Getting an old-school impedance bridge, worth the hassle? - Page 1
Forum section: electronics / testgear

author: Vincent
author_rank: Regular Contributor
timestamp: September 13, 2022, 01:18:18 am
content: (Maybe this belongs more to the Beginner section?)I'm in the process of buying some of the basic test equipment that I still don't have and while I can pretty quickly determine whether or not I need a particular piece ("If you can't articulate why you need something, then you probably don't need it"), I'm a bit on the fence when it comes to impedance measuring instruments. I already sorta know how to figure some things out without such equipment, say track bad electrolytics. And it seems to be a better approach to test the coils/caps at their actual working frequency anyway. But I still wonder how useful an LCR meter/bridge is from a general perspective. Or is it in fact such a specialized tool that the average hobby electronics lab is no worse off without one?Now my budget isn't that severely limited but I certainly wouldn't mind using the older manual instrument if it means saving a few $$$. I drive stick everyday, so doing the stuff manually isn't a problem.In the case it deserves a spot on the bench, any idea as to which model would be the best choice, again for sorta general purpose? I like the Leader LCR-740's compactness and 40kHz external oscillator capability. Gen-Rad models on the other hand are known for their excellent build quality (and stability it seems?). ESI ones don't seem all that bad either. Then there are a number of less common makes and models. There's even a Philips PM6301 on eBay right now which I'm tempted to buy, even if just as an expensive toy to play with LOL.

author: wn1fju
author_rank: Frequent Contributor
timestamp: September 13, 2022, 11:35:15 am
content: LCR meters are not typically required in a home lab situation, but once you acquire one, you will say to yourself, "how did I ever live without one of these?" They are particularly useful, for example, in identifying (unmarked) components from your junkbox.I have had experience with a lot of these: ESI 296, Genrad 1658, Genrad 1685, HP 4260A, HP 4261A, HP 4262A, HP 4265SA, HP 4271A, HP 4274A, HP 4332A and Sencore LC53. Many of these are automatic meters - insert component and read the display. But often they require an auxiliary test fixture or set of Kelvin leads. The manual ones, for example HP 4260A and HP 4265A, are a bit more difficult to use and I've found that you sort of have to know approximately what the component's value is before you start twiddling the (two) knobs. As you have said, ESI and Genrad make good LCR meters. So does HP. I would recommend the HP 4274A/4275A if you want a top-notch bench LCR meter. I'm not a big fan of Phillips, but perhaps that is because I've had some bad experiences in the past trying to fix some of their (non-LCR) test equipment.There are also a bunch of handheld models such as the Peak LCR45 or the BK Precision 880, or even some of the newer LCR "tweezers" if you are doing SMT work.Search through the eevBlog forums - many people have had recommendations.

author: nctnico
author_rank: Super Contributor
timestamp: September 13, 2022, 12:05:22 pm
content: IMHO an LCR meter is certainly useful but I'd get a modern (digital) one if your primary target is to actually use it for measuring components. And no, the 4274A / 4275A are not top-notch bench LCR meters. Been there, done that; it is old crap from the 80's. Technology has improved since then. GW Instek makes nice LCR meters for a reasonable price compared to Keysight.

author: precaud
author_rank: Frequent Contributor
timestamp: September 13, 2022, 01:24:06 pm
content: I'll split the difference between wn1fju and nctnico. The 4274A is not cr@p but is quite large for what it does. For bench use it would be nice if it had more freqs available, but for the high-speed sorting in an automated system that it was designed for, it is good enough. The 4275A is in a different class; covering 10kHz to 10MHz, it's a more specialized instrument. I don't use it a lot, but when you need to look at Z in that freq range, it is spot-on.For a good, inexpensive first lcr meter, take a look at the XJW01:https://www.aliexpress.com/item/3256801259913642.htmlA friend talked me into trying one and I've been quite pleasantly surprised. I have several HP and Wayne Kerr meters and this one is accurate, warms up quickly, and is so small and handy. It also accepts the same fixtures as the HP. I'm using it a lot more than I thought, like wn1fju said, for testing components in the junkbox, verifying parts retrieved from scrapped pcbs, or quick matching/sorting while assembling. FWIW, I bought mine from the vendor in the link.

author: wn1fju
author_rank: Frequent Contributor
timestamp: September 13, 2022, 01:28:26 pm
content: I really like my HP 4274A. The specs are generally fine for home use and seeing as I paid less than one-tenth the price of a new more modern LCR meter for it, it's good enough for me. True, it is a relic from the 1980's and is large and bulky, but at least one has a chance of repairing the unit unlike the modern stuff. I don't consider it "old crap from the 1980's," but that's just my personal opinion.I only offered it as a potential solution to the original poster since it appeared to me he was interested in older models and perhaps real old ones like the manual impedance bridges. The latter category I would, however, consider to be "old crap" unless you are talking about some of the specialized, extremely expensive bridges (Genrad, ESI, etc.) that were made decades ago and are metrology-grade.

