Decade resistor noob question - Page 1
Forum section: electronics / general

author: Ziginox
author_rank: Contributor
timestamp: January 14, 2022, 03:17:29 am
content: Hello everyone!The decade resistor I had ordered arrived today, but in checking it out I'm slightly confused. This is my first one of these devices, so I wanted to see if it's me or if there is something going on with the equipment.https://www.ebay.com/itm/323820046543This is the unit I purchasedWith all switches set on 0, I measure 1.3-1.4ohm (with the leads I'm using having a known resistance of 0.4 ohm, this is what I expect.)With the x1 ohm decade set to 1, I get the same measurement. So far, this all checks out as the specs say that it bottoms at 1 ohm, not 0Where it gets weird is this. If I dial in, say 32 ohm on the switches, I get 31.3-31.4ohm. Taking the lead resistance into account, I'm getting an entire ohm less than is set on the switches. I get this on two different (portable/handheld) meters.As a sanity check, I even dug out the old Guildline 9575 (Schlumberger Solartron 7055) and did a four-wire measurement. On that, if I set 32 ohm, I get 30.9 ohms. Dead on what the other meters were getting. If I dial in 904 ohm, I see 903 ohm.My question, then: Is this expected behavior? Should I be seeing almost exactly one ohm lower? Or is there something going on with this resistor?Thanks in advance!

author: Vgkid
author_rank: Super Contributor
timestamp: January 14, 2022, 04:28:53 am
content: What you should do is null out the resistance of the decade box(and use 4-wire ohms) and step through each range, and calculate the error(excel is good for this)What is the tolerance for the unit?You could have an issue where the resistor error, and the switch contacts add to give a much larger error than expected.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 14, 2022, 05:00:43 am
content: A seven decade decade box is an extreme technical challenge and I wouldn't expect perfect performance, particularly at this price point.Look up low tempco resistors. Resisters with a 5 ppm per degree tempco cost large fractions of a dollar and truly achieving perfect performance over seven decades requires fractions of a ppm.

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2022, 05:18:09 am
content: What do you get if you dial in 900 ohms?

author: Ziginox
author_rank: Contributor
timestamp: January 14, 2022, 05:48:52 am
content: Quote from: Vgkid on January 14, 2022, 04:28:53 amYou read my mind on graphing out the response, I've attached a graph from excel of 0-100 ohm.As for four-wire measurement, that's what the pictured Guildline meter is doing (as mentioned in the post)As for contact resistance, I could definitely see that happening, but what's confusing me is that I'm readinglowerresistance with all meters. If the contacts (or any other part of the construction) were playing a part, I would expect the readings to be higher than selected.Quote from: bdunham7 on January 14, 2022, 05:18:09 am900.56, which is within spec. If I click it to 905 ohm, though, I get 904.51. You'd think I'd get 905.5something, if anything.Anyway, as for the chart, you can see that it reads consistently lower resistance if the x1 ohm decade is on anything but 0. Seems a bit strange, right?

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2022, 05:53:57 am
content: OK, as I suspected, your issue is in the 1R decade. So set it to all zeros and then see what you get for 0,1,2,3,4,5,6,7,8 and 9. I assume your unit doesn't have a 10 position?

author: 1audio
author_rank: Frequent Contributor
timestamp: January 14, 2022, 05:56:32 am
content: I have several decade boxes and they all show 1 Ohm when set to 1 Ohm. It looks like the 1 Ohm resistor is shorted. It would make sense to open and check it. However contact the vendor first.

author: Ziginox
author_rank: Contributor
timestamp: January 14, 2022, 06:08:10 am
content: Quote from: bdunham7 on January 14, 2022, 05:53:57 amThat is also what I was suspecting. That is correct, it only has 0-9 on each decade. I realized the chart in my last post was off by one, as Excel had started at 1 instead of 0, I've attached a corrected version. As you can see, it reads essentially dead on for 1-9 ohm, but 0 ohm is reading as 1 ohm.Quote from: 1audio on January 14, 2022, 05:56:32 amI will definitely contact the vendor now that I have consulted with all of you, but may take a peek inside first.EDIT: Never mind, the note it came with has a strict "do not open" warning. Although I doubt its enforceability. Better safe than sorry...Thanks everyone!

