Can I use my spare potentiometer (same as P1) to regulate the voltage from my wall transformer at 14v?
what is the current rating of your potentiometer?
Current rating = sqrt (power rating / pot resistance)
eg.
10k 400mW vol pot has a current rating of:
Imax = sqrt(0.4 / 10000) = 6.3mA
We all know that to get long life from a resistor, that we never run it at average power that approachs the maximum rating.
Half power may give good life
Some would go to quarter power ( = 50% of max current rating) for guaranteed long life.
Current rating = sqrt (power rating / pot resistance)
eg.
10k 400mW vol pot has a current rating of:
Imax = sqrt(0.4 / 10000) = 6.3mA
We all know that to get long life from a resistor, that we never run it at average power that approachs the maximum rating.
Half power may give good life
Some would go to quarter power ( = 50% of max current rating) for guaranteed long life.
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the pot r is variable between 10 ohm and 2 Mohm. I think I should use it at 13 ohm to have 10 V from 14 V. But because the trafo it's 300 mA I need 1 W. Instead the pot is 1/2 watt
2M pot and 1/2W using the formula gives current rating = sqrt(0.5 / 20000000 = 0.50mA
The current rating at maximum power of the 1M pot is 0.5mA !!!!!!!
As suggested earlier you can adopt either 0.35mA (half power), or 0.25mA (quarter power), as your long life current limits.
The whole track and the wiper are rated for 0.7mA.
just using a tiny part of the track does not change the current rating.
Can you actually adjust a 2M pot to 13r?
What resolution is the next step? 1r or 175r?
The current rating at maximum power of the 1M pot is 0.5mA !!!!!!!
As suggested earlier you can adopt either 0.35mA (half power), or 0.25mA (quarter power), as your long life current limits.
The whole track and the wiper are rated for 0.7mA.
just using a tiny part of the track does not change the current rating.
Can you actually adjust a 2M pot to 13r?
What resolution is the next step? 1r or 175r?
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the earlier mA values are stated correctly, but this one should have said "The whole track and the wiper are rated for 0.5mA."
Great.
Can you work with a current limit of <=0.5mA?
sorry, I had a problem with the language. I took the 1/2 watt value from the datasheet. I thought it was related not to the maximum resistance. So I can't use the pot. To calculate the resistor to drop down to 10v I nedd the watt dissipated by the jfet and the voltage...
I've tried measuring the current putting the ammeter in series with drain and +9V and g-s to -9v. The result is 7.7 mA
I have a problem, I have damaged a channel. I tried the fault. I checked the resistors, the thermistors, P1 and P2, I replaced Q1 and Q2, Q3 and Q4. The test with the powercord with lightbulb is ok.
Idss Q1 3.02 mA and Q2 3.00 mA, Q3 VGS 4.04 V, Q4 VGS 3.77 V.
When I move P2, DC output rises quickly and P1 has no effect.
The other channel is ok.
You have any suggestions?
Thanks
Idss Q1 3.02 mA and Q2 3.00 mA, Q3 VGS 4.04 V, Q4 VGS 3.77 V.
When I move P2, DC output rises quickly and P1 has no effect.
The other channel is ok.
You have any suggestions?
Thanks
In post #14234 you stated you had one working channel, compare the resistance values with the non working channel, you should find the fault fairly quickly.
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Have you checked the functionality of P1?
P1 may need a couple of more turns than P2 - Q3 VGS 4.04 V, Q4 VGS 3.77 V.
Measure the value of R2, replace it if it is out of tolerance.
That means the two channels are working identically.
Or, is there a difference you have not revealed?