Need advice on power supply circuit for foam cutter

[BrianS]

I need some expert advice on a power supply circuit. Normally I would
have asked my friend who was an electronic wiz, but he's just passed
away
this year so I need a new electronic expert. I know this is probably a
lot to ask, but I figured there might be some kind soul who might lend
a little of their time. I would gladly exchange what knowledge I have
on other subjects such as AutoCAD, Linux, programming, etc.

I want to make a power supply circuit for a foam cutter. I've been
researching them on the Internet and found a number of different plans,
but I don't know enough about electronics to judge which one would be
best and if any of them could be modified (safely) to fit some of the
parts I already have. My skills are limited to soldering and following
a good set of instructions, not design.

I've found these different designs:
http://members.fortunecity.co.uk/slmohr/rcinterest2A.htm
http://www.uoguelph.ca/~antoon/gadgets/hotwire/hotwire.html
http://nsrca.org/technical/tip_tricks/foam_cutter/foam_cutting_power_supply.htm

From the sites I've read many recommend that you do not use nichrome

wire. So I have some stainless steel wire (0.020in DIA) left over
from a different project. I have no idea what ohms/foot rating it has.
Also I have a working transformer that is 120V input and 12V 8amp
output. Most of these designs call for 24V transformers.

My questions are:
1. Which design do you think would give the best control of the wire
temperature?
2. Can any of these designs work with the wire and transformer that I
already have?
If so, what changes to the components (if any) would be required?

Also this is the third time I tried posting this message (over the past
two days) to google groups so if there are two other copies that I
can't see for some reason I am sorry.

Thanks in advance
Brian

 

[Charles Schuler]

Brian, to avoid building anything, have you considered a variac and a
step-down transformer. If you want to save money, you could probably pick
these up for a few dollars at a hamfest flea market.

 

[[email protected]]

I need some expert advice on a power supply circuit. Normally I would
have asked my friend who was an electronic wiz, but he's just passed
away
this year so I need a new electronic expert. I know this is probably a
lot to ask, but I figured there might be some kind soul who might lend
a little of their time. I would gladly exchange what knowledge I have
on other subjects such as AutoCAD, Linux, programming, etc.

I want to make a power supply circuit for a foam cutter. I've been
researching them on the Internet and found a number of different plans,
but I don't know enough about electronics to judge which one would be
best and if any of them could be modified (safely) to fit some of the
parts I already have. My skills are limited to soldering and following
a good set of instructions, not design.

I've found these different designs:
http://members.fortunecity.co.uk/slmohr/rcinterest2A.htm
http://www.uoguelph.ca/~antoon/gadgets/hotwire/hotwire.html
http://nsrca.org/technical/tip_tricks/foam_cutter/foam_cutting_power_supply.htm

wire. So I have some stainless steel wire (0.020in DIA) left over
from a different project. I have no idea what ohms/foot rating it has.
Also I have a working transformer that is 120V input and 12V 8amp
output. Most of these designs call for 24V transformers.

My questions are:
1. Which design do you think would give the best control of the wire
temperature?
2. Can any of these designs work with the wire and transformer that I
already have?
If so, what changes to the components (if any) would be required?

Also this is the third time I tried posting this message (over the past
two days) to google groups so if there are two other copies that I
can't see for some reason I am sorry.

Thanks in advance
Brian

I havent looked at the linke, so cant comment there.

All thats needed is a bit of nichrome heater element wire and a supply
to run it, any low voltage supply that gets it hot but not burning.
Thats it. (I wouldnt choose ss myself.)

Were you wanting to add some kind of accurate wire temp control?


NT

 

[[email protected]]

I need some expert advice on a power supply circuit. Normally I would
have asked my friend who was an electronic wiz, but he's just passed
away
this year so I need a new electronic expert. I know this is probably a
lot to ask, but I figured there might be some kind soul who might lend
a little of their time. I would gladly exchange what knowledge I have
on other subjects such as AutoCAD, Linux, programming, etc.

I want to make a power supply circuit for a foam cutter. I've been
researching them on the Internet and found a number of different plans,
but I don't know enough about electronics to judge which one would be
best and if any of them could be modified (safely) to fit some of the
parts I already have. My skills are limited to soldering and following
a good set of instructions, not design.

I've found these different designs:
http://members.fortunecity.co.uk/slmohr/rcinterest2A.htm
http://www.uoguelph.ca/~antoon/gadgets/hotwire/hotwire.html
http://nsrca.org/technical/tip_tricks/foam_cutter/foam_cutting_power_supply.htm

wire. So I have some stainless steel wire (0.020in DIA) left over
from a different project. I have no idea what ohms/foot rating it has.
Also I have a working transformer that is 120V input and 12V 8amp
output. Most of these designs call for 24V transformers.

My questions are:
1. Which design do you think would give the best control of the wire
temperature?
2. Can any of these designs work with the wire and transformer that I
already have?
If so, what changes to the components (if any) would be required?

Also this is the third time I tried posting this message (over the past
two days) to google groups so if there are two other copies that I
can't see for some reason I am sorry.

Thanks in advance
Brian

I havent looked at the linke, so cant comment there.

All thats needed is a bit of nichrome heater element wire and a supply
to run it, any low voltage supply that gets it hot but not burning.
Thats it. (I wouldnt choose ss myself.)

If yourea hobbeast youve probably got assorted wall warts and other
supplies around.

Were you wanting to add some kind of accurate wire temp control?


NT

 

[[email protected]]

No, constant voltage is better. When the temperature drops, so does
the resistance, which causes the current to increase (when voltage is
held constant), which is what you want to combat the temperature
dropo. A constant current control would not benefit from this small
feedback.