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2022, 06:38:17 am
content: Quote from: Ziginox on January 14, 2022, 05:48:52 amYes, it seems very strange and personally I'd have a peek regardless, unless you are just getting a refund and trying a different product. There may be some baked-in design issue here. If '10' reads correctly, that means that it is reading 9 ohms more than the zero position, which makes no sense with any normal decade box design. And then '11' reads 9 more than '1'. Can you confirm that? I think I understand your graph, but I just want to make sure you are showing absolute numbers with no offset or zeroing.

author: Ziginox
author_rank: Contributor
timestamp: January 14, 2022, 06:54:20 am
content: Quote from: bdunham7 on January 14, 2022, 06:38:17 amI can confirm, that is exactly what I am seeing. The line on the graph is B-A, where B is the actual reading and A is the setting on the decade box. I've attached the entire spreadsheet, if you would like to see.

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2022, 06:57:59 am
content: I should have asked before, but what do you read if you short the two DMM leads together right at the decade box?

author: Ziginox
author_rank: Contributor
timestamp: January 14, 2022, 07:12:01 am
content: Quote from: bdunham7 on January 14, 2022, 06:57:59 am0.02 ohm on the Guildline (four wire) meter, 0.4 ohm on my two 'portable' multimeters (BK 2703C and an old Micronta/Radio Shack thing.)I accounted for the 0.4 ohm resistance when using those meters, of course.

author: jonpaul
author_rank: Super Contributor
timestamp: January 14, 2022, 04:20:05 pm
content: The IET units are resistor sub boxes and NOT precision decades.See the specsJon

author: Ziginox
author_rank: Contributor
timestamp: January 14, 2022, 04:30:34 pm
content: Quote from: jonpaul on January 14, 2022, 04:20:05 pmIt's not IET, the company that made it is EDFM. As for specs, the manufacturer quotes 1% but I'm reading 10% out.https://www.edfmusa.com/product/7-decade-resistance-substitution-box/

author: jonpaul
author_rank: Super Contributor
timestamp: January 14, 2022, 04:56:23 pm
content: perhaps marked EDFMUSA but junk from China

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2022, 05:31:57 pm
content: Quote from: jonpaul on January 14, 2022, 04:56:23 pmWe're perfectly capable of making our own junk, thank you!

author: Ziginox
author_rank: Contributor
timestamp: January 22, 2022, 10:38:50 pm
content: Small update, I did a little more measurements with my shiny new DE-5000 LCR meter in DC resistance mode as yet another check, and measured essentially the same results I went up to 120k this time. I've attached another chart, seems to definitely be only the 1 ohm decade causing issues.Now that I have an LCR meter, I used it to test the capacitance box (which was rated 5-10% tolerance) and ended up being 16% out on the x10 uF range.The lower ones seem fine, though. The knobs on it were also not fully tightened from the inside.I've sent the manufacturer/seller a message, we'll see how they respond. I think I may end up returning and keep an eye out for some secondhand IET units, or maybe the Eisco one? You really do get what you pay for, it seems.

author: Ziginox
author_rank: Contributor
timestamp: March 03, 2022, 01:40:54 am
content: To close this thread out:I returned them. Both of the decade boxes from EDFM just didn't seem to be working properly. To his credit, George, who I assume is the owner, was very helpful with measurement concepts and methodology. However, even with that, it was apparent something just wasn't right with the readings from either device.I briefly had an IET CS300 capacitance decade box, with date codes inside from 1988. It was dead on for every reading, but went back because the seller didn't disclose that some of the teeth on the switch were broken. Still, it showed me that my testing methods were not flawed and there was indeed something funky with the EDFM capacitance box.As for resistance, I ended up grabbing an Eisco 6-decade box. Clearly, this device is targeted for classroom/educational use, but it has none of the weirdness the EDFM box had. (Ended up choosing this one because of the 1W rating, the IET boxes with 1W resistors are more than I wanted to pay. I may end up grabbing one of the normal power ones in the future, though...)So there you have it, folks. I much appreciate all of your experience and suggestions!