Where to put diodes on Leach amp?

[supernet]

I will use this kind of bracket to fix my TO 247 transistors to heatsink. Could I put the diodes for thermal compensation on this aluminium channel bracket or would be better to put them on heatsink?


Thanks,

Supernet

 

[BrianGT]

I would put them against the heatsinks, pushed with a similar smaller bar, next to the u channel bar holding the output devices on.

Here is a picture how I mounted my diodes. My leach amp has been running for over a year without any problems.

I stripped some 20ga wire, and put the stripped pieces over the diode leads, and put insulation over the solder joints to avoid touching the plate.

--
Brian

 

[Verbal]

Are these diodes for thermal tracking? If so, wouldn't you want to put them as close to the output transistors as possible?

 

[mlloyd1]

I wonder about that also. Maybe even using some TO-220 or TO126 package transistors instead of diodes and bolt them to the heatsinks using the power transistors mounting screws.

mlloyd1

Are these diodes for thermal tracking? If so, wouldn't you want to put them as close to the output transistors as possible?

 

[BrianGT]

Yes, they are part of the VBE multiplier stage. From this page:
"D1 through D4 are mounted in holes in the heat sink with the output transistors to provide negative thermal feedback to the VBE multiplier. The diodes cause the voltage across Q7 to decrease as the heat sink temperature increases to prevent thermal runaway in the output transistors. C12 improves the voltage regulation across Q7 at high frequencies. In addition, it prevents any oscillations that might occur because of the shunt-series feedback around Q7."

I built my Leach amp for credit with Dr. Leach at school, and talked to him about this when I was building it, and he didn't feel that it would cause any problems.

--
Brian

 

[supernet]

Nice idea Brian about fixing diodes.

But I do not understand the following thing: should these diodes act slowly as heatsink or fast ( like top of the transistor case)?


Thanks

Supernet