neovibe 2n3904

[wlrs]

My neovibe is working great but the 2n3904 is getting hot.? I understand it is the tranny for the bulb and I have check all the PCB traces and I all ways check my resisters before soldering them in all the values are right as well. but when i turn the trimmer pot up the 3904 gets hot. Oh I also put a heat shield on it to help keep it cool.. It doesn't get so hot that you cant touch it but it does get pretty warm.
One other thing How do I post pics? Please tell me.
Thanks.
walrus

[Aharon]

If you do a search you'll find quite a few posts on the subject incluing a list from RG with bulb driver transistor subs.
Aharon

[wlrs]

I searched until my fingers cramped but I found nothing on why my 2n3904 bulb driver is getting hot. Any of the dozen 2n3904 will get hot.
I did a compleate strip down and tested every part and everything passed with flying colors. No solder touching no mistakes on the board either. the board is a GGG. This build is to specs of the parts list from GGG..Oh I use a insulated power supply so that one side of the power suply souldnot be grounded.I also found out that using a 12 volt 25 Ma bulb will make the 2n3904 run a little cooler then the  12 volt 50 MA bulb.. Im stumped some help please.
Thanks..
walrus

[R.G.]

Here's the real info.

The lamp driver dissipates more power than anything else in the circuit. It's going to get hotter than the other transistors.

The only question is whether it gets too hot to work correctly. After the circuit has been running for perhaps 10-15minutes, can you touch the 2N3904 in question with the tip of your index finger and keep it there? If so, it's under 130F and it's probably OK. If you can't hold your fingertip there or if it instantly burns you, it's too hot.

The lamp driver has to absorb the full lamp current and as much voltage as the bulb does not need. If there is excess voltage, the transistor has to dissipate the voltage times the current. There is a pattern on the PCB for subbing in a to-220 power device package for just such emergencies. It's the set of three square pads next to the driver transistor.

If you have a high DC voltage driving the bulb, you can put a resistor in series with the bulb or collector of the driver transistor to eat up some of the excess voltage and lower the dissipation of the transistor.

So the pertinent questions are:
(1) what voltage is feeding the top (i.e. non-driver transistor) side of the bulb?
(2) how low does the collector of the 2N3904 swing when the lamp is fully glowing?

[SteveB]

I noticed the same thing on mine, but it continues to sound good to me. I say leave it as is.

Steve

[R.G.]

I noticed the same thing on mine, but it continues to sound good to me. I say leave it as is.

Good call.

As an aside, that transistor gets hot in the original univibe too. It's the single part most likely to be bad in my experience with repairing them.

[wlrs]

I put a heat shield on it from another transistor and it seems to calm the heat down. The 12 volt 25 Mu bulb help it out allot to..
Is there not a hardier transistor to use for this app.?
Thanks Guy's for the help.
Darrell AKA Walrus

[wlrs]

Are we not allowed to post pictures ? do you have to post some where else and post a link to it..
I did a search but no help there either.. any ideas

[micro]

I have not seen an option to upload pictures here. You must use a site like Imageshack or Photobucket etc.

[R.G.]

There is no option to post pictures. You do indeed have to upload somewhere else and link.