I am modifying a plasma ball circuit, to generate an arc of 10-15 kV (at very low current - barely tingles when you touch it.) The goal is to replicate the first radio transmitters invented - spark gap transmitters.
The circuit is very simple. A two transistor oscillator generates a square wave. This controls a TIP122 100V 5A Darlington which is on a heatsink. The darlington drives the primary of the transformer and a secondary has many turns. The result is about ~2kV 20kHz AC at a very low current which drives a standard plasma ball and makes it light up.
The problem is, 2kV isn't enough - I needed a bit more, around 10-15 kV, to get a big enough arc. So I hooked it up to my power supply and ran it off 12V instead of the 4xAA (6V) it uses. That worked fine and gave me enough output to be received on the other end using an AM radio about 2 metres away.
My power supply was current limited though; the circuit was pulling about 1.5A and the supply sagged to 9V (I had the current limit set at 1.5A - the supply is a CC/CV type.) I didn't make much of this because the transistor can handle 5A.
Then I plugged it into a 12.6V LiPoly battery, and it worked great - for about 2 seconds. The TIP122 then went up in smoke. I have replacements, I planned for this to happen, but obviously I don't want this to happen all the time.
So, the question is - how do I replicate the current limiting the power supply provided me with? Because that worked well and the TIP122 lasted forever using this method. I could build my own current limited supply powered from the battery, but that's cumbersome and the widget needs to be small. I was trying to think of a way to use resistors, but I'd require a 10-20W resistor and that would make the battery life awful and also require serious heatsinking.
Any ideas?
The circuit is very simple. A two transistor oscillator generates a square wave. This controls a TIP122 100V 5A Darlington which is on a heatsink. The darlington drives the primary of the transformer and a secondary has many turns. The result is about ~2kV 20kHz AC at a very low current which drives a standard plasma ball and makes it light up.
The problem is, 2kV isn't enough - I needed a bit more, around 10-15 kV, to get a big enough arc. So I hooked it up to my power supply and ran it off 12V instead of the 4xAA (6V) it uses. That worked fine and gave me enough output to be received on the other end using an AM radio about 2 metres away.
My power supply was current limited though; the circuit was pulling about 1.5A and the supply sagged to 9V (I had the current limit set at 1.5A - the supply is a CC/CV type.) I didn't make much of this because the transistor can handle 5A.
Then I plugged it into a 12.6V LiPoly battery, and it worked great - for about 2 seconds. The TIP122 then went up in smoke. I have replacements, I planned for this to happen, but obviously I don't want this to happen all the time.
So, the question is - how do I replicate the current limiting the power supply provided me with? Because that worked well and the TIP122 lasted forever using this method. I could build my own current limited supply powered from the battery, but that's cumbersome and the widget needs to be small. I was trying to think of a way to use resistors, but I'd require a 10-20W resistor and that would make the battery life awful and also require serious heatsinking.
Any ideas?