Re: Harwell rally 9 February
The HP 3400A is a true RMS AC voltmeter using a thermal converter pair in a feedback arrangement.
The converter pair is in a round metal can and is irreplaceable. Don't chuck it away if you decide to recycle the case, meter etc, there may be people looking for one because they got used in various other instruments.
They use twin load resistors, each with a thermopile - a series of hot junctions attached to them. Incoming signal (direct or via an amplifier) heats one resistor. A DC amplifier heats the other. The DC amplifier is driven by the difference in thermopile voltages so the DC amp's resistor tracks the temperature of the signal resistor. As the two sectiona are carefully matched, the DC voltage into the amp's resistor must match the true RMS into the signal's resistor.
A second generation one came out in the early 80's built as a sort of MEMS structure and fitted into a TO-5 type can. 1SB1-0021 if my memory serves.
David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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