Foam wire cutter power supply

[Rolf]

Fully Loaded........

Providing the transformer is nearly fully loaded it will appear to be a resistive load so this shouldn't be a problem.

Providing the transformer is NOT fully loades it will appear to be a reactive load and this could be a problem.

 

[duffy]

Why is that, Rolf?

 

[Hero999]

If a transformer is unloaded it's very inductive.

If a transformer is fully loaded it's nearly completely resistive.

If you use a 18V 100VA transformer with a 3.2Ω load the transformer will be very slightly overloaded so it will look like a resistor to the dimmer. A 100VA transformer is very unlikely to overheat if a 101.25W load is connected, especially in this case when most of the time the dimmer will limit the current.

If you add a snubber network and overrate the TRIAC, there's no reason why your dimmer can't be used to driver a slightly reactive load. I've seen phase control dimmers used to control the speed of universal motors before with no problems.

100W is a lot of heating power for a foam cutter, I doubt you need anything this powerful, a 6V 12VA transformer will probably give you enough power.

EDIT:

replace the pot value by 5K (500Ohms would do, too. Finding a wire 5K pot will be difficult.)

You'll need to increase the capacitor to 2.2µF if you want to reduce the resistor to 500R. If you can't find 5k, then it's probably more convenient to use 100nF and 10k.

 

[Rolf]

Why.........

Why is that, Rolf?

I am not sure what the theory is. But I guess it is inductive ringing during shutdowns, causing higher spikes than normal RMS voltages on components used not rated for it.
See the first post where the originator complain about blown dimmers. I stated in a later post that dimmers are not built for inductive loads (transformers).
But I have no idea what a "fully loaded" transformer is or how you could maintain such a condition under circumstances like we are talking about.

 

[melchead]

Here is a link to a triac control for an inductive circuit: **broken link removed**

Scroll down and you will see one similar to the one you posted. I would put it in the primary of your transformer.

Could you help with the values for this circuit please

 

[melchead]

The fact that you were replacing dimmer switches might be a clue. Are you using nichrome wire, or substituting copper wire on the cutter?

I am using stainless steel wire - on the 48" bow the resistance approx 3.2 ohm

 

[melchead]

If a transformer is unloaded it's very inductive.

If a transformer is fully loaded it's nearly completely resistive.

If you use a 8V 100VA tranformer with a 3.2Ω load the transformer will be very slightly overloaded so it will look like a resistor to the dimmer. A 100VA transformer is very unlikely to overheat if a 101.25W load is connected, especially in this case when most of the time the dimmer will limit the current.

If you add a snubber network and overrate the TRIAC, there's no reason why your dimmer can't be used to driver a slightly reactive load. I've seen phase control dimmers used to control the speed of universal motors before with no problems.

100W is a lot of heating power for a foam cutter, I doubt you need anything this powerful, a 6V 12VA transformer will probably give you enough power.

EDIT:

You'll need to increase the capacitor to 2.2µF if you want to reduce the resistor to 500R. If you can't find 5k, then it's probably more convenient to use 100nF and 10k.

I found that cutting EPP foam the ideal current flow on the cutting wire (with a 48" bow) was approx 5.2 amp and on normal polystyrene (EPS) about 4.6 amp

 

[RODALCO]

The variac and TX is the best option,

although I have successfully wound about 10 turns of 2.5 mm² wire around a 100 VA transformer ( which had enough space to do so ) and use about 10 cm of resistance wire and it works great.

If too hot, make the R wire a bit longer or remove 2 or 3 turns.

The simpler the more reliable.

 

[duffy]

Here's a simpler circuit from that document. Neither of them have a snubber, both are shown driving a reactive load, and this one appears to be an ordinary lamp dimmer circuit.

But by all means, put a snubber on it. Or use a "snubberless" triac.

 

[Rolf]

Could you help with the values for this circuit please

Will this help?
**broken link removed**

 

[melchead]

Will this help?
**broken link removed**

This is not an option because safety is a concern! OR do you suggest I place the transformer as a load? and then drive the cutter from the 24vac output

 

[melchead]

Here's a simpler circuit from that document. Neither of them have a snubber, both are shown driving a reactive load, and this one appears to be an ordinary lamp dimmer circuit.

But by all means, put a snubber on it. Or use a "snubberless" triac.

Can you help with the component values for this circuit please? I am still not an expert to calculate the values.

 

[Rolf]

Safety

This is not an option because safety is a concern! OR do you suggest I place the transformer as a load? and then drive the cutter from the 24vac output

I wold definitely drive the transformer as a load, that is the only way to make it safe.

Here is how I would test it:

1. Put just a 100w incandescent in the circuit. If it works go to step 2.

2. Wire in the primary but leave the light connected. If it works go to step 3.

3. Remove the light and keep your fingers crossed!

 

[duffy]

Can you help with the component values for this circuit please? I am still not an expert to calculate the values.

Melchead, don't let us throw you off track from that original circuit. It seemed like it would work to dknguyen, and it seems like it would work to me, too. Replace that triac with a MAC15SMG, the gate current's a little low for that other one. The cap is .22µF, not 220µF.

 

[melchead]

Melchead, don't let us throw you off track from that original circuit. It seemed like it would work to dknguyen, and it seems like it would work to me, too. Replace that triac with a MAC15SMG, the gate current's a little low for that other one. The cap is .22µF, not 220µF.

thanks i will try that triac. i got it to work in multsim by reversing t1 and t2 on the triac. i did not know that there is a difference or am i wrong?
please advise me
thanks

 

[duffy]

Yes! There IS a difference! That got me, too, one time. Seems like they ought to differentiate them better, doesn't it?

 

[melchead]

I got a BTB16 800 Triac that fits the specs you mention. Once I got it running realtime not simulation, then I would like to add a analog amp meter. scale 0 to 10 amp. Where can I include it in the circuit without influencing the current to my cutting wire?

What I did notice is that if I bypass the R1 (5ohm) resistor then it does not want to work. Why would this be? The circuit was designed to bypass this resistor when using the longer wire.

 

[duffy]

Analog circuit simulators aren't always worth a damn. Get more than a few parts and they hang, the waveforms rarely match what you see on the scope.

The amp meter is a good idea. Problem is, it's AC. You are going to need to add a shunt resistor to the circuit in series with the cutting wire, and then you have to rectify the (very small) voltage and read it. I would recommend getting a cheap analog multimeter that can read 10amps AC and making it a permanent part of the circuit.

 

[melchead]

The simulator did help me as it made it easy to change components. Besides I do not have a scope and a variety of components to try. Using a cheap multimeter would be a better alternative.

Thanks

 

[melchead]

Finally it's working!

Thanks guys.. for your help. I have learned a lot from this forum.

My foam cutter power supply seems to be working 100%. I am just not sure what happens if the triac fails? does it go open circuit between t1 and t2 or does it give the full 24vac?
I am asking because I noticed that it used to happen with the dimmer switch on the transformer that I used before this circuit.