Basic question about mutual inductance in transitor legs... - diyAudio
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Old 10th April 2016, 11:07 PM   #1
JdAo is offline JdAo  Portugal
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Default Basic question about mutual inductance in transitor legs...

I know that from a practical point of view it probably doesn't matter, because some transformers have the secondary wires twisted all over each other and doesn't seem to be a problem, but here goes anyway as this is a high power amp...
Is it ok to twist the emitter and collector (basically the rails with more current and so more prone to mutual inductance...) legs of high power output transistors (200W range) over each other so that they're right on top of each other and separated only by a thin layer of PTFE ? Won't there be any problems of mutual inductance on the feedback or whatever circuitry ?
I'm asking this because I'm finally about to install some replacement transistors for an old high end amp and the replacements have differ on the order of the connections and I don't like the idea of putting jumpers and cutting tracks on the PCB as I frequently redo things to improve them...
Thanks in advance
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Old 11th April 2016, 12:04 AM   #2
rayma is online now rayma  United States
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[QUOTE=JdAo;4678492]Is it ok to twist the emitter and collector legs of high power
output transistors over each other so that they're right on top of each other /QUOTE]

Are you talking about changing the relative lead positions for a device to the pcb,
for something like a TO-247 type package? That should be fine, and if done carefully
can even reduce the lead inductance slightly. Or, are there wires on the leads?

Last edited by rayma; 11th April 2016 at 12:15 AM.
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Old 11th April 2016, 12:13 AM   #3
JdAo is offline JdAo  Portugal
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"Are you talking about changing the relative lead positions for a device to the pcb,
for something like a TO-247 type package? That should be fine. "

Thnaks, yes that's exactly what I mean, except that it's for MT-200 Sankens (200W each). The replacement transistors have different order on the legs. The legs would be right on top of each other really, only a very thin layer of PTFE between them, that's why I was worried, I recently bought an old school 200mhz CRT oscilloscope and amazed at the small parasitic signals you can see on that thing (only starting to learn though...).

Thanks
Cheers
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Old 11th April 2016, 12:23 AM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by JdAo View Post
it's for MT-200 Sankens (200W each). The replacement transistors have different order on the legs. T
he legs would be right on top of each other really, only a very thin layer of PTFE between them,
that's why I was worried, I recently bought an old school 200mhz CRT oscilloscope and amazed
at the small parasitic signals you can see on that thing
If the collector and emitter leads are adjacent on the MT-200, exchanging them should work out ok.

Some of those squiggles on the scope are due to the ground lead and clip, though.
Try using the voltage probe without the tip, and instead using the small accessory
spring ground pin that many probes come with.
http://www.caltestelectronics.com/ct...ssories/CT2714
terminology - What is the name of this springy type oscilloscope probe accessory? - Electrical Engineering Stack Exchange

Last edited by rayma; 11th April 2016 at 12:30 AM.
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Old 11th April 2016, 12:32 AM   #5
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I have changed transistors in amps before with different transistor pin outs.
I just twist the legs around each other leaving a good gap.
I didn't even insulate the legs and it worked fine.
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Old 11th April 2016, 12:53 AM   #6
jcx is offline jcx  United States
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don't rely on pfte, especially "plumbers tape" when any pressure can be developed - pfte creeps forever

insulating sleeving isn't often pure pfte for that very reason
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