Author Topic: 2n3906 transistors from Tayda  (Read 1185 times)

Rob Strand

Re: 2n3906 transistors from Tayda
« Reply #20 on: November 25, 2020, 07:13:41 PM »
Quote
if you want to lower the gain to be about 1/3rd, just swap the collector and emitter,
The reverse gain is usually only 1 to 10.   (You can also get problem with the BE junction conducting if the voltage exceeds about 10V.)

There's plenty of circuits which would happily work with hFE = 400.
The internet:  answers without the need for understanding.

PRR

Re: 2n3906 transistors from Tayda
« Reply #21 on: November 25, 2020, 10:39:49 PM »
Very OLD Ge transistors were sometimes symmetrical. Same gain both ways. (They did not have enough control to optimize the doping.)
  • SUPPORTER

iainpunk

Re: 2n3906 transistors from Tayda
« Reply #22 on: November 26, 2020, 08:51:39 AM »
Quote
if you want to lower the gain to be about 1/3rd, just swap the collector and emitter,
The reverse gain is usually only 1 to 10.   (You can also get problem with the BE junction conducting if the voltage exceeds about 10V.)
that's not what my experience with reverse BJT's is, a BC547 with an Hfe 318 in reverse measured Hfe 93, and a BD139 with gain of 85 measured 32 in reverse. that is in circuit, not in a DMM or component tester.
yes, reverse breakdown is a risk, but not so prevalent in 9V circuits, and enough datasheets list the max reverse BE voltage rating that you can account for that with series diodes/LEDs to lower the potential voltage on that junction

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail
snail man [x11]
snail man is fuccing real

antonis

Re: 2n3906 transistors from Tayda
« Reply #23 on: November 26, 2020, 04:23:03 PM »
I can ensure you, Iain, for a bunch of damaged BC317s conversely fitted in 15V CE (grounded E) amp circuit..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

Rob Strand

Re: 2n3906 transistors from Tayda
« Reply #24 on: November 26, 2020, 05:41:34 PM »
Quote
that's not what my experience with reverse BJT's is, a BC547 with an Hfe 318 in reverse measured Hfe 93, and a BD139 with gain of 85 measured 32 in reverse. that is in circuit, not in a DMM or component tester.
yes, reverse breakdown is a risk, but not so prevalent in 9V circuits, and enough datasheets list the max reverse BE voltage rating that you can account for that with series diodes/LEDs to lower the potential voltage on that junction
Interesting.  I've measured the values in test set-ups quite few times and I always get low values.  That's for small signal transistors all the way up to large TO-3 packages.

I only *use* transistors in reverse at low voltages, for example as controlled resistors.   The other motivation for measuring reverse beta is to come-up with spice models (many spice models have dubious reverse parameters and don't work correctly in reverse).

I'd have to check out what's happening.   It's possible BE leakage, promoted by breakdown is helping.
The internet:  answers without the need for understanding.

iainpunk

Re: 2n3906 transistors from Tayda
« Reply #25 on: November 26, 2020, 06:32:55 PM »
Quote
that's not what my experience with reverse BJT's is, a BC547 with an Hfe 318 in reverse measured Hfe 93, and a BD139 with gain of 85 measured 32 in reverse. that is in circuit, not in a DMM or component tester.
yes, reverse breakdown is a risk, but not so prevalent in 9V circuits, and enough datasheets list the max reverse BE voltage rating that you can account for that with series diodes/LEDs to lower the potential voltage on that junction
Interesting.  I've measured the values in test set-ups quite few times and I always get low values.  That's for small signal transistors all the way up to large TO-3 packages.

I only *use* transistors in reverse at low voltages, for example as controlled resistors.   The other motivation for measuring reverse beta is to come-up with spice models (many spice models have dubious reverse parameters and don't work correctly in reverse).

I'd have to check out what's happening.   It's possible BE leakage, promoted by breakdown is helping.
now im starting to doubt my measurements, it could have been a fluke, we measured the gain of the BD139 in an amplifier circuit based on the currents through the pins, and the BC547 was at a practicum where we had to measure Hfe of a transistor, among other things.

a good transistor to use in reverse is the BF245, since the gain is roughly the same both ways (its a JFET)

cheers, Iain
« Last Edit: November 26, 2020, 06:34:32 PM by iainpunk »
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail
snail man [x11]
snail man is fuccing real

Axldeziak

Re: 2n3906 transistors from Tayda
« Reply #26 on: November 27, 2020, 07:31:54 AM »

a good transistor to use in reverse is the BF245, since the gain is roughly the same both ways (its a JFET)


You just saved me a ton of time and worry on a completely unrelated project where a BF245 was suggested as a sub but was reverse from the original schematic. Thanks!

Little things like this are why I read this place everyday.

iainpunk

Re: 2n3906 transistors from Tayda
« Reply #27 on: November 27, 2020, 10:47:20 AM »

a good transistor to use in reverse is the BF245, since the gain is roughly the same both ways (its a JFET)


You just saved me a ton of time and worry on a completely unrelated project where a BF245 was suggested as a sub but was reverse from the original schematic. Thanks!

Little things like this are why I read this place everyday.
not gonna lie, this made my laugh.
there are a lot of JFETs where the source and drain are interchangeable, the BF245 is my personal favourite because it sounds very good in an SRPP style circuit.

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail
snail man [x11]
snail man is fuccing real

Yazoo

Re: 2n3906 transistors from Tayda
« Reply #28 on: November 28, 2020, 12:38:59 PM »
After all that, it turns out I owe Tayda an apology. While I did get an order from Tayda recently, the 2N3906s I bought for the same project came from an eBay seller, not Tayda.

So, grovel grovel, sorry Tayda. :-[