Resistor Value - Page 1
Forum section: electronics / beginners

author: OpenCircuit
author_rank: Regular Contributor
timestamp: May 10, 2017, 06:18:09 pm
content: DMM indicates .30 ohms each removed from circuit.http://resistor.cherryjourney.pt/indicates they should be 10k-15k.These are situated in parallel between the outputs of the rectifier bridge on primary side of a SMPS.Even if the red are orange bands they should still be 138k w/20% margin.Appreciate any info. Thanks

author: Ian.M
author_rank: Super Contributor
timestamp: May 10, 2017, 06:38:54 pm
content: Brown - 1Red - 2Silver - /100Red - 2%0.12 Ohm 2%I'd also bet your DMM isn't very accurate below 1 Ohm.

author: OpenCircuit
author_rank: Regular Contributor
timestamp: May 10, 2017, 06:43:59 pm
content: Quote from: Ian.M on May 10, 2017, 06:38:54 pmWhat about the black band?BrownRedSilverRed?Black?

author: OpenCircuit
author_rank: Regular Contributor
timestamp: May 10, 2017, 06:49:57 pm
content: Quote from: Ian.M on May 10, 2017, 06:38:54 pmOK. So do not utilize the tolerance band.

author: Ian.M
author_rank: Super Contributor
timestamp: May 10, 2017, 07:05:14 pm
content: Black - Red - Silver doesn't make sense at the start of the resistor code so it must be read the other way, (which happens to be top down, the preferred direction for vertical resistors). The fifth (or sixth) band is often confusing - extra bands after the tolerance tend to be proprietary, and may indicate temperature coefficient, reliability or identify a fusible or non-flammable resistor. You'd have to know what make they were to be certain.From your description its unclear where the resistors where in the circuit. If they were a parallel pair in series with the + terminal of the bridge it would make more sense than what you described.

author: OpenCircuit
author_rank: Regular Contributor
timestamp: May 10, 2017, 07:09:01 pm
content: Quote from: Ian.M on May 10, 2017, 07:05:14 pmYes, this is close. Neg side of the rectifier bridge TY.

author: Ian.M
author_rank: Super Contributor
timestamp: May 10, 2017, 07:15:33 pm
content: I'd bet they are still OK but its possible their value is drifting high.Tack solder them in series and re-measure across the pair. I'm betting you *WON'T* see 0.60 Ohms. Also see what your meter reads with both probes touched to one resistor lead (but not directly touching each other.

author: OpenCircuit
author_rank: Regular Contributor
timestamp: May 10, 2017, 07:25:40 pm
content: Quote from: Ian.M on May 10, 2017, 07:15:33 pmIan, I tacked them together and get .23 (UT61E). I think they are fine as you mentioned, thanks for commenting as I wasn't sure. Getting the same .3 with my Fluke 114 and the UT61E when resistors are separated.

author: Ian.M
author_rank: Super Contributor
timestamp: May 10, 2017, 07:59:21 pm
content: Edit:Yes they are fine. Your previous 0.60 Ohm reading was probably dirty contacts on your meter leads.If you have to check low ohm resistors at all often, it looks like you need to build yourself a low-ohm meter. Basically a 100mA current source and a rail-to-rail OPAMP configured for x10 gain with differential inputs so you can use Kelvin probes and get the actual voltage across the resistor with 100mA forcing current. Readout is on a DMM's 1.999V range direct in milliOhms.Another option if you've got one is to use an ESR meter. They usually aren't that accurate but are better than most handheld DMMs under 1 Ohm, and many are designed to keep the voltage low enough not to forward bias a Silicoon junction - a big advantage when checking current sense resistors between a transistor emitter or source and ground.

author: james_s
author_rank: Super Contributor
timestamp: May 10, 2017, 08:16:29 pm
content: I think you're barking up the wrong tree here. These resistors are likely used to limit inrush current and act as a fuse. If they're not open circuit then they're not the problem. A low ohm meter is handy but not really needed here. What is wrong with the device you're working on?

author: Zero999
author_rank: Super Contributor
timestamp: May 10, 2017, 10:13:20 pm
content: Given the description of their position in the circuit, there's no way they could be such a high value so 0.12Ohm is the only thing which makes any sense.Another thing to note is they're wire wound resistors. If you look at them closely, you can see the coil underneath the ceramic coating.

author: OpenCircuit
author_rank: Regular Contributor
timestamp: May 11, 2017, 03:21:51 am
content: Thanks for the comments.Quote from: james_s on May 10, 2017, 08:16:29 pmYes, I have moved on.https://www.eevblog.com/forum/repair/smps-primary-side-short/OK, I see how I got it wrong. Website author athttp://resistor.cherryjourney.pt/has a -band and a + band button to set first. I should have hit the -band button.