-Robert Scott
Ypsilanti, Michigan

The ideal would be constant adjustable temp. ie the supply monitors
wire R and adjusts supply to keep it steady, and the dial sets temp.
Then you can set the temp perfectly for all plastics and workpieces.


NT

 

[BrianS]

NT,

Would any of the listed circuits come close to the "ideal" setup you
mentioned? If not how would that be accomplished? I know that I
couldn't design such a circuit.

Thanks for the replies.
Brian

 

[Mike]

NT,

Would any of the listed circuits come close to the "ideal" setup you
mentioned? If not how would that be accomplished? I know that I
couldn't design such a circuit.

Thanks for the replies.
Brian

Just my 2 cents worth.

Assuming that constant voltage is the way to go then the 2nd circuit
utilizing the LM350K is your best choice of the three. The author
states the current regulation is provided by P3. The truth is that
there is no current regulation. The LM350k in this circuit is
configured as a voltage regulator and P3 simply provides for adjusting
the output voltage. 10k is a bit high for P3, 5K would be better. The
data sheet for the LM350 says that 240ohms is ideal for R2. Although
it really doesn't make that much difference, if you have to buy it,
buy a 240ohm. He also describes R3, and I may be losing it, but I can
find no R3 in the circuit and I don't see where one is needed. He is
correct that the LM350K will get very very hot if the resistance of
your heating element wire is low. Use the lowest voltage transformer
you can and still get enough voltage to get the wire hot enough. If
the wire resistance is less than about 5ohms then a 12.6V 5A
transformer would be fine for 3A maximum current while generating much
less heat.

The circuit at the first link could be much more efficient, but has
problems. The biggest problem is that there is no feedback that is
necessary to regulate the output voltage. The same is true of the 3rd
circuit

Mike

 

[BrianS]

As I said in the first post I have a 120VAC -> 12V 8amp transformer.
Some of the replies mentioned using a variac. Could I use this
transformer with the variac? Would it be a shock hazard?

I would like to use cutting bows that are from 8" to 24" long. (Maybe
longer ones in the future).
I just want a flexible system that would work well, be safe and not too
expensive. In other words I don't want to assemble a circuit and not
have it work later on. Or buy a variac use it for awhile and then
later need to build a circuit. As I said, this really isn't my
expertise.

Brian

 

[Rich Grise]

As I said in the first post I have a 120VAC -> 12V 8amp transformer.
Some of the replies mentioned using a variac. Could I use this
transformer with the variac? Would it be a shock hazard?

Only if you leave the terminals exposed.

I would like to use cutting bows that are from 8" to 24" long. (Maybe
longer ones in the future).
I just want a flexible system that would work well, be safe and not too
expensive. In other words I don't want to assemble a circuit and not
have it work later on. Or buy a variac use it for awhile and then
later need to build a circuit. As I said, this really isn't my
expertise.

If you have the resources to get a variac, or have one on hand, I'd say
try this first - I did a little plastic-cutting experiment where I took
a few inches of #36 NiCr wire, and clipped it to a 7VDC, 1A wall wart,
and it cut styrofoam like hot butter:
http://www.abiengr.com/~sysop/images/CutFoam-640x480.jpg
(the gouge that looks like a raggedy hole that's been cut in half is a
raggedy hole that's been cut in half - a remnant of an earlier experiment
with styrofoam. Hey! Hot copper pipe! - Oops, off on a tangent. Never
mind.

Of course, the wire flexed a little, and steel would probably need more
current than nichrome to get the same temp, but that's pretty easy to
test, given the variac, of course.

Use a bunch of fuses, like 1A from the mains to the variac, 1A from the
variac to the 12V transformer, and 10A on the transformer secondary.
Seriously, it's to protect the transformer in case there's a short-circuit
type of mishap with the cutting wire.

Have Fun!
Rich

 

[[email protected]]

NT,

Would any of the listed circuits come close to the "ideal" setup you
mentioned? If not how would that be accomplished? I know that I
couldn't design such a circuit.

Thanks for the replies.
Brian

I'd forget all that, just plug the wire into a transformer so it works.
The other stuff is just making life a lot more difficult.


NT

 

[Fred Bloggs]

As I said in the first post I have a 120VAC -> 12V 8amp transformer.
Some of the replies mentioned using a variac. Could I use this
transformer with the variac? Would it be a shock hazard?

I would like to use cutting bows that are from 8" to 24" long. (Maybe
longer ones in the future).
I just want a flexible system that would work well, be safe and not too
expensive. In other words I don't want to assemble a circuit and not
have it work later on. Or buy a variac use it for awhile and then
later need to build a circuit. As I said, this really isn't my
expertise.

Your transformer is about a 10:1 step-down. Among other things this is
also the ratio of secondary to primary current. Someone mentioned 3 Amps
through the wire as a good number. That makes for 0.3A in the primary or
0.3*90=27W (allowing for wire voltage drop). So place a standard bulb
socket in series with transformer primary, a connect the hot wire
directly across the secondary. Then use either a 15, 20, or 25Watt bulb
in the socket for best effect.
View in a fixed-width font such as Courier.


..
.. /--\
.. | |
.. |bulb|
.. | |
.. \--/ 120:12
.. __ ----
.. --| |----------| |--------
.. --|__|--. ---- )|| ------o
.. | 15,20,25 )||( /
.. | )||( \ ss
.. | L M H )||( / wire
.. | )|| ------o
.. | watts )||
.. '---------------------
..
..