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| Current source in Princeton Electrometer repair |
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| charles.ouweland:
--- Quote from: Alex Nikitin on June 08, 2024, 09:01:15 pm ---The circuit would make sense if pin 3 of T9 and T10 is an emitter. A reverse biased emitter junction can operate as a very low current capable zener at about 6-7V. Alex --- End quote --- This is an excellent reply. I desoldered one of the 2N3565's and put it in a transistor tester. I double checked everything and you are right, I had the c and e of T9 and T10 mixed up. I tried to measured T9's IB but my meter showed 0.0µA. A very low current indeed ::) --- Quote from: fzabkar on June 06, 2024, 01:04:41 am ---Measure the voltages. For a start, the base of T11 should be sitting at 1.8V below the 9V supply. Emitter to Base Breakdown Voltage (BVEBO) = 6.0V (min) at IC = 0, IE = 10uA --- End quote --- See attached revised schema with voltages. T9's VEB is only 5.0V and there is actually almost no T9.IB current running, so I'm still a bit puzzled as to how this is supposed to work. I measured the current coming out of T11's collector and it's 0.37mA (not .16mA) regardless of when the collector is connected to 0V or to -9V |
| fzabkar:
There must be around 1 or 2 uA flowing in the base circuit. T9 and T10 set the voltage on the base of T11, and this voltage should be reasonably constant. This then determines the current flowing in the 7K5 resistor. This is the kind of circuit I'm used to seeing: |
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