Switching power supply with light resistive load

Thread Starter

T3STY

Joined Oct 4, 2016
25
Hi,

I am trying to build my own hot-wire polyester cutter. I am using a switching power supply rated at 12V@1.5A, and a stainless steel wire (SS316L) that is long just as needed to get at around 10 Ohm.

The problem,is that the load resistance is not enough, and triggers the short-circuit protection. In fact, the power supply goes to 0V as soon as I connect the 10Ohm wire.

I am looking for possible solutions... I would like to avoid using a dummy load to not waste power.
A possible solution that came to my mind is to use a linear voltage regulator like a good old LM317 (or LM338) - actually, varying the output power could come in handy -but that would defeat the purpose and efficiency of a SMPS...

Do you guys have any tricks for this issue?
 

ericgibbs

Joined Jan 29, 2010
10,012
actually, varying the output power could come in handy -but that would defeat the purpose and efficiency of a SMPS...
hi T3.
If the Wire resistance is 10R and the SMPS is specified as 12V at 1.5A it should not shut down.?
If you have some 2Amp diodes on the bench, you could try a couple in series with the wire.
E
 

Thread Starter

T3STY

Joined Oct 4, 2016
25
Hi :)
I am not sure why it shuts down. The fact is that as soon as I connect the wire I can feelthe wire getting warm (not even near hot, I can definitely hold it in my hand without getting a burn). But this lasts for 1 second or 2, after which, the power supply goes dead. Measuring with the multimeter gives 12V before connecting the wire, and 0V as soon as I connect it.
The power supply works just fine for other appliances (for instance, an 18650 Li-ion battery charger that requires 12V@1A).

I will try as you suggested with some diodes, maybe it does the trick...
 

Thread Starter

T3STY

Joined Oct 4, 2016
25
The wire is just the same used in electronic cigarettes, so basically, I just unwind it from the plastic roll and measure with the multimeter in parallel with the two ends of the wire (multimeter set on 20R scale), until I achieve the desired resistance. I found that the length of the wire at around 10R works perfect for me.

I forgot to mention earlier, I also have a 4.5V @800mA power supply at the moment, still a switching one, and it works. It's just that the power is not sufficient to get the wire hot enough (it's warm enough, but it still doesn't burn my hand), and it takes forever to make the cuts... Not to mention that the wire cools down so easily, even with my breath will cool it...

Edit:
I meant to say 200 Ohm scale, the lowest I have on the multimeter.
 
Last edited:

Thread Starter

T3STY

Joined Oct 4, 2016
25
No, unfortunately I only have two 12V switching power supplies, the kind that are included with modems/routers.
 

Reloadron

Joined Jan 15, 2015
5,425
You are sure the wire is 10 Ohms? Using 316 Stainless Steel wire:
22 Gauge / 0.644mm
0.695 Ohms/ft at room temperature

24 Gauge / 0.51mm
1.101 Ohms/ft at room temperature

26 Gauge / 0.405mm
1.760 Ohms/ft at room temperature

28 Gauge / 0.321mm
2.803 Ohms/ft at room temperature

30 Gauge / 0.255mm
4.450 Ohms/ft at room temperature

32 Gauge / 0.202mm
6.953 Ohms/ft at room temperature

34 Gauge / 0.16mm
11.21 Ohms/ft at room temperature

36 Gauge / 0.127mm
17.80 Ohms/ft at room temperature

Room temperature being about 68 Degrees F. So just for example 10 feet of Awg 24 would be 10 * 1.101 = 10.1 Ohms. You mention your supply as 12 V 1.5 A. So while in the example using 10' of AWG 24 you would get 12 / 10.1 = 1.188 Amps which should work but using that same wire in a 5.0' length we get 5 * 1.01 = 5.05 Ohms so now we have 12 / 5.05 = 2.376 Amps. What gauge of 316 stainless are you using and what length? Sounds like your power supply is folding over because it can't provide the needed current for your load. Should that be the case you can either get a PSU with more current capability as mentioned or if possible increase the gauge # of the wire.

Ron
 

Thread Starter

T3STY

Joined Oct 4, 2016
25
Hi Reloadron,

I am using 29gauge SS316L.
I am quite unsure now whether my multimeter was working correctly. I'm sure I read the value "10" on the multimeter when I measured the wire.
I don't know the exact length of the wire, I have only "eyeballed" the length and measured it to be 10R. I am not at home right now so I can't verify everything, but I will do so as soon as I return.

