CurtZHP

Strange issue with a bipolar power supply
« on: May 01, 2020, 03:43:03 PM »
I've got this bipolar power supply that uses adjustable voltage regulators.  As built, it's supposed to put out +/- 18VDC as well as 48VDC phantom power.

I've built this same supply before and used it successfully.  This time around, something's not right.  Or maybe I'm just not testing it properly.

The phantom section is just fine.  It puts out about 52VDC.  The bipolar rails are screwed up.  Each regulator (LM317 and LM337) sees about +/- 38VDC on their input pins.  On the output pins, I get about 35VDC on the LM317, and I get about -10VDC on the LM337!

Thinking something was up with those chips, I replaced both of them with brand new ones.  Still the same problem.  I double-checked the adjustment resistors to make sure I had them in the right places.  I double-checked the orientation of the protection diodes, D103 and D104. 

The LED's and their respective resistors are not installed yet.  Made sure none of my caps were in backwards, even though I didn't get any fireworks.  I figure BR1 is fine since each regulator is getting the identical +/- input voltage.

I removed C116 and C117.  Here's where it gets interesting.  Once I did that, LM337 started giving me -22VDC output.  That's more like it.  But LM317 still shows over 30VDC on the output.  That's a head-scratcher.

Mind you, this supply is not currently connected to anything.  Being unloaded, I would expect to see voltages a little lower or higher than specified, but not this far off!

Now I'm starting to worry about the last one I built!  Unfortunately, I no longer have it to compare to.  (That gadget's now in someone else's studio...)

Electrons don't read schematics.


Audio1Man

Re: Strange issue with a bipolar power supply
« Reply #1 on: May 01, 2020, 06:11:49 PM »
C117 is BACKWARDS. + should be to common and - to ref terminal.
Duke

abbey road d enfer

Re: Strange issue with a bipolar power supply
« Reply #2 on: May 01, 2020, 06:33:25 PM »
The phantom section is just fine.  It puts out about 52VDC.  The bipolar rails are screwed up.  Each regulator (LM317 and LM337) sees about +/- 38VDC on their input pins.  On the output pins, I get about 35VDC on the LM317, and I get about -10VDC on the LM337!
Th eresistor values are very wrong! The LM317/337 require 5mA to regulate correctly. If you had connected the LED's, maybe you wouldn't hace noticed this issue.
The datasheet recommends 240 ohm resistor between Out and Adj, which draws 5ma so the circuit is somewhat loaded.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

CurtZHP

Re: Strange issue with a bipolar power supply
« Reply #3 on: May 01, 2020, 07:05:36 PM »
C117 is BACKWARDS. + should be to common and - to ref terminal.
Duke


No wonder it worked better when I took that out!

 :-[
Electrons don't read schematics.

CurtZHP

Re: Strange issue with a bipolar power supply
« Reply #4 on: May 01, 2020, 07:07:19 PM »
Th eresistor values are very wrong! The LM317/337 require 5mA to regulate correctly. If you had connected the LED's, maybe you wouldn't hace noticed this issue.
The datasheet recommends 240 ohm resistor between Out and Adj, which draws 5ma so the circuit is somewhat loaded.

240 seems like an oddball value.  Would 220 be close enough for rock-and-roll?
Electrons don't read schematics.

abbey road d enfer

Re: Strange issue with a bipolar power supply
« Reply #5 on: May 01, 2020, 07:24:20 PM »
240 seems like an oddball value.  Would 220 be close enough for rock-and-roll?
Actually I've often used somewhat higher values, like 470r. As long as there is a significant load, it works well enough.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

CurtZHP

Re: Strange issue with a bipolar power supply
« Reply #6 on: May 01, 2020, 07:39:49 PM »
I also noticed that, for the LM337, the recommended R1 value is 120Ohms.

But, like you suggested, as long as I'm in the neighborhood....

(and 3.2K is definitely NOT in the neighborhood!)
Electrons don't read schematics.

CurtZHP

Re: Strange issue with a bipolar power supply
« Reply #7 on: May 01, 2020, 08:31:11 PM »
Went digging through the parts bins and managed to find some resistors whose values were at least in the same ZIP code as what the spec sheet called for.  Swapped them out and retested.  +/- 19.2VDC.  That's unloaded, mind you; so I think I'm good.

Thanks again, guys!  I shudder to think what I would do without you.
Electrons don't read schematics.

Whoops

Re: Strange issue with a bipolar power supply
« Reply #8 on: May 14, 2020, 01:39:57 AM »
Reference your schematic to the JLM PSU schematic it uses the same regulators and it's really well done and prooved


 

Related Topics

  Subject / Started by Replies Last post
3 Replies
1649 Views
Last post July 04, 2005, 03:54:28 PM
by bcarso
3 Replies
1386 Views
Last post January 10, 2007, 09:33:48 PM
by Flatpicker
13 Replies
7283 Views
Last post March 11, 2009, 12:23:06 PM
by nielsk
9 Replies
440 Views
Last post April 05, 2020, 12:35:10 PM
by jdurango