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Old 26th February 2011, 05:58 PM   #1
phrarod is offline phrarod  United States
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Default Need Help. PSU not voltage down from design

I just did the maiden switch on of my pre-amp project that I've been working on for some time. I had it working fine on a board and a regulated supply.

Now its in chassis with dual mono choke loaded non-regulated supplies. I did all the modeling in PSU designer II prior to ordering the transformers. I included the model here along with the schematic.

One mistake I made in the model was just not showing the bleeder resistor.

Otherwise, I'm having 2 issues. Overall its down 29VDC. Normally, I use C1 to adjust voltage when I was using the old transformers. Now C1 makes no different whether its the value in the PSU model or all the way up to .1uF.

Second, the trans is supposed to be 200VAC output and its measuring 390VAC. Since its almost double then something is off. It was modeled as a full wave but it seems like something is wrong. I am going CT (5) to ground.
Attached Images
File Type: jpg PSU-only.jpg (27.7 KB, 142 views)
File Type: jpeg PS schematic.jpeg (128.4 KB, 146 views)
File Type: jpg power-trans-schematic.jpg (71.5 KB, 145 views)
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Old 26th February 2011, 06:28 PM   #2
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Hi there,

Quote:
Originally Posted by phrarod View Post
Second, the trans is supposed to be 200VAC output and its measuring 390VAC. Since its almost double then something is off.
the transformer datasheet you show says 400V CT, so 390VAC sounds ok to me. Don't be confused by the 200VAC shown in the PSUD screenshot. For full-wave rectification, the value shown in PSUD is for one half of the winding - that means if PSUD says 200VAC in full-wave mode, the transformer needs to be 200-0-200 or, written differently, 400VAC CT.

Greetings,
Andreas
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Old 26th February 2011, 06:34 PM   #3
phrarod is offline phrarod  United States
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Thanks Andreas!

Well the good news is it makes music and although my PSU VAC is up significantly from when it was on a board (.045 now .145) output VAC is .009 whereas on the board I had it at .003.

But there doesn't appear to be excess destructive hum. I'm sure I can work on that but I need to try and get it up to the specs. Any ideas why changing C1 no longer affects voltage? I need to raise it 29VDC at the tube plate.
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Old 26th February 2011, 06:39 PM   #4
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Hi,

the winding resistance of the trafo should equal the resistance of the secondary winding PLUS the reflected resistance of the primary winding. Did you take that into account?

Other things could be slightly off, too; for example the inductance of an iron-core choke is not constant but is a function of the average current in the choke...

Kenneth
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Old 26th February 2011, 06:56 PM   #5
phrarod is offline phrarod  United States
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I had Bud at O'netics do all the calculations. What I don't understand presently is why the circuit won't respond to C1 as it did in the past and in the PSU model. Previously putting in .045 or .1 made a bid difference in voltage. Now it makes zero difference.
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Old 26th February 2011, 07:58 PM   #6
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I think a drawing of your previous "board" setup would help in understanding what is different now and why...

Greetings,
Andreas
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Old 26th February 2011, 08:20 PM   #7
kevinkr is offline kevinkr  United States
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How does the choke differ from what you had before?

Schematics and other information showing the prototype and the finished design would also be helpful.

Are you drawing more or less plate current than before. Choke not regulating? (Since this is essentially a choke input supply)
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Old 26th February 2011, 08:28 PM   #8
Knarf is offline Knarf  Denmark
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There is something odd about your design. Are you attempting to make a choke input filter? 100nF is nothing when used as the first capacitor in a cap input filter. Might as well not be there. I just ran a quick and dirty SPICE simulation, and at that low value for the input cap, the overall output voltage behaves very much like a choke input filter. With a choke input filter you would also be fighting the need for the minimum, critical inductance of the choke, which might lead to further strangeness etc.

Try increasing the first cap to 10-30uF and check again.

Edit: If this is supposed to be a choke input filter, then the expected output voltage would be (transformer secondary voltage RMS times 0.9) minus (load current times total loss resistance (transformer, rectifier, choke)). With Vrms equal to 195V (390/2), then being 29V too low does sound quite plausible. 195 * 0.9 = 175V. The low secondary voltage could be because your mains voltage is a bit low.

- Frank.
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Last edited by Knarf; 26th February 2011 at 08:35 PM.
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Old 26th February 2011, 08:30 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Knarf View Post
<snip>

Try increasing the first cap to 10-30uF and check again.

- Frank.
As Frank says in his post, but even 1uF should make a small, but discernible difference. His choke concerns more eloquently echo the concern I expressed in my last post.
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Old 26th February 2011, 08:32 PM   #10
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Hi Frank,

I agree with all you said above, except

Quote:
Originally Posted by Knarf View Post
Try increasing the first cap to 10-30uF and check again.
This will essentially turn the choke-input supply to a cap input supply and rise the output voltage significantly, approximately from 0.9 U_in to 1.4 U_in. This will probably be more than wished for...

Edit: The first post says it is going to be choke-loaded (see first diagram).

Greetings,
Andreas

Last edited by Rundmaus; 26th February 2011 at 08:34 PM. Reason: Added last sentence.
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