KrisBlueNZ
Sadly passed away in 2015
Yep, you're spot on there.
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Heres some more questions
1. What wattage are the resistors?
2."Then measure the voltage between points E and F and adjust R6 to get 66 mV" I only need to do this once right?
3."After adjusting R6, wait ten minutes for the heatsink temperature to stabilise, then readjust and repeat" Readjust and repeat tell what?
4. Will any op-amp work just as long as it can supply 10mA and work with 12-36V?
Thanks again for the help
Dan
1. What wattage are the resistors?
2."Then measure the voltage between points E and F and adjust R6 to get 66 mV" I only need to do this once right?
3."After adjusting R6, wait ten minutes for the heatsink temperature to stabilise, then readjust and repeat" Readjust and repeat tell what?
4. Will any op-amp work just as long as it can supply 10mA and work with 12-36V?
Thanks again for the help
Dan
KrisBlueNZ
Sadly passed away in 2015
Good questions.
1. All resistors should be 0.5 watt metal film, 5% tolerance or better (metal film resistors are normally 1% anyway), except for the two 0.33 ohm resistors, which should be 5 watt wirewound 5%.
2,3. No, the recommended way to adjust the quiescent current (measured as the voltage between points E and F) for a class AB power stage is to keep readjusting it until it's close to (within say 5~10% of) the desired current when the heatsink temperature is stable. Wait at least 5 minutes between each adjustment. Each time you adjust it, the heatsink temperature will change, and after 5 minutes you'll see that the current has gone in the other direction from the previous adjustment, but by a smaller amount, so you're getting closer to the correct setting each time.
4. Actually I think you should use one with no crossover distortion in the output stage, such as MC34071. If you have trouble finding the MC34071 in a through-hole package, you may find it easier to get the MC34072, which is just the dual version.
No worries.
1. All resistors should be 0.5 watt metal film, 5% tolerance or better (metal film resistors are normally 1% anyway), except for the two 0.33 ohm resistors, which should be 5 watt wirewound 5%.
2,3. No, the recommended way to adjust the quiescent current (measured as the voltage between points E and F) for a class AB power stage is to keep readjusting it until it's close to (within say 5~10% of) the desired current when the heatsink temperature is stable. Wait at least 5 minutes between each adjustment. Each time you adjust it, the heatsink temperature will change, and after 5 minutes you'll see that the current has gone in the other direction from the previous adjustment, but by a smaller amount, so you're getting closer to the correct setting each time.
4. Actually I think you should use one with no crossover distortion in the output stage, such as MC34071. If you have trouble finding the MC34071 in a through-hole package, you may find it easier to get the MC34072, which is just the dual version.
No worries.
2,3. Can you give me step-by-step instructions on this one? I'm just not getting it.
Thanks
Dan
Thanks
Dan
KrisBlueNZ
Sadly passed away in 2015
OK. Turn R6 fully anticlockwise. Connect a voltmeter between points E and F. Don't connect anything to the output rails. Power up the circuit from 12~18V DC.
Slowly turn R6 clockwise while watching the meter. The reading will increase. When it reaches 66 mV ±10%, stop turning R6. Leave the circuit alone for at least 5 minutes.
When you return, the heatsink will be warm and the meter reading will have dropped. Adjust R6 again for a reading of 66 mV ±10% and leave the circuit for another 5 minutes.
When you return, the meter reading will have changed again. Adjust R6 for 66 mV again and wait.
Each time you adjust R6, you should see that you don't need to turn it as far as last time. In other words, you are getting closer to the correct setting. After a few adjustments, the error should be within ±10% and you're done.
Slowly turn R6 clockwise while watching the meter. The reading will increase. When it reaches 66 mV ±10%, stop turning R6. Leave the circuit alone for at least 5 minutes.
When you return, the heatsink will be warm and the meter reading will have dropped. Adjust R6 again for a reading of 66 mV ±10% and leave the circuit for another 5 minutes.
When you return, the meter reading will have changed again. Adjust R6 for 66 mV again and wait.
Each time you adjust R6, you should see that you don't need to turn it as far as last time. In other words, you are getting closer to the correct setting. After a few adjustments, the error should be within ±10% and you're done.
Ok cool. One last thing "Power up the circuit from 12~18V DC" do you mean input or output voltage?
Thanks
Dan
Thanks
Dan
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