Electronics > Metrology

ESI 277 capacitance bride (1963)


I have a 277 but I can not find information. http://www.pa4tim.nl/?p=4587 for pictures. Someone made some mods and the result is that the measurements do not make much sense. I wonder if someone knows this bridge.

are you going mormon you horn dog ? ;D just don't go into the jacuzzi on the honeymoon

That bridge would have been attached to an ESI 860 1Khz generator/detector.  I have attached images of the ESI 860 + 290/291B bridge connections as a guide plus a blurry photo of the 291B bridge instructions. The 291B is like the 277 but it can also do resistance and inductance.

Here is a link to the manual for the ESI 701 capacitance system that includes the more advanced variable frequency 861A generator/detector.


There is an 860A manual for sale here: http://www.vintagemanuals.com/manual/ESI/860A

Basically you want a transformer coupled generator output (the bridge probably can work 100Hz to 10KHz) plus a microvolt AC detector with a bandpass filter matching the generator frequency. Trouble is it may be very hard finding a shielded transformer with very low capacitance suitable for the bridge.

You will probably find one of the capacitance-under-test connections is directly connected to one of the detector inputs. The other detector connection is usually connected to 277 ground inside the detector box. Everything else has to be floating. That is the only point that anything in the bridge system connects to ground. If the signal source is not properly isolated from ground, the bridge will not work properly. If you use an ungrounded battery powered low distortion generator in a grounded metal box to eliminate the transformer issue, you may be able to get away without a bandpass filter on the detector, but the bridge null will always be sharpest with a sharp bandpass filter in the AC microvolt detector.

When the bridged is balanced, the voltage on the capacitance-under-test connector that goes to the detector will be exactly equal to ground voltage and so you get a true 3-wire precision capacitance system. It nulls out errors caused by all cable and bridge capacitances to ground on this pin. If you do not have the correct ground connections, the bridge will not be accurate.

The circuit of the bridge is very simple, and with the help of the 291A instructions, you should be able to work it out very easily.


Thanks Richard.
It that your own unit ? ESI used a very nice way to couple the units.

There is not much info about the 277, I asked Henry Hall after I got the 277 (about 3 years ago) and he told me he knew the unit because it was a competitor for the GR1608 at the time he was designing that and one had the manual but could not find it. But I think he mixed the 277 up with the 291.

I have two very capable uV detectors and a generator. Both General Radio. ( http://www.pa4tim.nl/?p=3398 they are part of my GR-1620 set that was used in the GR standards lab until that closed)
I have a very big soft spot for GR stuff. I repair test and calibration gear for a living and that is a nice way to combine playing with my collection of old component-test gear. Much of that gear still outperforms many modern instruments.

I'm in a niche market and  I repair the stuff nobody else can or will repair and often that involves a lot of component testing (most of it lacks any form of documentation) And I have a lot of component testing gear, commercial and self made ( like capacitor leakage meter,
, opto-testers, opamp testers, inductor saturation tester,  fixtures for my Tek 576 curve tracer (fi to test relais) Little pcb's made to test parts like 78xx regulators and things that help me in some way like an electronic adjustable and reset-able fuse. http://www.schneiderelectronicsrepair.nl/ there you see some of the things I repaired. 

I own the ESI 707 system  - I just got the pictures of the 291B system from the internet.

The Genrad generator has a problem in that the GR-1620 bridge has a built in isolating transformer, but the ESI bridges tend to put that in the generator instead. The Genrad generator looks like it has a grounded unbalanced output that just will not work.

If you can post the 277 circuit here - just a quick hand drawn circuit, we can help explain how to use the bridge. It will be very similar to the blurry "C" in the 291B instructions. How they handle the ranges is the key. If they have a different capacitor for each range, then it probably means that the voltages across the legs of the bridges is always equal, and that makes it much easier using a conventional center-tapped transformer from the generator. It will be easy to use a multimeter to measure the resistances and capacitances in each leg.

This bridge has nowhere the accuracy of the GR-1620 - it is only 4 digits, but it is still a great piece of engineering. It is a 12005 count bridge whereas the Genrad is probably a 1200000 count bridge. But it wouldn't surprise me if it is pretty close to a 0.01% accuracy in practice, and the controls make it efficient to use.


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