power dissipation

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Power dissipation is the voltage drop across a device multiplied times the current flowing through the device. This is true for passive components (like resistors) and active components (like voltage regulators and transistors).


+5 VDC fixed linear regulator. Supply voltage = 12 VDC. Current flow through device = 0.1 A.

Voltage drop accross device will be 12 VDC - 5 VDC = 7 VDC
Dissipation = 7 VDC X 0.1 Amps = 0.7 Watts

I am building the holton n-channel amp and I wish to calculate the power dissipation with supply rails of 84 volt dc rails. Will there be a voltage drop when the supply is loaded? I am guessing that the amp will put out 500-600 watts into 4 ohm. Is the correct so far?

Thank you,


Yes because the power supply and the mains wiring have a finite resistance, there will be a voltage drop @ full load.... how much depends on all these series resistances.

well power into any ohmage can be calculated thusly ....

(VxV)/(Rx2)=RMS Power

Where V = the supply voltage and R = the load resistance of the speaker.

Now assuming no voltage drop the following would be true....

(84x84)/(4x2) = 882W RMS

But this would probably drop to around 650W or maybe lower depending on the factors listed above. Also, you must ensure that all devices are kept within their SOA to minimize device failures and therefore, further derating of output power maybe required to satisfactorily achieve this.
It depends on the thermal resistance of the heatsink/s you'll be using, also if you be using mica or similar insulators, the inductive natural of the load etc etc etc. also may i suggest if you have not already purchased those fets that you seriously consider using IRFP244/254/264 instead as they have a bit more headroom on the supply voltage....

Since you mentioned Anthony Holton's N-channel amplifier, I wish to state the following:

I constructed the N-channel amp with 3 pairs of IRFP250s, all matched very closely. I used an on-load voltage of + - 84, set the quiescent as suggested by the designer; heatsinks were over-rated and there was a fan for additional cooling. I built four PCBs and discovered the following:

1. The pre-regulator transistor, 2SC3298(B) gets too hot for the touch and a small heatsink needs to be attached to keep it under limits.

2. The voltage drop across the output resistors are not at all equal despite the matching of MOSFETs for each bank. I used 3 x 1ohms 1watt resistors for each output device.

3. When there is a slight increase in the AC voltage, the output stage fails.

4. I isolated the input stage with Fast Recovery Diodes and had separate decoupling for input and output stages.

5. More numbers of output devices PROBABLY need to be paralleled to keep the dissipation in each of the output devices within their limits as well as to increase output power.

6. I have tried voltages as low as + - 30 volts and have one module working off about 50volt rungs without any problem.

(P.S. I have 3 PCBs which have been damaged due to higher voltage operation.)

You could search the Solid State Forum here to view a picture of the populated PCB which I have posted a week ago.


The devices you have suggested are better rated that the ones I used, but note the effect of paralleled device capacitance which might make the high frequency roll off much quicker.
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