• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

help identify resistor

gophert

Well-Known Member
Most Helpful Member
Hi can anyone help identify this resistor? Its burnt out (open) and the colors are tough to read. Any help would be appreciated.

Thanks
it would be more helpful if you could let us know what device it was in and, even more helpful, show the circuit.

my first guess is a red, black, black, yellow (plus a tolerance color). So, 2M. Or Brown, black, black, yellow (1M). However, it is unlikely to see so much damage on such a high value resistor unless it is used In some high voltage device or damaged by a lightning strike.
 

jbreda01

New Member
Thank you. I don't have a circuit diagram but its from a 220v 3kw patio heater with 3 power settings and remote control. The resistor goes between the 1n4007 diode and blue capacitor near the top right of the board. One end goes to the microchip (KW618C or KB6228201) and the other to the right lead of the blue capacitor. Top and bottom pics of the board attached.
 

Attachments

Nigel Goodwin

Super Moderator
Most Helpful Member
It appears to be feeding zero-crossing pulses to the chip?, so is likely to be a high value resistor - so 1Meg is probably about right, assuming the colours haven't changed (and they often do).

As it's a high value, are you sure it's O/C? - did you test it on a high enough range?.
 

throbscottle

Well-Known Member
That blue capacitor is actually a thermistor - look at the back of the board.
 

jbreda01

New Member
Thanks for everyone's help here. The resistor was open, and burnt quite badly so colours were not true. I thought it might be 100 ohms but after all the feedback it seems that 1 meg is the right choice. I'll try it today. Many thanks again to everyone, its appreciated.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks for everyone's help here. The resistor was open, and burnt quite badly so colours were not true. I thought it might be 100 ohms but after all the feedback it seems that 1 meg is the right choice. I'll try it today. Many thanks again to everyone, its appreciated.
To be fair, modern 1% resistors are quite hard to read the colours on anyway, and colours very often change when overheated.

Assuming it is 1 Meg? - it's not really a value which you would expect to 'burn up', as it's power dissipation should be pretty low.

But fitting a 1 Meg to try should 'do no harm', as if it's supposed to be 100 ohm a 1 Meg is effectively just an O/C 100 ohm :D
 

JimB

Super Moderator
Most Helpful Member
Assuming it is 1 Meg? - it's not really a value which you would expect to 'burn up', as it's power dissipation should be pretty low.
True.

But the thought occurs to me that resistors also have a voltage rating, and that resistor will have been subjected to mains voltage for long periods.
It seems reasonable that overvolt operation will eventually result in failure.

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
True.

But the thought occurs to me that resistors also have a voltage rating, and that resistor will have been subjected to mains voltage for long periods.
It seems reasonable that overvolt operation will eventually result in failure.

JimB
Resistors do have voltage ratings - and historically resistors in such circuits do go O/C, but they don't generally 'burn up' as there's not enough power dissipated in them.

Most common failures in such circuits was due to putting two smaller resistors in series, and these VERY commonly fail (it's one of the common design flaws), I don't know if it's down to voltage ratings, or just poor voltage sharing across the resistors - but removing the two in series, and replacing with a single higher wattage resistor, means they never fail again. It's worth mentioning that a 1W resistor usually has a higher voltage rating than a 1/2 W one.
 

Les Jones

Well-Known Member
Most Helpful Member
Looking at the picture of the component side in post #3 it looks like more components have also failed. I have traced out part if the circuit. The power supply uses the yellow capacitor in the top right hand corner to drop the voltage for the electronics. It lookes like the discharge resistor in parallel with this capacitor has also failed. The fuse and the 47 ohm resistor in series with this capacitor have also been removed so I assume that these have also failed. The smaller yellow capacitor and the blue varistor are directly across the mains after the fuse. The fact that the varistor looks to be undamaged seems to rule out a severe high voltage transient on the mains as being the cause of the problem. Some history of the problem would be helpful. For example could there gave been a buildup of dirt on the board that has caused a flashover.

Les.
 

jbreda01

New Member
Basically there was a loud pop while the heater was on and a number of components cooked. The resister in question was burnt and open as was the 47 ohm 2w resistor plus the fuse and a land pattern that has a wire across it now. Anyway I replaced everything I thought was bad but when i connected it up the resister in question (1 meg) starting smoking so I turned it off. Obviously there is more wrong with the board so I may just give up and connect the heater to an on off switch. First time I have used this forum and I find the helpfulness of everyone is fantastic.
 

Latest threads

EE World Online Articles

Loading
Top