Russell McMahon wrote:
>> ... from NEC I believe,
> 
> ie 2SD882 on your diagram.

Forgot to fix the message after noticing I had the reference marked on
the diagram from back then. It's a 2SD882 of course (took me forever to
find, since all that was written on the front was D882...)

> 
>> ... I replaced it with a BD437, and the inverter worked , but the 
>> transistor got pretty hot
>> (certainly hotter than the original). ...
> 
> 
>> Here's the schematic, with the original transistor:
>> http://marcansoft.com/subidos/ccfl_inverter.png
> 
> It's not clear why it would be getting hotter as it's not obvious that 
> the BD437 is inferior in any important ways. Current gain seems the 
> most likely culprit.
> 
> The claimed current gain is down somewhat -  say 40/?70? min/typical 
> at 2 amps compared to 60/160 at 1A for the original. But Vsat is 
> superior. Vce is slightly lower but should not matter. It may be that 
> they select on test the 2SD882 or that they get ones which have gains 
> in the higher spec sheet range - its not uncommon for devices to be 
> improved with time and move towards the higher end of a spec range. If 
> current gain is the issue then selecting from a batch of BD437s with a 
> beta-meta (sounds catchy) may be useful.

Would you recommend another transistor as a replacement? I have access
to a moderately-sized electronics supplier here in spain, so they should
have most common stuff, and they get free shipping from digi-key so I
can get anything that they carry (with a delay of a week or so, which is
a pain but I can handle in this case)

> 
> Yu MIGHT be able to get a logic level FET working there - you'd 
> probably need a from gate (was base) to ground. This would solve any 
> current gain issues but may have other problems. I'm not sure what 
> power/current level you are working at but an Rdson of 0.1 ohm or less 
> would be OK and quite a bit more may be OK.
> 
> Vdrive transformer.3_5 is unknown of course but probably enough to 
> give 1 to 2 mA in the 5K6 so may be in the  5-10 volt range, which 
> would make a FET viable. Something like an XXX3055L low voltage logic 
> level-cheapo N Channel MOSFET would be an OK place to start. But 
> measure Vt.3_5 and see what sort of drive voltage is available. Even a 
> non-logic FET may work.

Could I set up a non-logic FET together with some booster transistors to
bring the level up to what the FET handles? I have a bunch of IRF510's
here, and lots of random low-power transistors. Otherwise I'll try with
some logic-level FETs.

I was also wondering... Could the problem be current being dissipated
into the transistor some awkward way, that the original tolerated (i.e.
shorted) better? Like, something a diode could fix. I don't know exactly
how these oscillators work, but could I have some back-EMF ending up
where I don't want it?

-- 
Hector Martin (hector@marcansoft.com)
Public Key: http://www.marcansoft.com/hector.asc

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