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.
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.
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