MPSA42 troubles
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Thomas Kummer
Feb 8, 2019, 7:35:14 PM2/8/19
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My understanding is that a MPSA42 is an NPN transistor and the MPSA92 is a PNP. My understanding of how an NPN transistor works is that when the base “P” is turned on the circuit from collector to emitter is shorted. When the base is off the circuit is open. I’m trying to use the decimal point on a Nixie tube rather than an LED for the alarm. So, I have the collector hooked to the tube the emitter hooked to the ground, and the base hooked to 5V signal, I also have a 10K resistor between base and 5V. However, no matter what the decimal point stays lit!! Even when the signal is off. The only time it isn’t lit is if the base is open, if I even touch the base the decimal point lights up!! The transistor has the the imprint A42 B331. It appears to be a knock off and that could be where my problems are coming from. I’ve tried looking for a datasheet, but to no avail. Any help would be appreciated.
John Rehwinkel
Feb 8, 2019, 8:58:04 PM2/8/19
to 'Spirit's lab' via neonixie-l
> My understanding is that a MPSA42 is an NPN transistor and the MPSA92 is a PNP. My understanding of how an NPN transistor works is that when the base “P” is turned on the circuit from collector to emitter is shorted. When the base is off the circuit is open. I’m trying to use the decimal point on a Nixie tube rather than an LED for the alarm. So, I have the collector hooked to the tube the emitter hooked to the ground, and the base hooked to 5V signal, I also have a 10K resistor between base and 5V. However, no matter what the decimal point stays lit!! Even when the signal is off. The only time it isn’t lit is if the base is open, if I even touch the base the decimal point lights up!! The transistor has the the imprint A42 B331. It appears to be a knock off and that could be where my problems are coming from. I’ve tried looking for a datasheet, but to no avail. Any help would be appreciated.
Probably some sort of leakage. Maybe add a 10k pulldown resistor from the base to ground. If that doesn't work, try disconnecting it from 5V and ground the base directly.
- John
Thomas Kummer
Feb 8, 2019, 9:08:16 PM2/8/19
to neonixie-l
I tried a 10K and I tried a 2.2K, and a 1K, and then would ground the base, and still the decimal point lit up!! I took the resistor off the base and touched it with my finger, and the DP lit up!!! I’m 100% convinced that these are just cheap knockoff Chinese counterfeits as I tried through about 3 more transistors, and same thing happened, from now on I’m strictly sticking with Digi-Key for my components, no more eBay gambles I’ll just shell out the extra ~$10-$20.
David Forbes
Feb 8, 2019, 11:12:24 PM2/8/19
to NeoNixie
An NPN transistor has about a 0.5V diode drop from base to emitter when turned on with a few milliamps of base current. Do you measure this?
On Fri, Feb 8, 2019, 7:08 PM Thomas Kummer <tmkumm...@gmail.com wrote:
I tried a 10K and I tried a 2.2K, and a 1K, and then would ground the base, and still the decimal point lit up!! I took the resistor off the base and touched it with my finger, and the DP lit up!!! I’m 100% convinced that these are just cheap knockoff Chinese counterfeits as I tried through about 3 more transistors, and same thing happened, from now on I’m strictly sticking with Digi-Key for my components, no more eBay gambles I’ll just shell out the extra ~$10-$20.
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Thomas Kummer
Feb 9, 2019, 12:00:56 AM2/9/19
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No, but I just did, and it dropped 4.7V
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David Forbes
Feb 9, 2019, 10:49:30 AM2/9/19
to NeoNixie
Sounds like you have some transistor-shaped objects. I only buy parts from Digikey and Mouser, and the occasional Adafruit purchase.
This is why.
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GastonP
Feb 9, 2019, 1:10:38 PM2/9/19
to neonixie-l
Did you verify that you verified that the layout is E-B-C? Or is it B-C-E?
David Forbes
Feb 9, 2019, 1:22:45 PM2/9/19
to NeoNixie
That's a good point. I was a bit surprised that the transistor said "A42" and not "MPSA42", as it should. Perhaps it's a 2SA42!
The MPS series was started by Motorola, which is no longer in the component business.
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Thomas Kummer
Feb 9, 2019, 7:00:52 PM2/9/19
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When I plug it into a multimeter’s hFE slot for E-B-C I get a reading of 140 hFE, when I plug it into the B-C-E slot I get a quick 1600 hFE, and then just a 1 in the far left digit of the screen almost like a processing/measuring reading, like what the multimeter does when you measure V and you first put it on the V source.
Sent from my iPhone
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gregebert
Feb 9, 2019, 8:43:55 PM2/9/19
to neonixie-l
This is where a curve-tracer really comes in-handy. Sadly, even old used ones are a bit expensive compared to scopes from the same era.
I've been tempted many times over the years to design my own, but like most projects I dream-up I never get time to build them.
Thomas Kummer
Feb 13, 2019, 9:43:27 PM2/13/19
to neoni...@googlegroups.com
Yup, it was just counterfeit/junk transistors. I got my real MPSA42’s in the mail today, and I got them to work almost flawlessly.
Sent from my iPhone
This is where a curve-tracer really comes in-handy. Sadly, even old used ones are a bit expensive compared to scopes from the same era.I've been tempted many times over the years to design my own, but like most projects I dream-up I never get time to build them.
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