Resistor getting very hot

[DerSamoaner]

ZD3 was broken and R40 became very hot.

After replacing ZD3, ZD4 R40 and R39 i measured the temperature with heat cam. R40 and R39 become very fast hot. About 120°C(250F).

Is that normal ?

I think the original ones were 5 watt resistors, length: 0.94 inch, diameter: 0.29 inch. 270Ohm.

The amp works fine, but i tested only for 20 min.

 

[Perry Babin]

Larger resistors will run somewhat cooler due to having more surface area.

Look up the datasheet for the resistors you used to see if they are on the safe side of the derating curve.

 

[DerSamoaner]

I will probably have to test the amp over a longer period of time.

But 7W or 10W resistors are a bit too big.

I'll see which size still fits

 

[JMFahey]

120C is quite comfortable for a power resistor.

Inn any case, besides thermal cam results, please post voltage at each end of each resistors.

Also, which ones ended staying on thyen PCB?

The greyish printed value ones or the whitish 3 colour band ones?

By the way, the top one does not look 270 ohm by any means.

But we need measured voltage values to calculate actual dissipation.

 

[DerSamoaner]

The color has changed due to the enormous heat. It is still a 270 ohm resistor.

Across the resistor are 20V.

What voltages do you still need?

 

[Perry Babin]

That's only 1.5w. The resistor is derated to about 65% of rated at 120C.

 

[bansuri]

Isn't there a fan in that enclosure?

 

[DerSamoaner]

There is a fan, yes. But the fan is not permanently on. Probably temperature controlled.

 

[bansuri]

next to the NTC on the heat sink there is a 0805 resistor marked 222 somehow strange position, maybe the fan should be permanently slowly on and boost when the the temp is rising.

 

[Perry Babin]

How hot are the Zeners running?

Does the fan operate on output level as well as thermal?

 

[JMFahey]

The color has changed due to the enormous heat. It is still a 270 ohm resistor.

Across the resistor are 20V.

What voltages do you still need?

Thanks, that´s enough.
As Perry said, it´s dissipating WELL below ratings.

Not sure how the old one could have heated so much.
What´s the supply voltage in that amplifier?
Just to reach 5W, let alone surpass it, we need 36V across that resistor.
That´s why I asked for voltage at each end

 

[Perry Babin]

If I'm not mistaken, the 15v Zener shorted and that caused the voltage across the resistor to push it to overheat.

 

[DerSamoaner]

I think Perry is right. The amplifier should be new and only had the error with the broken diode.

The "222" resistor was probably added after the fact.

I will heat up the ntc and see when the fan starts running.

I will now test the amp over several hours.

 

[DerSamoaner]

When I heat the ntc, the fan starts immediately.

2 hours room volume without problems.

Bench test to clipping point passed.

Dynamic test (5-20A current) until the fan starts up also worked.

Probably normal that the resistors get so hot.