Help to identify a resistor value - Page 1
Forum section: electronics / beginners

author: Vendest
author_rank: Newbie
timestamp: September 01, 2023, 12:54:03 pm
content: Hi, seeking support from the forum members as I found it difficult to use the resistor color code chart.The damaged resistor comes from a 110/240Vac to 24Vdc power supply mounted inside a humidifier.The circuit was energized while in contact with water.

author: magic
author_rank: Super Contributor
timestamp: September 01, 2023, 01:04:25 pm
content: It looks like black, brown, grey, silver, gold.Ordinarily it would be 018·0.01 which is 0.18Ω, 5% tolerance.But I haven't seen so many bars on 5% resistors, so not entirely sure.0.18Ω is however a likely value, because this looks like current sense resistor, between the source of the switching FET and the negative rail of the primary.If this resistor is blown, it's highly likely that the FET is also shorted and possibly its gate resistor too and even the control IC. Definitely check those things before applying power.

author: Vendest
author_rank: Newbie
timestamp: September 01, 2023, 02:08:41 pm
content: Quote from: magic on September 01, 2023, 01:04:25 pmThanks for your advice, I'll make sure to check the FET and other components as you mentioned.The underside of the board seems to have heat marks.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 01, 2023, 02:55:06 pm
content: There is something weird here.Gold is not a "number", so you have to start reading from the other side.Normally resistors never ever start with black (Zero) because it's much more logical to shift the exponent ring.Because of it's rough grey surface I am guessing it is a fusible resistor, so it's meant to blow after some fault occurred, so it's likely more stuff is damaged.

author: Vendest
author_rank: Newbie
timestamp: September 02, 2023, 02:51:47 am
content: Quote from: Doctorandus_P on September 01, 2023, 02:55:06 pmSo should I read 8110% tolerance?

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 02, 2023, 04:27:25 am
content: AC-M6-POW-XR seems to be a common PCB on Aliexpress, etc. Here's a pic with a resistor from a different manufacturer. Again, it has that odd color code:(0.18 Ohms 1%)

author: magic
author_rank: Super Contributor
timestamp: September 02, 2023, 05:55:24 am
content: Quote from: Doctorandus_P on September 01, 2023, 02:55:06 pmSeethis thread.A multiplier of 0.001 has only been standardized in 2016; earlier resistors with 3 significant digits were coded with leading zero if necessary.What I haven't seen before is a resistor with 5% tolerance and three significant digits - typically they only have two.Quote from: Kim Christensen on September 02, 2023, 04:27:25 amThis looks like ordinary 0.18Ω 1% with 3 digit code. I have seen similar resistors and so have others in the thread I linked.Quote from: Vendest on September 01, 2023, 02:08:41 pmYep, it was connected between the FET and the negative rail. That's the current sense resistor and sub-1Ω values are usual here.AFAIK in flyback converters the exact value isn't even critical and it can be higher, only power output will be reduced proportionally.What must not be done is installing a lower value; this may cause transformer saturation and another short circuit through the FET.Check if the FET isn't shorted (but it probably is), check D27 and nearby resistors.Identify the SO8 chip and check if it doesn't have shorts to ground on pins such as FET drive output or current sense.editWhat's the TO92 device near this resistor, it looks unusual.It should be suspect too if it has connections with the blown resistor.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 02, 2023, 07:36:25 am
content: TO92 item near the optocouple looking like it's a typical feedback circuit with PC817, resistor dividers and TL431 voltage reference in a TO92 case.PC817 possibly could die too, and even TL431 (I mean sometimes voltage reference causes troubles).PS: sometimes, when you are not in mood to investigate, it is easier and faster to dumbly replace all components in a high voltage area and all semiconductors in feedback circuit (817 and 431).

author: magic
author_rank: Super Contributor
timestamp: September 02, 2023, 07:43:51 am
content: IC31 should be TL431 but so far there is no evidence of problems with it.I'm talking about Q21. No idea what it is.

author: u666sa
author_rank: Frequent Contributor
timestamp: September 02, 2023, 12:32:16 pm
content: 0.18 Ω ±5% (J)

author: donlisms
author_rank: Frequent Contributor
timestamp: September 02, 2023, 03:38:38 pm
content: Isn't that first band blue? Dark blue, yes - but I would agree black makes absolutely no sense (hah!), and blue is a sensible choice, especially know that colors are... a little... variable these days?Blue, brown, gray, silver, yeah? Then gold? So... 6.18 ohms, 5%.

author: Vendest
author_rank: Newbie
timestamp: September 02, 2023, 04:51:57 pm
content: Thanks again for the information, I have decided to order a replacement board on Aliexpress and will try to repair the existing one to keep as a spare.I'll post an update on the faulty components.

author: Andreas
author_rank: Super Contributor
timestamp: September 03, 2023, 05:04:39 am
content: Hello,The resistor looks like a special "fusible resistor" (flame proof).Unfortunately the markings are not standardized and vary from manufacturer to manufacturer.Most probably more than only the "fuse" is damaged.with best regardsAndreas

author: gamalot
author_rank: Super Contributor
timestamp: September 03, 2023, 06:05:37 am
content: It is 0.18ohm.