Resistor ID

Could one of you electronics gurus (@emuboy, @Kyle ?) help me identify this resistor please?

My cousin brought a gadget over from the States and my brother plugged it directly in to 230V here and funnily enough it isn’t working… upon opening it up this resistor had fallen off the PCB and is fried.

It is a simple device with 4 diodes, 1 capacitor and this resistor feeding a small 120V DC, 11W motor.

I’m not sure about the colours but if it is White-Black-Black then according to a chart on the net it should be 9 Ohms, right? It is 11mm long, so 1 Watt I think.

Any tips appreciated! Thanks

I’m colourblind, so not much luck helping you with the colours ( I’ve never been able to read a resistor band in my life)

But I can’t see why you would have a resistor if the motor is 120v anyway.?

As a current limiter?

I’m wondering if is a fuse?

Couple of questions - have you tested the resistance of this with a multimeter?

Also - is the capacitor still good - what are the ratings on that - sometimes that will have issues as well with over-voltage…

Maybe worth bringing it along to electronics night tomorrow??

It certainly became a fuse :slight_smile: It might’ve served to limit current or motor speed. I wouldn’t trust a resistor that blew itself off a board to have the correct resistance still. I’m also not great with color, you might have luck with some IPA to remove the deposited smoke.

Checking the cap is still good is important. A short failure mode might’ve even been the cause instead of a double power problem. The motor might’ve been damaged as well.

The motor can be modeled as an inductor, resistor, and voltage source for back EMF but without those properties, I’d have a hard time figuring out a good value ahead of time. A guess and check method might work if we can’t narrow down the colors.

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True dat.

Based on the size it looks like a 0.5W resistor:

Colors are very hard to see/tell…

Enhanced version:

I am making this up…!

Could be miles off! Even have a different number of bands… Could be reading it backwards, etc…

Don’t seem to be many 90k resistors out there - so this is most likely wrong… but you get the idea!

I think Tom had it with a 11mm 1W :slight_smile:

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Andrea, Andy, Kyle, thanks so much for your analysis :pray::pray:

The resistor is showing infinite resistance and does look a bit burned so think it has blown. The motor shows 400 Ohms across its terminals so I’m hoping that is ok. The capacitor is rated at 250V (pic below) so I’m hoping that’s ok too - I can’t remember how to test them with a basic multimeter but might be able to youtube that later.

I would definitely pop in to Electronics Night but at the moment am 60 miles away (with cousin and resistor too). I’ll try some alcohol on the resistor later and see if that sheds any more light…

Thanks again guys.

Ya- Dang crappy chart I found!!

At a guess, assuming this nothing else that I can’t see in the picture.

The PCB looks to contain a bridge rectifier smoothing cap and a current limiting resistor with a power rating of 1 Watt
110-volt coming in rectified is about 155 V the motor is rated at 11 Watts so 11 / 155 equals 70mA
P=IV so 1/0.07= 15V

So 15/0.07 =200R or less.

180R at a complete guess but a photograph of the label on the motor and the back of a PCB and whatever is going on on outside that photo would be useful so I could provide a better guess.

It might easily be a 24V motor which is being run on the other side of the power Curve let me just do that calculation.

155-24=131V P=V^2/R so 132^2/1 =18K

So 180R or 18k depending on which side of the hump we are operating on the voltage rating on the motor will tell you.

And before anyone tells me that’s not the correct way to do the math, I know, what I’ve done is a quick back of a fag packet approximation


Probably all made up but it sounds good :wink:

Your fag packet convinced me :smiley:

Cheers Joe, appreciated. The motor says on it ABJ Motor DR395M12-001, DC120V 11W, On 5min. I think you are right about the circuit - sketch below. I will see if I can find a 180 ohm to put in… :boom:

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Oh well I was completely wrong about what the resistor is for , it’s a bleed resistor not a current limiter.

New back of a fag packet calculation coming up.

Do not put a 180R resistor in it Will go flash bang wallop

P =V^2/R
155^2 /1 = 25K

So i would put in a 56k or a 47K so it doesn’t get to hot

This strikes me as an unusually low value for a bleed resistor, but they might easily be an American reg coming into play that I don’t know about 1M would be a normal value in my opinion

56k will give you a Tc of about 5S

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Just to repeat do not put in in a resistor below 47 k.

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Perfect thanks Joe :pray:

Joe, so sorry I sent you an incorrect circuit diagram, perhaps that explains why the values seemed weird?! :man_facepalming: :grimacing:

I managed to extract the PCB to get a better look and the resistor is actually in line with the AC feed to the diodes.

Is it back to 180R?!!

Back to 180R , remember that was a DC approximation, the actual waveform is quite complicated 180R should be the highest value you can put in without resistor self-destructing. You’re probably going to have to try a few different values below 180R , to get the correct voltage across the motor.

Very hard the model circuit like that without knowing the exact characteristics of the motor and the load on the motor.

But I can assure you you are the muck around with it until it works approach has a long and glorious history, I happen to know that the guidance system on a surface-to-air missile currently being used in Ukraine was perfected by the muck around with it until it works approach.

I would order a selection of values between 68R and 180R


Thanks Joe.

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My approach would be slightly different: could you ask on Reddit to see if any owners of a non-fried version could open it up and post a picture?

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