A couple of months ago I built the catode throbber as described here. Since then it has worked perfectly, throbbing two 30cm cc's and two 10 cm cc's. But, after cleaning my case and rerouting some cables I made my big mistake. I connected the CC's directly to my "power hub" since I was going to make a case for the throbber, and i reversed the polarity for two of the three inverters I have. Naturaly this resulted in pop's and sparks and two destroyed inverters. ( a blown cap and a demeged transistor, trying to repair them of course..) But, I figured it was time to update so I bought three new inverters, these were nice boxed ones, from the same company. (Sunbeam) Got them and hooked it all up, tripple checking polarity this time.. And it works! Sort of.. First, the throbbing was a lot "different" then before.. Long period of bright and very short period of dimming and brightening again. Ok, the inverters are different, not much to do about it.. So I messed around with the pot's, trying to get it just perfect when I tuched the metal tab of the transistors of the throbber. Ouch! It's burning hot!! Before, with the other inverters it never even got warm! Why? Adding a HS to the transistor would help, but why have it suddenly decided to get hot? VERY hot! Trying to be logical, I tuned the throbber to max output, so all CC's would be as bright as possible with no visible throbbing thinking that the trasistor probably produce heat as they remove exessive power when the CC's are dimmed. Then powered down and let the circuit cooled. Started it back up, and still the got urning hot after a few seconds! I just don't get it...
There are a few odd effects here. The different throbbing is one and the hot transistors another. The only link I can see is if the transistor is pulling a high base current from the LM324, overloading the opamp, which would be due to a high current draw by the new inverters. Try running the throbber with just a led and series resistor as load, if the transistor still gets hot you've a faulty TIP3055. Are the new inverters the same model as the old ones? Any spec on the box?
Well, the inverters are from the same manufacturer, but not the exact same model. The ones I used before was from a complete dual kit from Sunbeam, that type just heatshrink protected. The new inverters are also from Sunbeam, but the ones with a case. But to make that kind of heat, doesn't that mean the inverters must really pull some power? Well, I'll take a look at the circuittrying different things. Just one small detail, could the circuit still "sort of" work even if the transistor is faulty?
Ok, the throbber is no wired with two blue leds happily yhrobbing away. So far no heat what so ever.. So, my conclusion would be that the throbber works as it should, but that the new inverters pull a whole lot more current then the previous ones. But why? The same manufacturer, powering the same CCFL tubes at what seems like the same brightness.. Hmm.. So, what's the solution? Adding a HS?
Seems to be the only option. A TO220 case heats up by around 60C per watt with no heatsink, so if yours is very hot but not raising blisters, a small 20-25C/W heatsink should calm it down. I found a few specs on inverters here showing currents up to 750mA, which with a 3v drop on the transistor would give you a couple of watts of heat. That's probably way extreme as you've only got a single lamp on the end, but an inverter designed to run 2 lamps will take more power than one designed for a singleton.
