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Resistor overheated and melted color code, how do I determine type?

by quinnvanorder
Tags: code, color, determine, melted, overheated, resistor, type
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Jan29-12, 08:34 PM
Sci Advisor
P: 4,027
You should test the rectifiers. You have to use the "diode" function on a multimeter to do this though.

On the red one I pictured above, it is just above the "ON" marking of the on-off switch.

This gives a higher test voltage than the "OHM" ranges.

The big capacitor can initially be measured in-circuit. If it measures very low resistance (like less than 10 ohms) then it has failed.
These capacitors do not usually get slightly dented. They fail dramatically and stay failed.

It appears to be adequately rated, so it probably hasn't failed. However you have to check.

You may be able to measure some higher resistance across the capacitor and this may be due to other components in the circuit.

Can you try to read all the writing on the "3 wire regulator". I don't think we know for sure that is what it is.

You could consider a fuse later if this ever gets fixed.
Jan29-12, 09:19 PM
P: 8
Quote Quote by vk6kro View Post
Can you try to read all the writing on the "3 wire regulator". I don't think we know for sure that is what it is.
the writing is as such
You could consider a fuse later if this ever gets fixed.
How do I determine the correct one?
Jan30-12, 04:40 AM
P: 5,462
Here is my analysis of the situation using available data.

There is no transformer mentioned and the control circuitry requires a DC voltage very much lower than mains either.

The tub or can capacitors shown are low voltage types. This applies particularly to the larger black one.

The integrated circuit requires a low DC supply voltage.

VK6kro if you have identified it this should tell us this voltage.

This DC voltage will be supplied from the three legged black device which I think we are all now agreed is a voltage regulator.

We can therefore type the voltage regulator from the standard supply voltage of the IC.

Since there is no transformer either

The mains is directly rectified by IN4003 diodes and dropped by the burnt resistor to low voltage input to the regulator.

This is unlikely as the board is laid out so that this would mean exposed high voltage components leads.


The mains is dropped by the burnt resistor and the low voltage inputs to 1N4001 rectifiers and the filter capacitor to provide a DC input to the regulator.

Either way, Beware, the exposed top of the burnt resistor could be at high voltage.

Please check what each end of that resistor is directly connected to.

1N series rectifiers are rated for 1 amp current so that will also be the rating of the regulator.

Please also check what other components the heater terminals are directly connected to on the board.
Further there should be a rating plate somewhere on the apparatus, stating the wattage voltage and current, or at least two of these. What does the plate say?

Please also check what the three legged device, we are calling a regulator, is connected to as it still might be a thyristor controlled from the IC and supplied by the four rectifiers. ie one method of mains control is to rectify it to DC and drive the load (heater) through a controlled switch from this rectified DC.
In this case there will be no regulator.

Either way the burnt resistor will act as a voltage dropper to provide the DC supply for the IC.
However its value will be determined by whether or not is passes the heater current or just the IC supply current.

A final word of warning.

The multimeter proposed has separate (high) current sockets and an on/off switch.

You are dealing with mains here.
If you leave the meter on a resistance or current range and switch it off and then later switch it back on to try to measure the mains you will instantly destroy the meter, probably with a loud bang and much smoke.

Always return the range setting to a safe position ( a high voltage setting) after use.
Jan30-12, 09:16 AM
Sci Advisor
P: 4,027
The micro data sheet is here:

It uses a supply of 2.3 to 5.5 volts.
Feb4-12, 02:52 PM
P: 8
Ok. I have purchased the multimeter as suggested to prevent damaging someone else's tools, so when it arrives I will measure all the things and post results. Thanks again for all of the help!

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