author: nctnico
author_rank: Super Contributor
timestamp: September 13, 2022, 01:32:09 pm
content: Quote from: precaud on September 13, 2022, 01:24:06 pmFor such measurements, I use an LF network analyser. A bit for versatile compared to an LCR meter and it can do impedance versus frequency sweeps.QuoteThat is a good one as well. I have bought mine many years ago as a replacement for the 4274A I had.

author: precaud
author_rank: Frequent Contributor
timestamp: September 13, 2022, 01:55:13 pm
content: Quote from: nctnico on September 13, 2022, 01:32:09 pmSame here. But LCR meters are more for spot (single-freq) measurements. And the 4275A was very nice to have on hand to verify and fine-tune the VNA fixturing and compensation math.And as wn1fju said, if it breaks one has a decent chance of being able to repair it. I got mine as a fixer-upper for pretty cheap.

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2022, 02:07:36 pm
content: What's a decent LCR nulling bridge worth ?What makes it more useful than a good set of SMD smart tweezers ?

author: TimFox
author_rank: Super Contributor
timestamp: September 13, 2022, 02:32:14 pm
content: Quote from: tautech on September 13, 2022, 02:07:36 pmHow about measuring a nominal 1 uF polyester (mylar) capacitor?

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2022, 11:18:32 pm
content: Quote from: TimFox on September 13, 2022, 02:32:14 pmWell if I had one I might. Can't a DMM with Cap mode do that ?

author: TimFox
author_rank: Super Contributor
timestamp: September 13, 2022, 11:22:26 pm
content: Will it measure Q or D?I use that parameter to verify if a foil capacitor is polypropylene or polyester.Either a bridge, a network analyzer, or an LCR meter can do that.

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2022, 11:35:18 pm
content: Quote from: TimFox on September 13, 2022, 11:22:26 pmWell the topic is old-school impedance bridge and the only one I've ever used was a manual nulling bridge that certainly couldn't measure Q or D and as with my now fading amount of repair work SMD smart tweezers can do everything and more that the nulling bridge could and in a fraction of the time.Sorta goes like this: Is the measured value within tolerance spec ? Is the measured ESR value suitable for the cap application ?Like with any tool confidence and knowing what to expect grows with use and for my use I'd have/use a pair of SMD smart tweezers any day over an old-school impedance bridge.Technology has moved on.YMMV

author: TimFox
author_rank: Super Contributor
timestamp: September 13, 2022, 11:38:08 pm
content: I own several "old-school" impedance bridges, from General Radio, Wayne Kerr, Hewlett-Packard, and others.Each and every one of them can measure Q or D or parallel conductance or ESR.Which one did you use that only measured capacitance?I was replying to the post above, where you asked about using a DMM in cap mode.

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2022, 11:58:19 pm
content: Quote from: TimFox on September 13, 2022, 11:38:08 pmTBH I can't remember as a buddy lent me it for a few months to characterize a pile of recovered inductors some 15 years back before I splashed out on a pair of ST-3's that IMO have been far more useful and convenient than an old school RLC bridge. All I remember of it was in needed feeding a 9V battery occasionally much like these now old smart tweezers from a time when rechargeable versions hadn't emerged so I need keep a card of LR44's handy.Normally I can get them for $2/card of 10 so of insignificant cost.I've been tempted to get some of Shannons new tweezers that many of us had an input into their design/feature set but IMO he hasn't got the tip design quite right for them to be robust enough to penetrate solder oxides and conformal coatings without deformation whereas these old ST-3's tips are as tough as the day I got them all them years back although with a little less gold plating on them now.There's no doubt if you need a proper LCR meter there are many available from Asia at reasonable costs these days however after years of using LCR SMD smart tweezers I couldn't be without this far more convenient form factor.

author: precaud
author_rank: Frequent Contributor
timestamp: September 14, 2022, 12:26:19 am
content: If youreallywant an old-school manual bridge, I recommend the GR 1608-A. It was the "Caddilac" of manual LCR bridges. Superb build quality. I had one for many years. I think you'll find any of the digital meters mentioned here to be more versatile and useful. IIRC, the largest capacitance it would measure at 1kHz is 1200uF.

author: Vincent
author_rank: Regular Contributor
timestamp: September 14, 2022, 04:16:59 pm
content: 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.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.

author: TimFox
author_rank: Super Contributor
timestamp: September 14, 2022, 04:29:10 pm
content: 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" circuithttps://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.

author: Vincent
author_rank: Regular Contributor
timestamp: September 18, 2022, 04:11:57 pm
content: Just googled it... Not sure I wouldn't be bothered by those two eyes staring right at meBut 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!