21-01-2018, 01:10 PM
For early bridge basics look at Wheatstone, De Sauty, Schering, Wein etc.
Lawrence.
Lawrence.
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STC Resistance Capacity Bridge
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(21-01-2018, 12:28 PM)Terry Wrote:(21-01-2018, 07:25 AM)Crackle Wrote: ... if you look at S3, a DSDP switch (or change over switch) intended I believe to switch from resistance measurement to capacitor measurement, you will see that the 2 central common terminals have been bridged together. There are 2 other wires, one from R3 R2 R1 common and one from the switch itself which are soldered together. I cant work out how these would have been fitted to the switch. Because I found a similar method used in the internal / external battery change over switch and the wiring from the external socket removed altogether, and was wired as a single pole isolator switch, on one supply leg, to disconnect the battery by adding the copper bar across the common terminals. In other words it was not wired as a double pole change over as it should have been to change from int. to ext. battery supply. I assumed that the R / C change over switch had also been changed by adding the small bar across the common terminals. Terry It is difficult for me to convert the wiring drawing into a theoretical circuit drawing when I dont know or understand how the circuit works. That is the task that I am hoping someone will help me with. All I can do at the moment is copy what I see in the hope that someone can help me. thanks Mike
21-01-2018, 03:24 PM
Actually Mike it is probably better that you do not have a pre-conceived idea of what the circuit looks like. You will be less likely to make assumptions and get it wrong. From what I've seen of your previous work on here you are more than capable of the task.
Print out your layout diagram. Number the connections of every component 1,2,3,4 as required. Do the same when you draw the component on the schematic. As you mark the connections between components on the circuit put a cross through the wires on your layout diagram. The first iteration of the schematic will look terrible as it always does but with a bit of tidying up you'll get there. Al
Tuned for maximum smoke .
Mike, it's the switchery that will be the hardest, best to view it in terms of a Wheatstone bridge, look at the one here:
https://www.electronics-tutorials.ws/blo...ridge.html First, resistance: Substitute R1 and R2 with the main pot track and make point "C" the wiper, call R4 R range and call R3 Rx, now replace the battery with an AC sinewave source (your transformers secondary) then connect the 'phones across Vout (between points "C" and "D") That's the set up for the unknown R measurement (Rx). Next, capacitance (non electrolytic): As above but replace R4 with a capacitor, call that capacitor C range, replace R3 with a capacitor, call that capacitor Cx, that's the set up for the unknown C measurement (Cx). Next, capacitance (electrolytic): as above but replace R4 with a capacitor in series with the PF rheostat, that's the set up for measuring Cx and the power factor at the test frequency. Lawrence.
21-01-2018, 07:04 PM
Mike, sorry I haven't found the relevant stc stuff yet, but I'm convinced that your instrument has been mucked about with! In the mean time I've put that old wireless service manual article in a new thread for members interest. I think even if we find the manufacturer diagram you would have a hard time getting it back to its original internal condition, it may be better to rework it using the components.
After such a nice cosmetic restoration it would be a shame if it didn't work well too! I will keep looking, but if its much before ww2 I may be at a loss. Andy.
Hi Lawrence, thanks I am starting to get the picture, I seem to remember doing this with a Wheatstone bridge in physics at school, but that was using massive rheostats and galvanometer and long ruler with wire. (I think)
Andy. Your article is very interesting and after a few reads hopefully will fill in lots of gaps in my knowledge. I am off to look for a pair of suitable phones, and to experiment with the voltage for the buzzer (reduce it from 9 volts to see when it stops buzzing) , as at the moment it is rather loud and accompanied with lots of arcing. Just back from testing the voltage, and the buzzer sounds happy at 3v and the arcing is negligible, it seems to have bedded its self in and starts reliably now at 3v. the OP voltage goes up to 9vAC. I have tested it with a 1k resistor and a .01uF cap and I can get a null in each case which seems to correspond to the setting on the dial, (give or take a factor of 10 or so) At 3 volts the current draw is 100mA I have a loudspeaker transformer connected to the phones socket and my bench speaker as the sounder. But the buzzer is very distracting in the background. I am wondering if it was intended to measure resistances with DC and only switch in the buzzer for capacitors. I would think the battery drain would be significantly less if the buzzer was not operating on the resistor range. thanks Mike
22-01-2018, 12:01 AM
(21-01-2018, 08:24 PM)Crackle Wrote: I am wondering if it was intended to measure resistances with DC and only switch in the buzzer for capacitors. I would think the battery drain would be significantly less if the buzzer was not operating on the resistor range. How would you hear the null in the phones with DC? Yes they are distracting, that's why the electronic oscillator was being touted in that article. Anywho, pal Don has a capacitance/resistance bridge by STC that looks nothing like yours  but has very similar guts, the only thing different is the lack of that 200pf capacitor, and the second wafer on the range switch, he says its a compensation capacitor, I dunno but I stared at the diagram we worked up from his, and your layout picture, and there's no difference between them.Don claims his to be 1937 vintage and is wired with cotton covered rubber insulated wire throughout, but now sports a 555 chip oscillator. So, here you go!
22-01-2018, 07:02 AM
(22-01-2018, 12:01 AM)Pat Pending Wrote: How would you hear the null in the phones with DC? Yes you are correct I keep forgetting that. An analogue meter works well with it though, and I think, gives a better defined null than the audio because of the noise from the buzzer. Thanks very much for the circuit, I agree it does seem to be the same, I will check value of C2 again to see if it is low in value, which would explain the addition of C4. I wonder why the 2 wires were joined near to S3? Would it be OK to add that circuit into the Radio Museum? Thanks again Andy Mike
22-01-2018, 08:50 AM
By all means Mike, Just make it clear its not an official drawing though.
Cheers. Andy. |
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