Cannot Decide on a Resistor Value

[inuh]

Hi everyone,

In the attached pictures you can see a Bosch laundry machine power supply card which has failed resistors on it. I will replace those resistors, but I cannot decide on their value; are they 100K or 1 Ohm resistances? Furthermore, since they appear to be failed due to excess power, I am planning to replace them with 5W ceramic resistors, any thoughts on that?

 Thank you all in advance for your help and have a nice day.

[Zero999]

Do you know what caused the resistors to dissipate too much power and overheat?

It's highly likely the resistors failed because of a fault elsewhere and replacing them, will just result in them failing again.

[Audioguru]

Why did you guess 100k for the resistor value? Do a power calculation to determine a suitable value.
If the resistors are rated at 1W then a 100k resistor will need a voltage of 316V for it to be at its maximum allowed temperature and maybe 500V or more to scorch like in your circuit.
Do you think the circuit feeds them 500V?

I agree that whatever failed to cause the resistors to burn is still failed and will cause replacement resistor also to burn.

[MarkF]

Why did you guess 100k for the resistor value? Do a power calculation to determine a suitable value.
If the resistors are rated at 1W then a 100k resistor will need a voltage of 316V for it to be at its maximum allowed temperature and maybe 500V or more to scorch like in your circuit.
Do you think the circuit feeds them 500V?

I agree that whatever failed to cause the resistors to burn is still failed and will cause replacement resistor also to burn.

Besides, the bands look like red - black - brown or yellow would be 200 or 20K.

[inuh]

Thank you all for your comments,

Dear Hero999, this is the original board that was shipped in the product so failed resistors were not replaced before. I do not know exactly what caused the failure, but it is likely that general failure reasons like aging, corrosion, fluctuation on the power line or even poor design (although it is a Bosch/Siemens Home Professional product) caused the problem. This is why I am planning to replace them with "better" components.

Dear Audioguru, as I said this is a board that came in a Bosch product, so this is not my design, therefore, I do not know what is going on in the circuit, hence there is no chance of doing "a calculation". The resistors look like 2W job, so the engineer who designed this board thought it was sufficient, maybe it is an acceptable value for 5-year life span, but I hope to use this product for couple of 5 years:) It is possible that the voltage level to exceed 316V as line voltage here is 220Vrms. My assumption on their values solely based on their color codes which I cannot tell for sure.

Dear MarkF, I am confident that the first ring is brown, but I cannot tell if the last ring is yellow or gold, that is why I am asking for help:)

[jc101]

Have a goole around for the Bosch part number 9000661565 - there are some pics of PCBs on eBay and for sale etc.  Some of the pics may be good enough to help determine the colour.

[Shock]

Looks like Brown/Black/Brown/Gold.

There are several images worse than this one but they all seem to match the first and third band as far as I can tell. There is a few tricks to double checking aside from the obvious measuring it and looking at the underside. If they are open you can carefully remove, chop in half or break then sand or file etc and measure/calculate/add the remaining resistance together with a multimeter.

Edit: Actually this one looks even clearer.

[JS]

Looks like 100Ω for me too, are they connected to the diode bridge? what's the voltage rating on that cap?

You should check if the bridge and/or cap haven't failed short, they look like they broke due to some failure not just overheat, way too much destruction for slow over heating, and if that were the case I would expect much more signs of overheat on the PCB. They can stand quite a lot of heat before failing, over time they could go up in value slowly, I've seen resistor with over 50% of their original value running way too hot for decades and replaced them just because, not total failure.

JS

[Kevin.D]

If you carefully scrape away some of the cracked outer coating you may be able to expose the helical metallized
track around the resistor which you can probe onto directly. Sometimes measuring the resistance of the unfused
half (half as accurate as you can)  can give you an approximation of it's original value.
regards.

[inuh]

Thank you all for your comments,

Dear jc101, that is a great idea, thank you.

Dear Shock, thank you for doing the search. With the images you provided, I guess it is now for sure they are 100 Ohms.

Dear JS, the resistors are connected in parallel and they are connected to the diode bridge from the AC side. Please see the attached pictures. The cap is rated for 400V. The bridge looks OK, but the cap measures 7.5 Ohms in the circuit so I should remove it and measure again, thank you for this important warning.

Dear Kevin.D, it is a very nice suggestion but I guess you agree on that they are 100 Ohms based on the photos that Shock shared?

[xavier60]

Power Integrations SMPS ICs commonly fail. Check the TNY*** IC for Drain to Source short or low resistance.

[JS]

Before removing it from the PCB be sure to measure with the correct polarity, if measured backwards you won't blow a 400V cap with your DMM but you could be measuring the bridge diode. It isn't likely to be shorted without blowing off the pressure relief marks but it could happen. Also, there are no other signs of damage, for now you should take the resistors off, you know they are bad and the less stuff in the PCB the better to measure around, without the resistors and the jumper on L1 position which you could easily take out without much risk you can check the bridge almost unconnected.

JS