General Resistance RDS-76 vs. ESI DB877 Decade Resistance Box - Page 1
Forum section: electronics / general

author: garrettm
author_rank: Frequent Contributor
timestamp: February 16, 2025, 10:51:09 am
content: I was wondering if anyone has had experience with both General Resistance RDS-76 and the ESI DB877 decade resistance boxes?I'm considering buying one, but would like to know of any quirks or reasons to go with one over the other.Looking at the data sheets, the General Resistance RDS *seems* to have better specs: 15ppm/°C temp. co. for 10Ω and 1Ω decades, lower zero resistance per decade and 0.5W power rating for all resistors (the ESI has 22mW/step for 1MΩ and 220mW/step for 100KΩ), and a stated 25ppm yearly drift spec.The only thing the ESI box seems to do better is have an extra decade and a much more compact design.Are the General Resistance RDS specs a bunch of hot air? I find it hard to believe their specs are so much better than ESI's.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 16, 2025, 02:34:51 pm
content: More Discussion of this topic:https://www.eevblog.com/forum/testgear/decade-resistance-boxes/Both are good units. GenRad Instruments are very good and specs are not any sort of exaggeration.Biddle is another good name brand, there are others. I found a Biddle bridge at a Hamfest for $20, looks brand new. Most resistance bridges will have a connection for use as a resistance box, so that is another thing to consider.Old units typically have wire wound resistors that can be hard to solder if a wire breaks. Also old switches need gentle cleaning, but the switches used by GenRad are very good and expensive.

author: Overspeed
author_rank: Frequent Contributor
timestamp: February 16, 2025, 06:44:55 pm
content: HelloIf that a used one , best is to open to check over heating and contact wear , performances are given for new units and some user are more ''blacksmith ''RegardsOS

author: garrettm
author_rank: Frequent Contributor
timestamp: February 16, 2025, 10:18:47 pm
content: Quote from: Wallace Gasiewicz on February 16, 2025, 02:34:51 pmThanks for the link. I'm asking about General RESISTANCE, not General RADIO, though. Very similar names, especially when shortened--but very different companies.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 16, 2025, 11:22:05 pm
content: OOPS ,,,,,SorryBut the statement applies to General Resistance also... they made pretty accurate resistors for a lot of things...I have some of their resistors around here somewhere....

author: donlisms
author_rank: Frequent Contributor
timestamp: February 17, 2025, 01:33:49 am
content: I don't have an RDS-76. I have a couple of DB877's. Thesethoughts may be a little scrambled; it's my way. :-)There are, as usual, both apples and oranges in the specs. Ithink before you can conclude one is vastly superior, you haveto try to convert and compare carefully.And, I'd expect the real-world performance to be better than specs,except as altered by the ravages of time and abuse.The DB877 is made of manganin resistors, and I assume the RDS isalso. Manganin's TC of resistance is somewhere in a range, dependingon the heat treatment and range you need to cover. I would expectboth boxes to be similar in Real Life, regardless of the specs.They're both "mostly about 5ppm". I don't ever trust the numbersat the ends of the resistance scale.Assuming you buy used, long term stability is most likely alreadypretty well stabilized (because that's what manganin does), so thosespecs probably don't matter much. But the RDS probably wins the spec contest,claiming 25ppm per year versus the DB877's 200ppm per forever.Unless the RDS is 25ppm *very* year, which means the DB877 wins ifit's over 8 years old.You didn't say much about how you intend to use the box, but if youwant precision, you need to consider the power and the heat it willgenerate in conjunction with the resistance and power temperaturecoefficients. E.g. the RDS claims to have a power rating of 0.5watts per resistor at 125 degrees C, and at 5ppm per degree, solet's say 100 degrees * 5ppm = 500ppm or 0.05%. None of the otherspecs matter much at that point.The RDS power coefficient is 500ppm / watt or 0.5ppm/mW. The DB877is rated at 0.3ppm/mW. Setting a tolerance limit will set a powerlimit. ESI usually suggests 100mW, I prefer 10mW or less if Iexpect the manganin resistors to behave, which means 3ppm for theDB877, 5ppm for the RDS. But ESI is saying 500mW here, and I'mbaffled by that; it means 150ppm delta. The RDS would be 250ppm.The low zero resistance of the RDS, assume it's still valid inwhatever unit you pick up, suggests it has better switches, or atleast, those nice old-school switches with many parallel blades.They don't mention repeatability. In any case, I don't think I'drely on either of these boxes for very low values, which are anadventure of their own.The RDS accuracy is 0.01% or 100ppm, the DB877 is rated higher at300ppm long term. Both are probably better than they suggest; Iwould not place a bet on one being more accurate than the other,overall, once all the switch combinations are made.The DB877 claims 1mOhm typical repeatability. It uses stackedceramic switches with silver (plated?) contacts, which I believe(but am not sure) are from Centralab. The arrangement uses five2R resistors and one 1R resistor, to give a total resistance of 10Ror 11R per decade. This requires an additional switch contact toput the 1R resistor in or out for odd or even values. So itinherently has slightly higher zero resistance.The DB877 uses the traditional ESI concentric shaft switching; eachdial connects to another diameter of shafting, each connecting toa different switch layer. Thus... small. But if any of the shaftsget a little bent, the dial rotation tends to have tight spots.This is a usability issue rather than a resistance issue. (I haven'tyet had a chance to try to straighten mine, but I have many ESIinstruments waiting in the queue for this treatment. Seems doable.)It occurs to me that 22mW in a 1M resistor is about 150 volts; 220mWwould be about 470 volts. RDS says that the limit is 300 VDC maxper resistor. Similarly current at the low end; you have to dosome math. I suppose whether those ratings even matter depends onwhat you're working on, and then to find the limits, you'd need alittle python program to compute them for a given total resistancevalue and what the individual resistors are being subjected to.If I wanted a box to use for general purpose in-circuit substitutions,I'm not sure either of these would be my first choice.