Repairing a blood pressure monitor - finding this transistor

Discussion in 'General Electronics Chat' started by tom66, Sep 21, 2010.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I am repairing a cheap blood pressure monitor.

    I suspect that the pump control transistor may have been burnt out.

    But can anyone identify it?

    I looked on the internet and I found a part which may match: 8550D. It's a 1.5A PNP transistor. Which seems about right, until I figure that the microcontroller (the black blob - or COB) is trying to turn the transistor on with a diode drop (>600mV) on the base pin (#1). Also, I measured the pump current at only 180mA, so a 1.5A transistor is almost x10 overkill.

    I see no freewheeling diodes, so I added one across the pump terminals (just a 1N4148.) I suspect that may have been part of the cause of the failure of the device. The pump operates if you short collector to emitter.

    The other transistor appears to control the valve, but I can't verify the operation of the valve transistor yet because the monitor is supposed to pump up the arm and operate the valve - I think.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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  3. tom66

    Thread Starter Senior Member

    May 9, 2009
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  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Good question, maybe they got them cheap, or the control circuit was easier.

    Have you tried hand drawing a partial schematic of that area? Doing that often provides an explanation.
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
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    That's normal running current. What about startup current?

    Using a PNP is a perfectly good circuit design choice because the MCU can usually sink more current than it can source from its output port pin.
     
  6. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I thought NPN transistors were cheaper.

    Without knowing NPN or PNP it's going to be difficult to figure out the topology. I suspect collector is connected to the battery positive, emitter to the motor positive, base to the MCU through a resistor.
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
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    You better find out before you have a heart attack due to not having a pressure tester. ;)

    NPN typically are cheaper. They could avoid using a driver or other parts by using the uC to sink the current via the PNP.

    The cost of the whole may justify the extra PNP cost.

    or

    This may be the reason its dead. ;)
     
  8. tom66

    Thread Starter Senior Member

    May 9, 2009
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    While trying to remove the transistor, I lifted a pad. (I only have a cheap iron and I think this might have been too hot.) I'm hopeful though that I'll be able to wire an NPN in by scraping some copper off. If that works, then I know it was a NPN. I know the pin configuration is base, collector, emitter, due to the traces: base goes to the MCU, collector goes to battery positive and emitter goes to pump positive.
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    It could very well be a house numbered NPN that just happens to be very close to a standard PNP number.

    They do have different designations at the bottoms of the package.

    I guess we will see.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Companies have been known to get great deals on "barrels" of parts that are soon to be discontinued or from another manufacturer that no longer uses them but has a warehouse still full of them.

    Usually the numbers at the bottom are a date code.

    I'll go with the sinking more current theory as well.

    Matters not, but I'd try to hand draw a schematic and take any measurements you can lest you endanger the driving circuitry.
     
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