pyrography power supply questions OT

Am trying to hack together a set up to power a wood burning pen.
The set up is similar to a hot wire power supply. Just a light dimmer
switch and a transformer.
After a little playing I managed to get it to put
out the right amount of power to heat the pen tip to red hot
and able to burn wood, leather...
The problem I now have is that after a few minutes the transformer
starts to heat up and I get that lovely "electrical stuff burning"
smell.
I am about to give up and just buy the right supply, but before I do,
just wondering if there is anything obvious I am missing.
Some of the details:
The dimmer needed a little adjustment to get the output from the
transformer down to about 4-6 volts. I've replaced the 150K trim pot
with a 330K resistor.
The transformer was originally set to put out bout 24V ac. and I've
taken 15 turns off the output winding. Output voltage at full power
is down to 19.5
I can lower this to less than 1 volt using the dimmer.
The winding resistance is about .3 ohms and that is the
approximate value of the burning tip resistance.
Before I started hacking the transformer, it had a 4 amp breaker
built in. So I assume that it was able to handle about 96 W
I assume that the transformer must dissipate the same about
of heat as the burning tip and this should be in the 100 W
range.
That doesn't seem to be too much but...
Appreciate any help, ideas...
DOC
Reply to
doc
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The power that the transformer can _deliver_ to the load is about 96w (24v x 4A). The power that it can _dissipate_ is about 5W (4A^2 x .3 ohms). In other words, the limiting parameter is the current. This transformer can only deliver 4A (at whatever voltage) without overheating.
What you can do is convert a MOT (microwave oven transformer). Remove the secondary and put a few turns of whatever guage wire in its place. The guage needed will be determined by the current required by the pen. E.g., #14 would do for 10A. You will get about 1v per turn of secondary.
See here for some MOT stuff:
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HTH, Bob
Reply to
Bob Engelhardt
The transformer does not need to dissipate the same amount as the burning tip. If the burning tip dissipates 100 watts and the transformer only 1 watt, then the burning tip would get hot and the transformer would stay cool.
I would recommend you strip off the rest of the output winding and then wind on enough turns to give you say six volts using much heavier wire. With the burning tip having a resistance of 0.3 ohms, the current would be 20 amps at 6 volts or 120 watts for the burning tip. If the output winding of the transformer is at least 10 gauge , then the output winding should not get too hot during intermittent use. You can use two strands of 12 gauge instead of one strand of 10 gauge.
To get 6 volts under load, the output of the transformer will have to be somewhat more than 6 volts under no load. Try about 8 volts.
You may need a bigger transformer if you can not get enough output turns on the transformer.
Dan
Reply to
dcaster
I use the supply for a thermal wire stripper to power a pyrograpy tip - it wants between 1 and 3 volts. The lamp dimmer puts a DC component into the transformer that may be unhelpful and a source of your problem, but more likely you have insufficient current capability - do not start with a 24V transformer, get a 6VCT transformer out of an old radio or something and use that with a bridge rectifier to give you 3VDC and then you will be close - or as another poster suggested, rewind the transformer secondary for about 3V out at much higher current
Reply to
Bill Noble
Have you tried just a soldering iron? It's just a heating element with a hot tip, after all.
Good Luck! Rich
Reply to
Rich Grise

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