Blown heated blanker controller. Trying to figure out these smd resistors - Page 1
Forum section: electronics / beginners

author: LeWidget
author_rank: Regular Contributor
timestamp: June 29, 2024, 03:47:48 pm
content: Hello,We have one of those heated throw blankets you plug into AC power. The other day it went *bzzzbzzzbbzz... pop*, so I opened it up to find what appears to be two resistors gone *poof*. (R7 & R14 in picture)luckily, we have a second blanker with what appears to be identical controller, so I opened it up to take a pictureAccording to the good controller PCB, R7 = 9531, & R14 = 103Online SMD resistor calculator saysR7 - 9531 = 95.3kohmR14 - 103 = 10kohmIs this correct? Would all I need is a 10k ohm & a 9.53k ohm? I had a google but there seems to be many types with many with different tolerances. Any suggestions on which ones I should choose?

author: MathWizard
author_rank: Super Contributor
timestamp: July 01, 2024, 09:43:17 pm
content: Yeah those should be the correct values. IDK the physical size, maybe 0805?? 5% or 1% tolerance is the most common, and on all sort's of stuff. Depending on the circuit, 2 10k's might be fine. You can probably find those on other old circuit boards, like if you had a TV or printer to scrap. Or even use through hole resistor's, just lay them flat on the PCB.I'd like to know where those resistors go to, and why they burned out. It could be their power rating was a bit low and they burned out over time. But why did they go at about the same time. Did anything else fail?I personally would map out some or all of the circuit, and after checking some stuff with a DMM, I might apply a low voltage DC as VCC to the control circuits. So there's nothing on the blanket plugged into AC, but then you might find if the chip and other stuff works ok.Does it use a Triac type device to AC power some heater resistor, and control it with high-voltage opto-coupler's ? Or is there some inductor's and step-down switching type PSU, that power's the heater from lower voltage DC ??The only similar thing's I've worked on are a hot-air soldering station, and a soldering iron. Both have Triac's in them, they are sort of like Silicon based relay's, for AC.

author: Konkedout
author_rank: Frequent Contributor
timestamp: July 02, 2024, 03:50:44 am
content: I concur with MathWizard that the blown resistors are likely to be a result and not a root cause. This means that if you replace them, the new ones will quickly be destroyed as the originals were.It is likely that a triac is used as a power control element, and I wonder whether the triac may be toast. If you are GenZ you can get an avocado and have some avocado toast.

author: golden_labels
author_rank: Super Contributor
timestamp: July 02, 2024, 05:40:38 am
content: 9531 = 953 · 101Ω = 9530 Ω. Not 95.3 kΩ.The package seems 1206 or 0805. They should be fine at 30–45 V, so whatever destroyed them so quick was much higher voltage. Agree with MathWizard and Konkedout: don’t connect power until you determine, what was the cause. What is a bit concerning is that R14 could pass current only to U1 chip. Unless we are missing some trace under that chip or you were particularly lucky and it went through C4 without causing visible damage.9530 Ω is an oddly specific value. In particular with 10 kΩ already being in BOM.

author: stj
author_rank: Super Contributor
timestamp: July 02, 2024, 09:02:26 pm
content: lolthis post is a copy-paste from BadCaps.nethe got the same reply there too but i guess he didnt like the answer.

author: LeWidget
author_rank: Regular Contributor
timestamp: July 05, 2024, 02:01:42 pm
content: Quote from: MathWizard on July 01, 2024, 09:43:17 pmQuote from: Konkedout on July 02, 2024, 03:50:44 amQuote from: golden_labels on July 02, 2024, 05:40:38 amSorry for the delay, much appreciate the feedback I've attached new pictures showing front and rear of PCB. Strangely, I found a review on Amazon where a user posted (negative) because his controller burnt out too. Same controller, same resistors.https://www.amazon.co.uk/gp/customer-reviews/R3GYRIZ80YKHI1/ref=cm_cr_getr_d_rvw_ttl?ie=UTF8&ASIN=B001IRYFJGI unfortunately don't have a DC power supply nor know enough to apply it as DC VCC. I might have to scrap this I think. New blanket is maybe $40, I don't know electronics. Thought it might have been a simple resolder of resistors, but seems to be more than that from the comments I've read.Quote from: stj on July 02, 2024, 09:02:26 pmTechnically, BadCaps is a copy/paste from this forum lol. True, I didn't like the technical talk lol, thought it would have been a simple replacement of parts, but doesn't seem to be the case. Makes sense what the comments say though, can't test the main IC but, as no markings, don't know what readings I should be getting.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: July 05, 2024, 06:30:29 pm
content: Quote from: LeWidget on July 05, 2024, 02:01:42 pmBin it. Bin the bloody thing right now. Heated blankets are one of the more dangerous electrical devices in your home and a dodgy one can burn you or burn your house down very quickly. So appreciate the fact that this one failed dead, and don't give it another chance to fail hot.

author: Kurets
author_rank: Regular Contributor
timestamp: July 05, 2024, 06:43:12 pm
content: Seems likely that what has happened is a short from mains to the thermistor, taking out the bias resistor, series filter resistor, and probably MCU. Bin it is the correct call.

author: LeWidget
author_rank: Regular Contributor
timestamp: July 07, 2024, 07:33:18 am
content: Quote from: exmadscientist on July 05, 2024, 06:30:29 pmQuote from: Kurets on July 05, 2024, 06:43:12 pmThanks guys, I'll do that.