Questions about an unusual transformer - Page 1
Forum section: electronics / general

author: 99tito99
author_rank: Regular Contributor
timestamp: August 21, 2013, 05:13:15 am
content: Hi Folk: I bought a thermocouple vacuum gauge control on eBay (see Figure 1). If not familiar with TC vacuum gauge, it measures vacuum from the voltage produced by an electrically heated thermocouple junction (see Figure 2). At atmospheric pressure the air effectively conducts the heat from the junction an the TC produces a low voltage and at vacuum the heat is only lost by radiation and junction gets very hot and the TC register a higher voltage. The whole thing is analog with the voltages displayed on a moving-coil voltmeter.My question is about the transformer that provides the power. Figure 3 shows an overview, Figure 4 shows some marking and Figure 5 identifies some specifics. The laminations are welded together at the top (by the primary) and it has magnetic shunts.So my first thought and question is, is this a saturated primary core?Second, although the primary and secondary are wound on separate cores they are interconnected by two black wires (shown in Figure 5). Using an ohmmeter I determined the winding interconnections and show the resistant for each winding.My second question is: what is the purpose of the 0.04? winding that interconnects the primary and secondary?Figure 7 shows the basic circuit for the TC gauge controller. With the primary configured to the mains as shown, about 6.7 volts is delivered between taps 7 and 8. The Adj. pot calibrates the reading by varying the power to the heater and the Bal. pot balances the potential between the two legs of the heater.Final question: what is the purpose of the 1 uF cap (shown in Figure? Noise suppression (analog?), in-rush current, what?A thanks ahead of any consideration you may give. Cheers, Mark

author: megajocke
author_rank: Regular Contributor
timestamp: August 21, 2013, 05:36:25 am
content: That's a ferroresonant transformer/constant voltage transformer.http://en.wikipedia.org/wiki/Voltage_regulator#Constant-voltage_transformerThe capacitor is part of the resonant circuit. The interconnection between the two sections is probably used to achieve better regulation. I've seen it before in some transformer book, which I don't remember the name of right now.

author: johansen
author_rank: Super Contributor
timestamp: August 21, 2013, 07:30:45 am
content: that is some kind of both constant current AND constant voltage transformer.They have combined both operations into the same transformer core.Don't attempt to mess with the operation until you understand both the circuit operation and the the thermocouple-vacuum response. --and the meter's non linear display.

author: TerraHertz
author_rank: Super Contributor
timestamp: August 21, 2013, 03:16:43 pm
content: Pretty tricky and neat.When you think about it, the objective would be to produce constant _power_ input to the heater element, regardless of variations of the heater resistance, and mains input.Oh no wait... it's more complicated than that. As the vacuum increases, the power input to the heater has to decrease by a known curve. The heater must be allowed to get hotter, but not overheat. If constant power was put in, it would burn out at some point as heat loss by gas conduction diminished to near zero.So the resonant transformer's purpose is to produce an accurately reproducible power-to-load vs load resistance curve, while ignoring mains input variations.I have no idea how the transformer circuit actually does that.

author: 99tito99
author_rank: Regular Contributor
timestamp: August 22, 2013, 01:01:24 am
content: Quote from: megajocke on August 21, 2013, 05:36:25 ammegajocke, I think you've got it! I Googled around and found a bunch of info., some similar and some with more complex configurations.

author: 99tito99
author_rank: Regular Contributor
timestamp: August 22, 2013, 01:09:08 am
content: Quote from: johansen on August 21, 2013, 07:30:45 amThe unit as is has a three position rotary for 3 TC's. The plan is to add a five position for a couple of extra TC's. Also, I need to change the current limiting resistors because the TC heads I have, have different specs then the OEM. Even if all of that throws off the calibration a bit, that's OK. I just need to know when I attain max rough pump vacuum (with no apparent leaks) before I switch in a diffusion of turbo pump.And, if I didn't fiddle with stuff I didn't understand I wouldn't know as much as I do now and, I would have more stuff that still worked.Cheers, MarkPSI'm a big Ben Krasnow fan and have seen his Thermocouple vacuum gauge teardown.

author: johansen
author_rank: Super Contributor
timestamp: August 22, 2013, 04:17:39 pm
content: i would recommend building a constant resistance pirani gauge instead of messing with TC gauges, I actually did spend the money on 4 pirani tubes but found that its unnecessary, a low wattage high voltage light bulb is essentially the same thing. --the difference is the tungsten wire isn't gold plated to prevent any oxidation when oxygen molecules hit the hot filament.

author: 99tito99
author_rank: Regular Contributor
timestamp: August 23, 2013, 01:20:16 am
content: Quote from: johansen on August 22, 2013, 04:17:39 pmHi johansen:For my application TC or Pirani it's six of one half dozen of the other. They both work over about the same range the Pirani may having a bit better resolution. But, the point is moot as I have the TC heads and the controller and all is one in the universe.However, if you have a design for a constant resistance Pirani gauge please post a link, I might build one to boot.Cheers, Mark