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| Getting an old-school impedance bridge, worth the hassle? |
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| Vincent:
Thanks for the inputs everyone! To clarify things a bit more, getting a DE-5000 would probably be the simplest option, but I feel using it wouldn't have as much of an educational benefit as getting experience with the earlier instruments. But I'm aware of the limits of these relics, hence why I'll thoroughly shop around before making any purchase. I'm into salvaging components from scrapped electronics, so an impedance bridge would definitely be useful for IDing components, especially unmarked inductors, even if I just get ballpark values. I've had an interest in lighting technology for many years and at some point I was almost fascinated by carbon arc lighting. If I were to craft a ballast for a homemade carbon arc lamp an impedance bridge would almost certainly be a must! "Repairability" is an important factor in my decision-making process. I really enjoy the freedom and sense of accomplishment I get out of disassembling a broken device and make it work again. ;D Now I'm not sure what's the fair price for those. I certainly can't blow a thousand dollars on one. Some Genrad units on eBay don't seem too expensive, namely 1650As. Maybe I should start a WTB for a short list of models I'd be interested in. |
| TimFox:
Although I collect "old-school" impedance bridges, I admit that I use a DE-5000 for most relevant tasks. A typical old-school bridge uses a "Schering bridge" circuit https://electricalvoice.com/schering-bridge-advantages-disadvantages/ The balance equations are independent of frequency, but often the dial for the series loss is calibrated in Q or D at the default frequency, typically 1 kHz. Most can use an external generator to measure at other frequencies, but the Q/D dial must be multiplied by a frequency factor. My favorite bridge is a Wayne-Kerr B221, which operates at a fixed frequency of 1570 Hz (10 k rad/s). The two sections null the capacitance and (parallel) conductance separately, with no interaction. The most significant digits rely on switched turns on ratio transformers, which being integers do not drift. The internal standard capacitors are hermetically sealed, and are still accurate more than 50 years later. The best part is the null indicator, which comprises two dual magic-eye tubes that progressively close as null is achieved, with almost no lag. For inductors, the transformer is switched to negative capacitance, which corresponds to inductance. That is why the frequency was chosen: the arithmetic for the inductance that corresponds to a negative capacitance requires a reciprocal, and the manual included a table of reciprocals since calculators were not yet about. |
| Vincent:
Just googled it... Not sure I wouldn't be bothered by those two eyes staring right at me :-DD But more seriously I like the engineering behind it. Too bad the frequency is fixed... What about the Heathkit IB-5281? Probably the cheapest impedance bridge one can buy nowadays. It tests the smaller values at 100kHz. I don't doubt it does but I'm a little bit skeptical as to how accurate the instrument is at that frequency. Otherwise it doesn't look like a bad deal! |
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