Spare PSU from PC for wire cutter?

[GlorfindelRW]

I have a couple old 20-pin ATX power supplies from previous upgrades to my PC. I know that you can put a jumper on the motherboard connector to let you use this power supply without plugging it into the motherboard. (http://www.lumenlab.com/forums/index.php?showtopic=10558&pid=249648&st=0&#entry249648)

I don't really know my electronics very well, but since the pins on computer PSUs can output +12vdc, I was thinking about using one to create a device as follows:

- Has a few banana plugs for 12v to plug in chargers and other such things that use a 12v battery as a power source.
- Connect one 12vdc output thru a light dimmer switch for usage as a home brew hot wire foam cutter.

I searched around and couldn't seem to find anyone else doing similar... am I crazy, or would this (safely) work?

Edit: Just saw that light dimmers do not work with DC... is there a cheap alternative to use with the above setup, supposing that the rest of the idea is feasible?

[slipstick]

The first part of your idea works fine and there are hundreds, possibly thousands, of people already doing it.

For the hotwire part, since this is the DIY Electronics forum you could always build yourself a PWM switching "DC dimmer" using a MOSFET or two. But I couldn't be bothered and I just use a big heavy old brushed speed controller that will never go into a plane again with a simple servo tester to set the temperature.

Steve

[GlorfindelRW]

Thanks, slipstick.

A thought just occurred to me... what about using a fan speed controller? Such as this: http://www.newegg.com/Product/Product.aspx?Item=N82E16811998129

Interface: PCI
Output DC Variable: Manually Adjustable Voltage
DC Input Voltage: 12V +/- 10%
DC Output: 5.0V - 11.70V
Max Current: 1.2A

Or would I be in trouble because the system would pull too many amps for this to work?

[slipstick]

Or would I be in trouble because the system would pull too many amps for this to work?
Depends what type of wire you're using and how long it is. I've had hotwire cutters that need a lot more current than that (3A+) and also some that need more than 12V because of the length.

If you already have the cutter, measure what it uses. If you don't then "home brew" that first and then measure. Just guessing may not help :(.

Steve

[GlorfindelRW]

I don't have a cutter built or anything yet... but I saw nycrome wire replacements for a bow cutter at a LHS for about $2. I'm not sure how long it was though... might be too short.

[pburt1975]

Will a 120vac to 24vac @40va be enough to heat up a 24" piece of .025 stainless steel wire?
Just some stuff I have on hand already.

I cant find specs on the exact xformer I have, but all that match its rating are only rated for 1.66 amps max output. doesnt sound enough, but its not that much wire either??

[MarkusN]

A stainless steel wire of 24" and 0.025" dia at roughly 70 uOhm*cm has a resistance of about 1.35 Ohms. I.e. it will overload your transformer. Several times over.

More (or thinner) wire would actually be gentler on the transformer, but then, I have no idea what kind of power is needed to cut foam.

[Andrew0820]

I'm using a converted PC power supply for charging and foam cutting. The foam cutting control is a fairly simple PWM circuit using a single 27A MOSFET. The PWM control gives very fine temperature control. Cutting wire is type 302 .018" (~25 ga) stainless available from McMaster-Carr at $8.75/285'.

[GlorfindelRW]

Andrew, that looks exactly like what I was thinking about. Unfortunately, my knowledge of electronic components and circuits is very lacking except, perhaps, logic circuits. Is there a diagram somewhere that I could use to try and reproduce this?

Also... when you use it for charging are you just turning your dial to max, or does one of the two red leads bypass your circuit?

PS. I did just get a polycharge4 recently, and what I learned from this has helped in successfully rigging up a temporary power supply from a spare PSU, thanks! If anyone is curious, I used some wire I found laying in the garage (not sure what it came from) to jump the green pin on the 20/24 pin motherboard connector to a black lead, and then I plugged in a failed hard disk to create a load and get the voltage up... my voltmeter is too inaccurate to say what the exact output is afterward. It has worked well!

[Andrew0820]

Also... when you use it for charging are you just turning your dial to max, or does one of the two red leads bypass your circuit?


My charger plugs directly into the 12v outlet from the power supply. When used for cutting, the red/black jacks on the PWM controller are also attached to the 12v rail in the power supply. The leads to the cutting bow are plugged into the black jacks on the backside of the PWM control.

