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Low voltage AC power for thermocouple tube? Answered

I am trying to make a readout for a DV-6M thermocouple gauge tube to measure vacuum. The tube requires 380mV (0.38v) AC across its filament to operate, and anything more could burn the filament. (I was reading up on this tube, and found the filament has a resistance of 18ohms but if you tried to measure it with a multimeter you could burn the filament). I have modified a 555 tone generator circuit, to accomplish this, and have measured the output to be 380mV AC with my fluke multimeter, and 0mv DC. When I put an LED across the output leads however, the LED is able to light which should take around 3v, which makes me wary of trying to connect it to my tube. what is happening, and is it really producing 380mV AC or is something else going on?


Did you see this one ?

Was the LED fully lit ? Measure the current through it.

You really need a good reference oscillation, your O/P depends on the I/P....

Fantastic pointer on a very excellent  practical discussion of vacuum gauges.


I've seen some other stuff on Belljar but not that. Thanks.

The LED wasn't fully lit, but it was more than barely lit if that helps. I'll measure the current in a little bit.

What frequency ?

Remember a regular multimeter is only designed to read AC SINE waves correctly - all bets are off for non-sinusoidal waveforms. If you're really serious, then you'd use a true-RMS multimeter, but they cost.


My meter that I used is the "Fluke 115 true RMS multimeter" it read the frequency to be between 115 Hz. and 1.5 kHz. depending on the position of a potentiometer.

Have you got a driver circuit from somewhere ?

The usual configuration IME is a 4 terminal bridge, so the excitation is probably across the whole device.

Most vac gauges like this MUST NOT be used in ordinary pressures !

There was a schematic in a book that I tried, but I was having some problems with it so started seeing what I could make on my own. The one from the book worked pretty much the same way though. A 555 timer controlled the base of a transistor going to the primary of a small audio transformer (sp-33) and the secondary coil on the transformer was the output. with an 18 ohm resistor acting as the load at first you were supposed to adjust the output using two potentiometers one 500k between pin 7 and 8 of the chip, and one between pin 3 and the base of the transistor. once you got it outputing 380 Vrms it was supposed to be good but the whole LED thing worries me.

Are you referring to the actual thermocouple when you say the four terminal bridge?