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Old 30th June 2013, 12:35 AM   #3951
eboz is online now eboz  United States
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Quote:
Originally Posted by Salas View Post
Its normal to drift with temp due to the semis tempco in the Vref but not wildly. 2-3V. Leave it warm up for 15mins and then set for Vo. If it continues drifting in the tens of Volt then something's not right.
Yes.. something is not right, because it eventually drifts down -30 or more. Any advice on what to replace first?
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Old 30th June 2013, 12:37 AM   #3952
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The JFET, then the LED if it continues so.
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Old 30th June 2013, 12:38 AM   #3953
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P.S. Oscillation can do weird things like that also.
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Old 30th June 2013, 12:41 AM   #3954
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Does it behave the same both on the dummy load and on your electronics load?
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Old 30th June 2013, 12:54 AM   #3955
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Quote:
Originally Posted by Salas View Post
Does it behave the same both on the dummy load and on your electronics load?
Yes it does drop as it warms on the dummy load as well.i thought it might be the sensing since it was shorted on the dummy, so I wired it up for the real load which has twisted pairs for output leads. But they still had the same behavior, unfortunately.
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Old 30th June 2013, 12:58 AM   #3956
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So its not some bad interaction with the load. Proceed with the parts exchanging. Is the set current stable? What is the Vout you set for?
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Old 30th June 2013, 01:04 AM   #3957
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Quote:
Originally Posted by Salas View Post
So its not some bad interaction with the load. Proceed with the parts exchanging. Is the set current stable? What is the Vout you set for?
Current appears stable at 35mv. Input voltage is 340 and output should be about 305. 22ma excess current.
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Old 30th June 2013, 01:14 AM   #3958
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Try raise the current first in little steps. Maybe the excess is not enough and drags down the Vo setting. Just to exclude that possibility before changing parts anyway.
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Old 30th June 2013, 01:16 AM   #3959
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Quote:
Originally Posted by Salas View Post
Try raise the current first in little steps. Maybe the excess is not enough and drags down the Vo setting. Just to exclude that possibility before changing parts anyway.
Thanks.I'll give that a shot later tonight.
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Old 30th June 2013, 02:38 PM   #3960
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I would suggest you "test" the the CCS for reasonably constant current from cold to warmed up.

Attach a low voltage transformer as your power input. Attach a low value resistor across the output.
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