Anyone got a link to a circuit I can use to adapt a computer power
supply ( I know how to get fixed voltage out of it OK) to a variable
supply.It's gotta be simple and able to handle the power. This is in
regard to my Hotwire Foamsaw post now that I have seen the light and
moved away from a lamp dimmer and transformer.
The Gravel Pit
Visual Diary of an art metal caster
depending on the power supply, you can trick it by a variable resistor in
the reference feedback circuit - but if you try this with one of the newer
supplies it will be much harder. look for an older generation computer, or
if possible a linear not switchmode power supply.
Or, alternately, use a variable resistor on the output
It would help if you posted approximately what voltage and current
your hotwire requires.
A straight PWM circuit would be simple and quite inexpensive, 5 to 10
bux worth of parts if bought new. This could handle up to 50 amps
with no problem.
Depending on current and voltage, a "buck converter" is only slightly
more complex and has the property of magnifying current to the extent
that load voltage is less than supply voltage. It acts essentially
like a variable DC transformer. The trick there would be finding (or
making) an inductor that can hack the load current without saturating.
Nope. But I used to cut foam using a basic arc welder as the power
supply. I would attach the welding leads to the ends of the cutter
wire. I would start at the lowest setting on the front of the welder
and increase it until I got the right "feel" for the cutting process.
Too high and the foam would melt into a blob. Too low and the foam
wouldn't "cut" smoothly or easily.
Don, THANKS! I know there was something out there that worked like
this, just couldn't remember the term.
I need a power supply for that old 60Volt motor I posted about and
can't find a good 60ish volt transformer to work with.
Even though a "bucker" will certianly work.. IMHO a good ole Variac and some
diodes from hell are they way to go... I have an old computer tape drive
spindle motor mounted on my BP.. Some people think it's rated at 60VDC, some
think 90VDC (seriously I've see EXACTLY the same motor rated at both , I
mean same fricken part number)... I have it on a Varaic.. FWIW one of the
neat things about these old computer tape drive motors is that hey were
meant for continious hard duty and were meant to be air cooled (they used
the output of the vaccume system for the tape drive to blow into the
motor)... and yeah I managed to get the first stage vaccume assy (basiccaly
just a fan in a shroud) from the tape drive and I air cool it... I plow away
on the BP all day doing SERIOUS metal removal and the motor is cool as a
Checkout this url for some pictures of the Variac and Diodes from hell.. :-)
and although it shows an import lathe (my OLD lathe) I have 3 of these
motors and ..duh .. stuck one on the BP...
You'll also see a smaller version that I used to power a gearmotor as a
power feed on my old import mill/drill..... and yeah these are kinda old..
since I now have a BP and a Clausing :-)....
The PC supply will need a "bias load" on it to put out power. With a
PWM on the +5 output, it will shut down with no load on the +12.
A 250 watt AT (not ATX) power supply will ive 30 Amps at 5 volts (150
watts) or 11 amps at 12 volts (132 watts).
If you can find a 350 or 400 watt AT supply your output is almost
double. AT supply has a remote switch, while ATX is switched by the
mother board (more difficult to control)
You COULD use a saltwater rheostat on the output, but a PWM would be a
I'm sure you can do it, but I don't see ANY reason to use DC. All you
need is line isolation and safety precautions if you have much over 60
volts or so of either flavor.
As others have said, it would help if you told us how much power you
need; voltage range, current range. And maybe what the cold
resistance resistance of the installed cutting wire is.
I don't know. I have a 20 lbs e-core that I could wind as an
inductor, a PIC (which I have a bunch of) could provide the PWM for a
FET and a big diode later I have a supply.
Regulating the PWM should move the voltage around. I have a fair
number of electronics parts, this seems like a cheaper way of going
I think the issue is what kind of 'computer power supply' is it. It could
be a large linear frame, large Switcher capable of hundreds of amps, A general
linear that is highly filtered ? The number of letters in 'computer power
2^19 variations or 2**19 variations.
If a switcher - you can't use a variable transformer. Output regulation only.
If an Analog - then an input autotransformer 'Variac' (trade name) is most
Remember to draw out your circuit - know the hot resistance of your wire so you
can calculate the output regulator if it is a switcher. High current means a
high wattage tap. I suggest knowing the most and the least resistance and
steps - experiment in other words with the needed HIGH POWER resistors.
A switcher or an analog supply can be done this way - :
Once you find a good operating set - then make a set of two or three resistors
that you can tap into. In other words, switches won't work unless you happen
to a very high current switch.
Something like this :
+ ----/\/\/\-O-/\/\/\/-O-/\/\/\-O O~~~~~~~~~~~~~~~~------- -
The tilde or " ~ " equates to be the existing Nichrome wire or what you use.
The " /\/\/\ " are resistors - maybe 100 watt 5 ohm or maybe more.
The " O " are points where you bolt your heater wire (or a wire attached) to
the various points of the resistor tap.
The + and - are the supply points.
Remember work the numbers with the hot resistance value (voltage divided by
of your heater wire.
Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot"s Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
Maybe it's been suggested but a suitable 240-120 transformer could be used
for 120-60. Or just a 240/120 primary winding if the current rating is good
enough. A variable auto-transformer, such as a Variac, could also be a
possibility if you have one. Also, two 24V and one 12V transformer with the
secondaries in series.
12:10 pm, D>
I've thought of it, the issue is that the draws going to hit about
1.2KVA at the max. Thats a big transformer. I think I can do a buck
converter with mainly parts on hand. Maybe have to order a FET and
driver, but thats going to be like 3 bucks vs over a hundred.
I've got more PIC's than Carter has Liver pill's... Including a complete PWM
rig with current foldback and LCD display I did for a client to control high
power CO2 lasers...
I STILL hung the BFV on Diodes from hell.. :-)
What ever spins yer crank....
Dave, see Karl Townsend's post about AM radio noise. Could you look
at the PIC source code for the sensor and determine the duration of
the "read initiate" pulse to the sensor? The source code can be found
I'd like to determine the viability of putting an AM broadcastband RF
trap in the sensor line.