P3- P3A idle/ temperature question

[sakis]

...... P3-P3A have been extended disscused here i think but there a thing that troubles me .....

what happens in a P3 -P3A if the operating ( room ) temperature varyies like 20 degrees ?????

summer time in my office we have something like 40 degress and then winter time we have something like 18 .....

what happens then ????

[AndrewT]

The amplifier will run at deltaT above Ta (ambient temperature).
As Ta varies so does the amplifier temperature.
If the temperature compensation is set exactly to maintain the same bias at all operating temperatures then delta T is always the same.

If the amp is under compensated then bias increases as temp rises and deltaT increases with the increased dissipation.

If the amp is over compensated the bias reduces as temp rises and deltaT falls as temperature rises.

A slight overcompensation helps keep the output heatsink at nearer a constant temperature as ambient varies. The rest of the components will still follow Ta since they have no temp compensation.

[Jan Dupont]

Your P3-P3A will get 20 degrees hotter in the summer

However there are other aspects. The extra 20 degrees could meen that the sinks could be to small to handle the power....
And you could risk some thermal runaway...

My best suggestion is to heat up the beast while measuring the the temperature and the bias.

[Jan Dupont]

You beat me, Andrew

[sakis]

that vbe multiplier in P3 P3A is not located on the heat sink.....

so i think i have to measure with both temperatures to see what is going on ....

any other comments ????

[fotios]

Originally posted by sakis
that vbe multiplier in P3 P3A is not located on the heat sink.....

so i think i have to measure with both temperatures to see what is going on ....

any other comments ????

Hi Sakis and happy new year

Have you checked the temp of heatsink with a digital thermometer?
To find the right temp which developed from the iddle current, you must:
1) Power up the amp from cold state.
2) Before this you must short the input to gnd, and left the output open.
3) Wait for 1/2 to 3/4 of an hour, and now check the temp of heatsinks.
I will give you an example from my last AB class amplifier:
Iddle current=50mA per output device X 4 devices=200mA total per channel.
Temp of each heatsink=45 deg C after one hour of amplifier preheating according to the above reffered setup.
This measure has been done with 22 deg C ambient temp.

Regs
Fotios

[AndrewT]

and when the ambient temperature goes to 32degC the amplifier will run 10Cdegrees hotter.
If the bias remains at 200mA, the heatsink will now be at 55degC and the device temperatures will also be 10Cdegrees hotter.

It would be nice if the bias had reduced to 180mA at the high ambient temperature.
The heatsink would now be a degree or two cooler than the 55degC predicted. The transistor junctions would be more than a degree or two cooler due to the thermal resistance between junction and sink and the reduced heat flow (less power dissipation).

[fotios]

Hi Andrew and happy new year

Your remarks are correct.
My example is some incomplete, because i was reffered in the iddle current adjust procedure. An indicating point, it is that when my amp starts from cold, its iddle current is about 20mA per device. During the pass of time, it is increased at 50mA per device, but from this point and after it happens the Vbe multiplier compensation take-off. It keeps stable the Iqs in 50mA with little deviation from 47,5 to 52,5 mA. I have checked it for 1 hour after the first hour of heating. Also by driving the amp with 20-20000 Hz sweep under 8Ù dummy in half power, the temp of heatsink increased at 65 deg C, stable. Of course there are cases such as an ambient temp>35 deg C, where the temp of heatsink (consequently the junction temp of output devices) may exceed the 80 deg C. In such a case, a comparator based on an LM35 it cares to open the contacts of output relay disconnecting thus the load. I consider that a such thermal protection (either electronic or passive with thermal breaker) it is indispensable in each amplifier to be safe.

Regs
Fotios