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Old 10th January 2013, 02:38 PM   #8961
Salas is offline Salas  Greece
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You mean there is no drop over the setting resistor near it? Because with Vgs=4V its certainly on.
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Old 10th January 2013, 02:42 PM   #8962
zeeuw is offline zeeuw  Netherlands
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yes, there is a drop over the resistor, but only IxR
U is at drain and source the same

I expect a drop in voltage

or not?
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Old 10th January 2013, 02:55 PM   #8963
Salas is offline Salas  Greece
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You should have Vin-Vout across the CCS Mosfet. Can you set the output at all by turning the voltage setting trimmer?
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Old 11th January 2013, 11:42 AM   #8964
zeeuw is offline zeeuw  Netherlands
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just measured
Vin 44v
Vout 43,5v
50mA (0,5v over the 10ohm resistor)
when turning the pot to 100mA
Vin 43,5v
Vout 42,5v

possible mosfet is blown out?
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Old 11th January 2013, 11:50 AM   #8965
Salas is offline Salas  Greece
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Check Q2,Q3,Q5 for being OK first.
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Old 11th January 2013, 12:01 PM   #8966
zeeuw is offline zeeuw  Netherlands
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checked
are ok
measured also other channel,same reading

so I can set the mA, but no voltage change
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Old 11th January 2013, 12:06 PM   #8967
Salas is offline Salas  Greece
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Do both Mosfets show VGS reading? How much is the voltage drop across the series resistor under the trimmer? Is the trimmer correctly wired as a rheostat?
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Old 11th January 2013, 12:17 PM   #8968
zeeuw is offline zeeuw  Netherlands
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yes, Vgs = 5,37 v
voltage across r = 20,5 v
and is reostat
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Old 11th January 2013, 12:23 PM   #8969
Salas is offline Salas  Greece
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That means there is 6.12mA running in the resistive Vref and normally you could set between 21 to 43.5V. If the trimmer is indeed changing Ohmic value when you turn, then I would be very suspicious of Q2 and Q3 first when having VGS on Q4. Does Q3 show healthy Vbe?
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Old 11th January 2013, 12:43 PM   #8970
zeeuw is offline zeeuw  Netherlands
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the 5,37 was on Q1
on Q4 there is Vgs 2,2v
Vbe = 0,62v
still I do not understand
what should be normal readings on tippoints in the sence circuit?
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