I return to the subject.
As soon as it is switched on, the voltage across the emitter resistances has a value of approximately 0.7V (well over 0.33V optimal) and the current measured across the fuse holder approximately 1.5A. After about a minute it slowly drops to 0V and 0A. By increasing the quiescent current, the values start to increase again and then go down again. Switching off and on again does not change the behavior.
Do you have any advice for what to check or where to intervene?
Thank you all
As soon as it is switched on, the voltage across the emitter resistances has a value of approximately 0.7V (well over 0.33V optimal) and the current measured across the fuse holder approximately 1.5A. After about a minute it slowly drops to 0V and 0A. By increasing the quiescent current, the values start to increase again and then go down again. Switching off and on again does not change the behavior.
Do you have any advice for what to check or where to intervene?
Thank you all
Do you have the vbe multiplier in contact with the heatsink to sense temperature?
The voltage across the 68 ohm resistors should be a constant, if that is falling the vbe multiplier will follow by giving a reducing forward bias voltage.
Your posts reads as if the current always falls back to zero which suggests a real problem somewhere.
The voltage across the 68 ohm resistors should be a constant, if that is falling the vbe multiplier will follow by giving a reducing forward bias voltage.
Your posts reads as if the current always falls back to zero which suggests a real problem somewhere.
The voltage of about 0.7V is measured on the emitter resistance of a single branch of the amplifier and the current measured at the ends of the fuse holder about 1.5A always on the same branch. repeating the measures on the other side the behavior described does not change.
Help
Help
Yes, it is in close contact with the BDW83 and is screwed on.
There is a problem somewhere. The problem is that I don't have the conditions to identify it🙁
If you have 0.7 volts across a 0.22 ohm then you have a current of over 3 amps flowing. There is no other path available to make up the difference between the calculated 3A and your measured 1.5A
So something is amiss. It is even possible the amplifier is unstable and oscillating which would throw all DC measurements off. You need a scope check to confirm it is stable.
So something is amiss. It is even possible the amplifier is unstable and oscillating which would throw all DC measurements off. You need a scope check to confirm it is stable.
I'll double check everything tomorrow.
If the voltage across the 68 ohm resistors is not a constant, what can it depend on?
If the voltage across the 68 ohm resistors is not a constant, what can it depend on?
That's the big question... it depends on the basic circuit design, in other words how well the designer has done their job.
It is never going to be very thermally stable as far as I can see but I wouldn't expect it to go from 3A (or 1.5A) to zero. That suggests something is very much amiss somewhere.
It is never going to be very thermally stable as far as I can see but I wouldn't expect it to go from 3A (or 1.5A) to zero. That suggests something is very much amiss somewhere.
This is what I do not understand, when switching on the voltage across the resistance of one branch (but also on that of the other) is about 0.7V, but the current is 1.5A and not 3A for one branch, as for the other
Last edited by a moderator:
The amplifier worked divinely and for several years, until I thought about making improvements to the PCB and components, so I would have no doubts about the circuit.
If anything, I would think of some malfunctioning component.
But as I said, I don't have the knowledge to establish it at the table.
The only thing that, as a layman, I could do would be to double check all the components 🙁. I'd like to spare myself if anyone finds an explanation.
Last edited by a moderator:
So that leaves us with the possibility of instability. Ohms law can be relied on providing we can be sure the quantities are DC and so we know the meter reads them correctly.
If there is instability the meter readings are not true and can't be used. That is where a scope check is needed.
You could try as a test reducing the value of the 1k feedback return resistor. Running at higher gain should improve stability margins... its worth a try to see if you still get the same result.
The result may be inconclusive, however if the readings are different then something like instability is influencing the readings.
However if the result is the same then instability hasn't been disproved.
If you have rebuilt using different parts then you need to check carefully all values. That 22pF cap is critical. To big and it could oscillate.
If there is instability the meter readings are not true and can't be used. That is where a scope check is needed.
You could try as a test reducing the value of the 1k feedback return resistor. Running at higher gain should improve stability margins... its worth a try to see if you still get the same result.
The result may be inconclusive, however if the readings are different then something like instability is influencing the readings.
However if the result is the same then instability hasn't been disproved.
If you have rebuilt using different parts then you need to check carefully all values. That 22pF cap is critical. To big and it could oscillate.