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Old 1st Jul 2020, 2:39 pm   #1
60 oldjohn
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Default Which transistor

Which transistor can be used in reverse, ie E&C reversed with B staying the same. A bit like the FET 2N3819. Needs to be low power 2ma switching transistor.

TIA John.
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Old 1st Jul 2020, 3:56 pm   #2
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Question Re: Which transistor

For myself, I'll be surprised if such a transistor exists (apart from a faulty one!). But I'm curious: why do you ask this Q.?

Al.
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Old 1st Jul 2020, 3:58 pm   #3
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Default Re: Which transistor

There are a few that are pretty much the same both ways round, but I can't remember the type codes at the moment.
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Old 1st Jul 2020, 4:06 pm   #4
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Default Re: Which transistor

I recall an episode of the regular "In the Workshop" feature that used to appear in, I think, "Television", where a pair of reversed bipolar transistors was used to make a low power multivibrator that ran on low voltage supply with a low power consumption, but can't remember any details. It was the first time I had come across anything indicating that it was possible.
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Old 1st Jul 2020, 4:19 pm   #5
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Default Re: Which transistor

I seem to recall that there was a special transistor developed for use in early computers which would fit the bill, but like paulsherwin I forget its type number. I don't know whether this helps you at all. Possibly OC1??
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Old 1st Jul 2020, 4:52 pm   #6
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Default Re: Which transistor

Further to my last these transistors may have been used in commercial TV standards converters. Or am I barking up the wrong tree?
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Old 1st Jul 2020, 4:59 pm   #7
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Default Re: Which transistor

I think point contact types can be used either way on low voltages, but the gain is better the right way round.

EDIT:

There's a bidirectional one here (page 49) Brimar TS7: https://worldradiohistory.com/BOOKSH...e-Sinclair.pdf
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Old 1st Jul 2020, 5:12 pm   #8
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Default Re: Which transistor

Could you put two in inverse parallel? Bear in mind VBE is limited to but a few volts.
 
Old 1st Jul 2020, 5:19 pm   #9
60 oldjohn
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Default Re: Which transistor

Quote:
Originally Posted by Skywave View Post
For myself, I'll be surprised if such a transistor exists (apart from a faulty one!). But I'm curious: why do you ask this Q.?
Following on from the 2N3819 FET discussion I just wondered if a NPN bi polar existed to experiment with. Please don't anyone go researching it for hours it's not that important.

John.
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Old 1st Jul 2020, 6:08 pm   #10
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Default Re: Which transistor

It probably wouldn't have very good characteristics. To get good performance bipolar transistors are deliberately asymmetrical - highly doped emitters to get a high emitter efficiency, lightly doped collectors.

I guess a symmetrical valve wouldn't be very useful either?
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Old 1st Jul 2020, 7:45 pm   #11
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Default Re: Which transistor

I think (from memory) that OC139-140-141 [all NPN] were designed to be 'reversible'; and I also seem to recall that OC41-42-43 [all PNP] were at least a little bit 'reversible'. (And, of course, all are germanium, too).


But cosmic rays over many years may have corrupted my memory .


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Old 1st Jul 2020, 7:54 pm   #12
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Default Re: Which transistor

That's likely, they're used in switching circuitry, not analogue.
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Old 1st Jul 2020, 7:57 pm   #13
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Default Re: Which transistor

There was a bunch of 'reversible' PNP transistors produced back in the 60s and often used as things like relay/solenoid-drivers in first-generation electronic telephone exchanges: these invariably had low-gain [Hfe of 30?] but were cheap so obviously attractive to designers when you needed two of them for every subscriber-line.

CV7343/BCY30? I've got a bag-full of them here, manufactured by Texas Instruments Bedford if the codes are to be believed.
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Old 1st Jul 2020, 7:59 pm   #14
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Default Re: Which transistor

The OC140 as used in the sample and hold part of the analogue BBC C06/501 625 to 405 line standards converter.

John
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Old 1st Jul 2020, 8:00 pm   #15
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Default Re: Which transistor

2SC2878
Reversible but not symmetrical muting transistor:

https://electronics.stackexchange.co...ric-bjts-exist

David
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Old 1st Jul 2020, 8:01 pm   #16
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Default Re: Which transistor

ISTR that the OC140/141 was used in bucket-loads in the extraordinary BBC/Pye TV standards converters in the read/write role where the reversibility was useful.

Edit- looks like jjl has a better handle on it than me!
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Old 1st Jul 2020, 8:58 pm   #17
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Default Re: Which transistor

BCY30 series, they could be used with an NPN to make a level detector.

They were an alloy transistor and were limited in applied voltage in this mode as stated above.

Used with a ZTX 80 NPN in very many timers associated with power line auto reclosers produced by Reyrolle

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Old 1st Jul 2020, 9:28 pm   #18
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Default Re: Which transistor

Just did a quick simulation with anti parallel BC547s, the gain was a bit more than half of one transistor and symmetrical. It works.
 
Old 1st Jul 2020, 10:00 pm   #19
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Default Re: Which transistor

Quote:
Originally Posted by eddie_ce View Post
I seem to recall that there was a special transistor developed for use in early computers which would fit the bill, but like paulsherwin I forget its type number. I don't know whether this helps you at all. Possibly OC1??
Was this the OC140 used in the BBCs standards converters?

Keith

Beaten too it!!
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Old 1st Jul 2020, 10:46 pm   #20
60 oldjohn
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Default Re: Which transistor

Quote:
Originally Posted by merlinmaxwell View Post
Just did a quick simulation with anti parallel BC547s, the gain was a bit more than half of one transistor and symmetrical. It works.
Thank you, something to experiment with, I like it. When using breadboard I have back to back diodes save getting them wrong. I realise this may not work for all circuits. (This is not a fool proof fix) with the transistor, as I've said just something to experiment with.

John.
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