Re: Issue with homemade diode power sensor for HP meters
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Hi Tobias!?
Great experiment. Now add C2 & C3, c6,c7,c5 & c8.? These caps are part of the AC circuit and are used for balance in the autozero (C2 & C3). HP did not put in parts they did not need!
I have not studied the 436 input amp like you have, but would comment that the 220 Hz chopper frequency that switches Q1 & Q2 on/off alternatively needs to be accommodated in the bipolar AC amp. Also I note no source of DC for the bipolar to run on, it needs current from somewhere so it is getting it probably thru a negative bias on the emitter wrt the collector from the 436A otherwise how does the BJT work? In the emitter leg is a thermistor to compensate ambient mount temp. You have a pot there instead. I would think that you would want to adjust the detected voltage level before the FET gate circuit for calibration purposes.
I am sure the thermocouple arrangement produced a very small voltage.
I think a dead mount with your new input circuit is a great place to continue the experiment from. Re-use their carefully designed amplifier/fet gate circuit.
VBR & 73
Jeff Kruth
WA3ZKR
In a message dated 11/17/2018 12:49:37 PM Eastern Standard Time, tobias.pluess@... writes:
@Peter
thanks. Yes, exactly, the meters themselves are really cheap. I currently don't have access to some blown 8481A or similar sensors, so I cannibalized a sensor cable :-( because the connectors seem to be HP specific ones and are not available e.g. at Mouser or so.
But yes, as soon as I have a working circuit, I will it design such that it fits into the normal housing for those power sensors. I also thought of milling my own housing which would be similar to the original HP housing anyways.
The cool stuff is that Agilent and Keysight still use the same interface (at least for some power meters) as far as I know, so the sensor should work with any of those!
@Ed
?thanks also. Yes, I tried to study on the interface between the 436 and the power sensor. The resistor you are talking about is called the mount resistor in the 8481A manual. Here is the general schematic of this sensor:
The service manual tells which resistor value is used for the different ranges. I think it will be possible on the homemade sensor to add a little switch to change the mount resistor value and therefore the range the meter uses - such, the same sensor could be used for different power ranges. The 436A power ranges are very limited, as far as I remember.
However, what I don't understand in the interface between the meter and the sensor is how the autozero circuit works, and I am also a bit unsure about the working principle of the amplifier formed by Q1 in the sensor (see schematic) and the OpAmp in the meter. Do you know more details about how it works?
Best
Tobias HB9FSX
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