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Is there anything wrong with this PS design?
Is there anything wrong with this PS design?
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Old 2nd January 2018, 08:17 PM   #31
MarcelvdG is offline MarcelvdG  Netherlands
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I don't see any local supply decoupling at all for the DC op-amps. That might make them unstable, especially when the wiring to the supply is long. The LM317s should be fine, though.
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Old 3rd January 2018, 12:27 PM   #32
simonra is offline simonra  United Kingdom
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Quote:
Originally Posted by MarcelvdG View Post
I don't see any local supply decoupling at all for the DC op-amps.
I can't believe I missed that! Thanks... would 100nF X7R caps be a sensible starting point to decouple those amps without causing problems with the PS regulators?
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Old 3rd January 2018, 01:14 PM   #33
MarcelvdG is offline MarcelvdG  Netherlands
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Yes, that sounds very sensible.
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Old 3rd January 2018, 02:13 PM   #34
dotneck335 is offline dotneck335  United States
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In my research, I found that there is a pretty big difference between BRANDS of LM317 type three-terminal regulators. For example, in the datasheet specifications, the Linear Technology LT317 has 16 db better ripple rejection and 10 db less noise than the T.I. part. The LT317, of course is ~ $2 each compared to the 50 T.I. LM317; but for most projects only requiring a few, it's money well spent, IMNHO.
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Old 10th January 2018, 06:46 PM   #35
simonra is offline simonra  United Kingdom
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I'm a bit stumped... I've got some new boards made up (schematics & layout below) and I'm getting +-28V at the output with 1k & 12k resistors I was expecting 16.25V. I've double checked the orientation of all the polar components, the only thing I can think is that I damaged the regulators somehow - or I've made some other dumb mistake with the circuit, but I've been through it a few times now and can't see what the problem is.
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Old 10th January 2018, 07:09 PM   #36
MarcelvdG is offline MarcelvdG  Netherlands
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Minimum load current? If I remember it correctly, an LM317 requires 10 mA of load current to maintain regulation.

Last edited by MarcelvdG; 10th January 2018 at 07:14 PM.
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Old 10th January 2018, 07:13 PM   #37
simonra is offline simonra  United Kingdom
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Thanks, I hope that's the answer... I can't check right now but I just probed the outputs with no load.
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Old 11th January 2018, 07:27 AM   #38
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by MarcelvdG View Post
Minimum load current? If I remember it correctly, an LM317 requires 10 mA of load current to maintain regulation.
Some version of the lm317 work with only 5mA of output current.
That allows R3 to be 240r. If 10mA is needed then R3 becomes 120r
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Last edited by AndrewT; 11th January 2018 at 07:30 AM.
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Old 11th January 2018, 07:57 AM   #39
simonra is offline simonra  United Kingdom
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Thanks, I think I understand now... the minimum load current is achieved through R3 + R1, so if I make those 120R and 1400R respectively the output should be 15.8V with creating a load current of 10.4mA.
I guess to correct the mistake on the board I've already constructed I can just add 1k8 resistors from the +&- outputs to ground in parallel with the 13k already provided by R3 & R1.
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Old 11th January 2018, 08:51 AM   #40
AndrewT is offline AndrewT  Scotland
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Adding a parallel resistor avoids the need to risk damaging the PCB by removing an incorrect resistor.
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