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Old 20th November 2010, 07:54 PM   #1
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Default A couple of PSD questions

I was running some PSDII sims the other day and I have a couple of questions. I measured the unloaded output and the cold DC resistance of the PT. secondary and used those numbers for the simulation. The DCR seemed pretty low to me but I ran with it anyway. Even with a very small first cap PSDII was giving me an over current warning so I looked at the ratings on the transformer and the difference in output voltage that I measured open circuit minus the rated voltage at 200mA divided by the 200mA current indicated that the resistance should be much higher.

Is the impedance of the secondary under load going to be larger than the measured DCR cold?

I just realized I may have been calculating the resistance of the entire winding instead of one phase but I will need to go dig up my notes.

The second question is how to interpret over current warning. looking at the simulation results from time zero with no delay it looks like even with large resistance in series with the windings a very large current will flow in the first few mS. If the .75A limit on the 5AR4 is exceeded for just a mS or two is it damaging to the rectifier? Would an inrush limiter (thermistor) on the primary help to protect the rectifier?

How reliable are PSDII current results?
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Old 20th November 2010, 09:59 PM   #2
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Use the DC resistance for one side of the secondary winding.

Did you input the unloaded secondary voltage & DC resistance in the options area of the transformer drop down window by clicking on the dots to the right of the voltage input fields and let PSUD calcualte/estimate the voltage and DCR? Or did you input them as the the transformer voltage and DCR?

You can also turn on the "soft start" option in PSUDII and see if you still get the warning.

What is the rated current and DC resistance of your transformer? How much current are you drawing in the circuit?

Last edited by boywonder; 20th November 2010 at 10:02 PM.
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Old 21st November 2010, 06:43 PM   #3
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I put the unloaded voltage and measured DCR directly into the first window (did not click on the ....). I bet that was the wrong thing to do huh?

Tranny is rated at 760 CT at 200mA. I put the unloaded tranny on a variac and ran it up to 150 output and then measured the input voltage to find the ration and then multiplied by the ration to get the output at 120V in as I didn't want to have 800 volt leads hanging off the work bench. I ended up with 409-0-409 as a result. IIRC the DCR of the entire secondary was 65 ohms.
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Old 23rd November 2010, 03:02 AM   #4
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How much current does the PS need to supply? Is it close to the rated current of the transformer? What CLC or CRC values are you using?
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Old 23rd November 2010, 05:30 PM   #5
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Your "over current" warning is a function of your model, not a problem with the circuit. I assume you have several tubes down stream of the 5AR4. Those tubes will take several seconds to warm up so the current will ramp up slowly, not all burst into life in the first milliseconds. Use the stepped load feature for your current sink. You can time the actual warm up time for your tubes or just add a second or so in the model. So for, say, a pair of output tubes drawing 100mA, make the initial current draw 5mA and then after 1 second make the additional current draw 95mA.
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Old 23rd November 2010, 10:21 PM   #6
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Quote:
Originally Posted by Palustris View Post
Your "over current" warning is a function of your model, not a problem with the circuit. I assume you have several tubes down stream of the 5AR4. Those tubes will take several seconds to warm up so the current will ramp up slowly, not all burst into life in the first milliseconds. Use the stepped load feature for your current sink. You can time the actual warm up time for your tubes or just add a second or so in the model. So for, say, a pair of output tubes drawing 100mA, make the initial current draw 5mA and then after 1 second make the additional current draw 95mA.
Yes,the slow warm-up time of tube rectifiers will limit the initial (inrush) current peak to a much lower value. PSUD II has a clever feature to simulate this:
Just open the option menu and select soft start . Without doing this you will get repeated warning messages because the max.peak current of the tube will be exceeded with any but small input filter capacitors. Of course this is only valid for tube rectifiers,for simulations using solid-state rectifiers the soft start option must be allways turned OFF.
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Old 23rd November 2010, 11:05 PM   #7
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The circuit is...

PT:Rect:40uf:4H@60ohm:40uf:4H@60ohm:80uf:150mA sink:2.2kohm:30uf:10mA sink:2.2kohm:30uf:10mA sink So I am going to be loading the tranny close to it's rating but not too close. Circuit is a pair of KT-88s with 6N1P driver and a hand full of other preamp tubes.
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