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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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1st Jul 2020, 2:39 pm | #1 |
Dekatron
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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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My favourite text message "I'll be there in five minutes, if not read again" Last edited by 60 oldjohn; 1st Jul 2020 at 2:49 pm. |
1st Jul 2020, 3:56 pm | #2 |
Rest in Peace
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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. |
1st Jul 2020, 3:58 pm | #3 |
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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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1st Jul 2020, 4:06 pm | #4 |
Dekatron
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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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1st Jul 2020, 4:19 pm | #5 |
Hexode
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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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Eddie BVWS Member. Friend of the BVWTM Last edited by eddie_ce; 1st Jul 2020 at 4:20 pm. Reason: additional information |
1st Jul 2020, 4:52 pm | #6 |
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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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1st Jul 2020, 4:59 pm | #7 |
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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 |
1st Jul 2020, 5:12 pm | #8 |
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Re: Which transistor
Could you put two in inverse parallel? Bear in mind VBE is limited to but a few volts.
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1st Jul 2020, 5:19 pm | #9 | |
Dekatron
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Re: Which transistor
Quote:
John.
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1st Jul 2020, 6:08 pm | #10 |
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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? |
1st Jul 2020, 7:45 pm | #11 |
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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 . dave
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1st Jul 2020, 7:54 pm | #12 |
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Re: Which transistor
That's likely, they're used in switching circuitry, not analogue.
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1st Jul 2020, 7:57 pm | #13 |
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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. |
1st Jul 2020, 7:59 pm | #14 |
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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 |
1st Jul 2020, 8:00 pm | #15 |
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Re: Which transistor
2SC2878
Reversible but not symmetrical muting transistor: https://electronics.stackexchange.co...ric-bjts-exist David |
1st Jul 2020, 8:01 pm | #16 |
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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! |
1st Jul 2020, 8:58 pm | #17 |
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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 Ed |
1st Jul 2020, 9:28 pm | #18 |
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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.
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1st Jul 2020, 10:00 pm | #19 | |
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Re: Which transistor
Quote:
Keith Beaten too it!! |
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1st Jul 2020, 10:46 pm | #20 | |
Dekatron
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Re: Which transistor
Quote:
John.
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