about $100 and be done. I think using an Omron or Omega PID unit
is gross overkill if it were even suitable, but given the new information,
I'd tend to recommend against that approach.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
 
A

Asimov

"Robert Wolcott" bravely wrote to "All" (15 Jan 05 14:47:44)
--- on the heady topic of "Re: Thermal control question"

RW> From: "Robert Wolcott" <[email protected]>
RW> Xref: aeinews sci.electronics.repair:7310

RW> This is for temperature control of an LBO crystal used in doubling a
RW> yag laser (externally). I have heard that there is an optimal
RW> temperature for maximizing efficiency in these and I'm sure there will
RW> be some heat generated by the 1064nm beam within the crystal. My
RW> concern is that the beam's heat will actually drive the crystal's
RW> temperature over the optimum value and cooling might be needed, which
RW> would require a means of cooling as well as heating.

RW> How much does the temperature affect the conversion efficiency
RW> anyways? If it is ~20%, designing a good thermal control system would
RW> be worth the effort, but not if it is only 2-3%. For my purpose, an
RW> elaborate thermal control system might be overkill but it is still
RW> interesting to learn about nonetheless.


In that case you could use a couple of matched diodes, one to sense
the temperature and the other as reference. The voltage difference
between the 2 diodes will be aproximately linear with temperature at
about 2.5mV/'C. Add some gain and an offset and you can control
any set temperature. A PID is overkill for such a simple task.

A*s*i*m*o*v

.... If all else fails, hurl it across the room a few times!
 
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