PSU for Nichrome wire cutter

[giro1991]

Edit: before you read all this post, the problem was lack of isolation between the two ends of wire (the frame was live, thanks again to those who helped)

Hi, I have made a nichrome wirecutter, the wire itself was cheap enough so thought why not. I have transformers lying around I thought one ought to work. I tried batteries prior, 2x 3.7v 3000mA and this worked but only briefly, the power drained too quick and the batteries got hot too so proba not the best idea.



One PSU I tried is from a networking device, 240in, 3.3 (rated 12A) and 5v (rated 4A) out. I tried both outputs but the wire didn't heat up. COnsidering tutorials for wire cutters online suggests a car battery charger I hdoubted this tiny PSU would work anyway.

Another PSU is a battery charger, rated 6V 1A. Tried - no heat on wire.

Here are the specs of the transformers (all 240 primary)
First one has secondaries:
43-0-43
9-0-9
20-0-20
Its a big thing from 70w power amp, if anything I'd expect this to be suitable, however when I hooked nicchrome wire to one of the 43v secondaries, the 5A fuse on the primary side went.

Another transformer: UK to japan 100v step-down transformer, one primary, one seondary. Not tried this out of fear of bodging it.

Another transformer: twin 110 primaries connected at center tap to work on UK mains, whose secondaries are single 170-0 and 6.3-0. Connecting these up did not heat up the wire at all. The 6.3 warmed it up a little.

Another: twin 110 input again but with 2x 15v secondaries. Its 12Va. When I tried this I got nothing to which I though was odd. I tried the two secondaries in parrallel which probably wasn't wise and upon activating the circuit, smoke arose from the transformer.

The length of the wire is 300mm in length.
The resistance is around 5ohm (measured with MM)
However the specs of the wrie as follows from the sale page:
5.02Ω/m @ 20ºC
Approximate current needed to heat a horizontal length of wire in free air to:-
205ºC 2.5A
315ºC 3.3A
425ºC 4.0A
540ºC 4.9A
650ºC 5.9A

I know that Square law need to be applied to AC voltages to determine equivalent DC voltage, but from my calculations, I thought 20v AC should be sufficeint however when I tried the 20v from xformer #1, it did nothing. I'm tempted to just stick it into the bloody wall outlet.

How does anyone begin to even approach such a problem? Its just a flimsy wire!!

 

[stevenrotterdam]

Try a 12 volt halogen transformer. A big one.
If a toroid then you can increase voltage by putting on some extra turns.

 

[russc]

5 ohms per metre;
therefore 300mm = 5x0.300 = 1.5ohms.

Say you want 425ºC 4.0A -
you need 4.0x1.5 = 6volts.

Therefore a 6 volt supply that can supply in excess of 4 amps (for a safety margin, the calculated current will be an approximation).

For a different length of wire or a different temperature, substitute the appropriate figures.
Look up ohms law - that's all you need for this exercise.

AC or DC makes no difference. AC (RMS) voltage is usually defined as the equivalent DC heating voltage.

A disused computer ATX supply could be a useful power source; especially if it has auto shut-down on overload.

 

[russc]

1 - Are the ends of the wire insulated from the saw frame? looks to me as though the current will pass from one end of the saw frame to the other.
2 - You won't live long plugging that contraption into the mains - stay safe!

 

[giro1991]

1 - Are the ends of the wire insulated from the saw frame? looks to me as though the current will pass from one end of the saw frame to the other.

I actually did test but your question prompted me to test again, with a continuity tester, turns out the ends aren't insulated... DOH

 

[giro1991]

I put insulating tape around the black rod before sliding it back into the blue handle. Both ends are isolated now. 12v from spare PC psu and it works thanks fellas




(Near carpet, dangerous risks.........)

 

[russc]

Had you connected to mains, hand around the saw handle, muscle spasm would have prevented you letting go.
Even the 43v transformer (60v peak) could have been fatal.

 

[JMFahey]

Hi, I have made a nichrome wirecutter, the wire itself was cheap enough so thought why not.
I have transformers lying around I thought one ought to work.
I tried batteries prior, 2x 3.7v 3000mA and this worked but only briefly, the power drained too quick and the batteries got hot too so proba not the best idea. You shorted Li-Ion batteries, what could go wrong?

One PSU I tried is from a networking device, 240in, 3.3 (rated 12A) and 5v (rated 4A) out. I tried both outputs but the wire didn't heat up.
Another PSU is a battery charger, rated 6V 1A. Tried - no heat on wire.

Here are the specs of the transformers (all 240 primary)
First one has secondaries:
43-0-43
9-0-9
20-0-20
Its a big thing from 70w power amp, if anything I'd expect this to be suitable, however when I hooked nicchrome wire to one of the 43v secondaries, the 5A fuse on the primary side went.

Another transformer: UK to japan 100v step-down transformer, one primary, one seondary. Not tried this out of fear of bodging it.
probably an autotransformer, this one would have killed you.

Another transformer: twin 110 primaries connected at center tap to work on UK mains, whose secondaries are single 170-0 and 6.3-0. Connecting these up did not heat up the wire at all. The 6.3 warmed it up a little.

Another: twin 110 input again but with 2x 15v secondaries. Its 12Va. When I tried this I got nothing to which I though was odd. I tried the two secondaries in parrallel which probably wasn't wise and upon activating the circuit, smoke arose from the transformer.

I'm tempted to just stick it into the bloody wall outlet.

How does anyone begin to even approach such a problem? Its just a flimsy wire!!

You are a dangerous crazy experimenter, we can not mince words here.
You survived this one, you will soon die or at least burn your home or something.
This is not a Tech problem, but a devil-may-care attitude.
You should not be allowed to be within 10 meters of Electricity ... not even batteries since you *just* got within seconds of exploding 3.7V batteries or making them catch fire.