The value of RF in the schematic is 34K -- this drives the 555 timer around 115 Hz. I substituted a IRF540 in place of the IRF510. The 540 MOSFET is rated at 27A and is more than sufficient to carry the bow. The heatsink is really larger than needed since MOSFET temps are dictated by switch speeds as much as load -- I just had it in the junk box. At 115 Hz, it does not even get warm under load. D1 can be any of the 1N400x series diodes -- its primary purpose is to prevent backfeed into the MOSFET from a fan. This particular design will run from 0 to 100% duty cycle.

andrew

[pburt1975]

A stainless steel wire of 24" and 0.025" dia at roughly 70 uOhm*cm has a resistance of about 1.35 Ohms. I.e. it will overload your transformer. Several times over.

More (or thinner) wire would actually be gentler on the transformer, but then, I have no idea what kind of power is needed to cut foam.

Thanks MarkusN,
Ok, I also got a hold of an E string from an acoustic guitar. Its about 3' long.

Is there any way to make this transformer work with the wire I have?

It would be nice to because I only have a 1.5" sheet of the blue foam. Cutting wings out of that is a PITA and takes a ton of finish work, as I found out this weekend. Also I cant find sheets of this foam at 1/2" thick at lowes. Well I can but I dont need 50' x 3' of this stuff right now

[wwldo]

I'm using a converted PC power supply for charging and foam cutting. The foam cutting control is a fairly simple PWM circuit using a single 27A MOSFET. The PWM control gives very fine temperature control. Cutting wire is type 302 18 ga. stainless available from McMaster-Carr at $8.75/285'.
Andrew080,

Can this be used to regulate/adjust the voltage on my BL motor test/thrust stand..?
I using a Pyramid regulated power supply.

Thanks,

Larry

[Andrew0820]

Larry -

A PWM (pulse width modulation) circuit does not change the voltage level, but chops the existing voltage into chunks or pulses. If the input voltage to the PWM circuit is 12 volts, then the output device "sees" pulses of 12 volts and 0 volts. For a 50% duty cycle, the output is high 50% of the time and 0v 50% of the time -- these cycles of 50/50 are sent out about 115 times a second (the timing may vary, but the above circuit runs around 115 Hz). This circuit can be used to drive computer case fans, but these fans have the timing circuits built into the fan itself. Your brushless motor requires external timing -- usually provided by an ESC.

You could attach your ESC directly to a converted PC powersupply and drive your test stand like that, but the PWM circuit will not be a satisfactory control.

andrew

[wwldo]

Hi Andrew,

I have a good PS as described in my post, I want to use it as a Batt simulator so I need to be able to adjust the voltages for different batt sinulations, for use on my thrust stand.

Any sugestions....?

Larry

[Andrew0820]

Larry --

Here is a schematic for no frills voltage regulator (http://www.uoguelph.ca/~antoon/circ/ps-lm317.html) - see the schematic at the bottom of the page for a current bypass circuit. In place of C1, I would substitute a 2200uf electrolytic rated at 50v -- this will supress any ripple coming out of your power supply. Also, replace R1 and R3 with a 5K pot -- this should get you down to about 1.25v on output (minimum for the LM317). Additionally, if you will replace R2 with something in the neighborhood of 560 ohms, it will broaden your voltage adjustment across the full range of the 5K pot.

Use the NTE393 instead of the TIP2955 -- the NTE393 is rated at 25A continuous, 40A peak. The 2955 is rated at 15A continuous.

Both the power transistor and the LM317 are TO-220 packages and will need heatsinks for continuous loaded operation --

andrew

[wwldo]

Andrew,

That looks great, my PS is a regulated 13.8V DC at 12 AMPs constant and 14 AMPs surge...... so the way I see it is that will be the max AMPs I will be able to pull, is that correct..?

Would it be better to get a PC power supply for more AMPs or what do you suggest..?

Thanks for the help,

Larry

[Andrew0820]

Andrew,

That looks great, my PS is a regulated 13.8V DC at 12 AMPs constant and 14 AMPs surge...... so the way I see it is that will be the max AMPs I will be able to pull, is that correct..?



Larry --

While not always the situation, PS manufacturers sometimes overstate the capacity of their units. In any case, 12A continuous is the most you can expect to get. How much current do you need for your test stand?

A 350W Sparkle that I have in the office is rated for 18A @ 12v -- that's pretty hefty. To move much above 20A on the 12v rail, you really have to step up in wattage. Interestingly, the less expensive a PS is at a given wattage, the higher the current claims seem to be -- probably because the more expensive PSU manufacturers are more accurately stating their specifications.

Using your powersupply, to fully utilize the 13.8v output, drop R2 in the listed circuit back to about 470 ohms. I had set the 560 ohm rating based on a 12v output.

andrew

[wwldo]

Ok, I will stick with the PS I have, it will serve 90% of the testing I will be doing, when I need more amps I'll have to use a battery..........

Thanks for all the inputs/help...

Larry