The length however should be around 20-25 cm ( about 9 inches?). It's supposed to be a hand-held device.
If the resistance per feet you mentioned are correct, then the actual resistance might be way lower than 10R, which makes sense to trigger the short-circuit protection...

Thank you all for the tips, I'll come back as soon as I'm home and I have verified everything.
 

Reloadron

Joined Jan 15, 2015
5,425
I sort of figured for a cutting wire it would be 12" or less. So you can figure about 3 Ohms on a foot of what you have which works out as 12 / 3 = 4.0 Amps. That would explain quite a bit. No clue though why you measured 10 Ohms?

Ron
 

Thread Starter

T3STY

Joined Oct 4, 2016
25
Hi,

I have checked again and indeeded, there were some wrong measurements.
First of all, I have checked and the wire is 30 gauge, not 29.
Second, the wire of one of the multimeter leads was actually broken near the contact of the test lead. It caused the measurement to read about 6-7R more. I have replaced it and managed to get a correct reading on the wire of 4.8R - 5R (value was a bit unstable jumping between 4.8R and 5.1R).
The wire length is slightly less than 30cm.

By my calculations, on 5R load at 12V flows 2.4A current, which is way over the capabilities of the power supply I am using.

I think this answers the main question... Still, what would you suggest to limit the current at 1.5A, so not to exceed the power supply limit?
 

Reloadron

Joined Jan 15, 2015
5,425
I think this answers the main question... Still, what would you suggest to limit the current at 1.5A, so not to exceed the power supply limit?
Well things make sense now. However, here is the problem. We can limit the current by placing a resistor in series with the wire. The problem becomes how much current do you need to get the wire hot enough to cut the polyester material? OK using your numbers 12 / 5 = 2.4 Amps so we now have 2.4 Amps * 12 Volts = 28.8 Watts. OK so lets say we place a 5R resistor in series. Now we have R total = 10 Ohms so 12 / 10 = 1.2 Amps. So with the current limited to 1.2 Amps through the wire the voltage drop across the wire is now 6 Volts. So 6 * 1.2 = 7.2 Watts or about 1/4 of the power we originally had. So will that be enough heat to do the job and also the resistor needs to dissipate heat so you need a high wattage resistor, likely a 5 Ohm 20 Watt and even that will run warm.

I have no idea how much heat you need but what you have is a 12 Volt 1.5 Amp supply which is 18 Watts of total power and that assumes the supply will really deliver that much power.

Ron
 

Dodgydave

Joined Jun 22, 2012
9,155
Well things make sense now. However, here is the problem. We can limit the current by placing a resistor in series with the wire. The problem becomes how much current do you need to get the wire hot enough to cut the polyester material? OK using your numbers 12 / 5 = 2.4 Amps so we now have 2.4 Amps * 12 Volts = 28.8 Watts. OK so lets say we place a 5R resistor in series. Now we have R total = 10 Ohms so 12 / 10 = 1.2 Amps. So with the current limited to 1.2 Amps through the wire the voltage drop across the wire is now 6 Volts. So 6 * 1.2 = 7.2 Watts or about 1/4 of the power we originally had. So will that be enough heat to do the job and also the resistor needs to dissipate heat so you need a high wattage resistor, likely a 5 Ohm 20 Watt and even that will run warm.

I have no idea how much heat you need but what you have is a 12 Volt 1.5 Amp supply which is 18 Watts of total power and that assumes the supply will really deliver that much power.

Ron
Why bother wasting heat with a resistor, just double the length of wire, better still use a Constant Current source.
 

Reloadron

Joined Jan 15, 2015
5,425
Why bother wasting heat with a resistor, just double the length of wire, better still use a Constant Current source.
Yeah, double the length would work as long as with a wire cutter which he is making there is room. Maybe fold it over for twice the length in the same distance. My take is he wants to use his existing supply which I don't see putting out enough power. All of the wire cutters I used for cutting polystyrene and foam allowed for adjustable heat but the better ones were costly. Anyway, yes, there are plenty of options but I believe most will require more than 12 volts at 1.5 amps.

Ron
 
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