Foam wire cutter power supply

[melchead]

Hi Guys,
I need help to get a circuit to work. I found a triac controlled circuit on the web but I can not get it to work.
I used a dimmer switch to control the current on a 220/24VAC transformer that worked well but does not last. I replaced 3 dimmer switches before I decided there must be a more reliable way and safer way to cut foam. The attached circuit was used by a person that claims he has been cutting foam with it for 15 years. So I think it must work but perhaps he did not include the correct circuit. I cannot get his contact details to ask him.

Please help guys

Thanks

 

[dknguyen]

Looks fine to me.

 

[melchead]

I am not sure what type of capacitor is used here and also is the 470ohm resistor connected to the capacitor? or does the wire cross over to the wiper of R2? I also simulated the circuit in Multisim (electronic workbench) and even there it does not work.
I need at least 4.5 to 5.5 amp to cut EPP foam. The resistance of the cutting wire is about 3.2 ohm.

 

[dknguyen]

The 470 ohm resistor is connected to the capacitor. The lower end of the variable resistor does not actually have to be connected to anything in this case...as a short-circuit is formed across it. It serves to adjust the resistance of the RC combination thus changing the time that the time the capacitor takes to charge to a voltage sufficient to drive enough current through the 470R resistor to trigger the triac.

THe capacitor here is dealing with a 60Hz signal...super super low frequency. Which means any capacitor will work fine. But since it is bipolar AC, the capacitor must be non-polarized. However, the capacitance requried is very large for a non-polarized capacitor and the schematic is displaying a polarized capacitor (like an electrolytic). The likely explanation is that the reverse voltage across the capacitor never exceeds ~1.5V which the capacitor can withstand...but I can't exactly see why that is. Another exaplanation is that the author is using mF as microfarads (uF) instead of millifarads (mF), as some old notations use m in place of u. In which case you can get a non-polarized capacitor quite easily (like a ceramic).

The size of R1 is equal to the resistance of the length of wire that is NOT present in order to get the current you desire- which you probably already know. If not, now you know. So V=IR gives you 25Vrms = 5A*(Rwire+R1). Plug in Rwire and solve for R1.

 

[melchead]

I tried with the connection and without the connection

 

[dknguyen]

Don't know what to tell you. Circuit looks fine to me unless you are just simulating it improperly. Try lowering the value of the 200R or less (or just set it to zero ohms). There may not be enough drive current going into the gate. THe datasheet says a max of 125mA is required, and at 25Vrms = 35Vp, that means a resistance of 280R or less is required (assuming there is no base-terminal voltage drop, which there is so the resisistance is even lower.

 

[Leftyretro]

For best long term reliablity I would just use a small Variac to drive the 24v transformer. Simple and reliable always saves time and money in the long run.

Something like this would work well:

**broken link removed**

Lefty

 

[melchead]

In my case the mains supply is 220VAC to 24VAC and 50Hz signal. So what you are saying is that I should use a .22uF ceramic capacitor?

 

[dknguyen]

In my case the mains supply is 220VAC to 24VAC and 50Hz signal. So what you are saying is that I should use a .22uF ceramic capacitor?

Yes, I am- one rate for at least 50V since 25Vrms = 35Vpeak + some margin since mains voltage fluctuates.

(THe variac is a better solution assuming you can find one you can afford and bigger ones can support ridiculous amounts of current. THey can also outputs voltages at mains level so it it can be dangerous. You may want to put a bead of epoxy or something to act as a stopper to keep you from turning the knob up to too high near mains voltage when it's dangerous.)

EDIT: ANother note- variacs are variable autotransformers. Autotransformers meaning they are do not provide isolation from the mains like regular transformers. So you might want to stick a regular 1:1 transformer at it's output for isolation.

 

[melchead]

The variac would be the solution but the costs to South Africa with the current exchange rate would not make it viable. (far too costly for me) So I am forced to persue the current type of circuit to control the wire temperature. Do you perhaps have a different type of circuit that will work?

 

[dknguyen]

The circuit you have is fine.

 

[melchead]

Thanks for your help. I will follow your advice and give feedback when I got it working.

 

[kjennejohn]

OK, perhaps my ignorance shows, but the drawing says the 24Vac transformer is rated for 2 Amps and the user says the cutter typically uses 4.5 to 5.5 Amps. Don't we get a smoking transformer pretty quick? This must be running in REALLY short pulses. What am I missing here?

Later!
kenjj

 

[melchead]

Sorry but the circuit and supply voltage used in my country is 220VAC and the transformer I am using is rated at 10 amp. This is the circuit I got from the web . My supply is 24VAC @ 10AMP

 

[duffy]

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

 

[Rolf]

Dimmer Failure ..........

Hi Guys,
I need help to get a circuit to work. I found a triac controlled circuit on the web but I can not get it to work.
I used a dimmer switch to control the current on a 220/24VAC transformer that worked well but does not last. I replaced 3 dimmer switches before I decided there must be a more reliable way and safer way to cut foam. The attached circuit was used by a person that claims he has been cutting foam with it for 15 years. So I think it must work but perhaps he did not include the correct circuit. I cannot get his contact details to ask him.

Please help guys

Thanks

If you wired the dimmer to the primary of the transformer it would probably fail in short order because they are made for resistive loads.
If you could get another transformer like the one you have and wire the two primaries together, then you would have a isolation transformer and you should be able to use the dimmer on the secondary with the nichrome without any problems.

 

[Hero999]

I need at least 4.5 to 5.5 amp to cut EPP foam. The resistance of the cutting wire is about 3.2 ohm.

Use a 15V 70VA or 18V 100VA transformer, perferably the latter, larger is always better as you can easily reduce the temperature using the dimmer.

 

[Rolf]

Inductive Load TRIAC Circuit

Hi Guys,
I need help to get a circuit to work. I found a triac controlled circuit on the web but I can not get it to work.
I used a dimmer switch to control the current on a 220/24VAC transformer that worked well but does not last. I replaced 3 dimmer switches before I decided there must be a more reliable way and safer way to cut foam. The attached circuit was used by a person that claims he has been cutting foam with it for 15 years. So I think it must work but perhaps he did not include the correct circuit. I cannot get his contact details to ask him.

Please help guys

Thanks

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.

 

[Hero999]

If you wired the dimmer to the primary of the transformer it would probably fail in short order because they are made for resistive loads.

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

 

[Boncuk]

Unless you want to cut big foam blocks of 1m thickness the required current is really relatively low.

I used to cut 2inch foam using a normal household bell transformer rated 9V/1A. To cut I used the G-string of a guitar.

There is no temperature regulation necessary if the wire length is about 6 inches.

Just in case you didn't get the schematic right (missing junctions and last not least, switched life and fused neutral!!!!) here is a schematic not omitting junctions.

For the 0.22µF capacitor use a WIMA MKS-02-220n or MKS-2-220n (rated 100VDC/63VAC respectively 63VDC/40VAC).

The symbol used is correct. It's the normal ANSI symbol for non-polarized capacitors.


Boncuk

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