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Old 4th October 2004, 08:23 PM   #1
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Question PSUD2 question

What will happen in practice if I leave the PSU as it is when the psud2 warns me with the above message??

"A current sink has pulled the voltage below zero for more than 5 mains cycles, at time…"




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Old 5th October 2004, 04:18 AM   #2
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I don't think it matters. I've also seen this message from PSUD2, when simulating several stages of RC filtering with current taps as loads. I just ignore it. I think it's caused by the simulator assuming that the load is a constant current even when the voltage is ramping up from zero at initial turn-on, which is not true.
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Old 5th October 2004, 04:56 AM   #3
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If your load actually is a constant current sink, then the supply's voltage will drop to zero as it is powering up, just as the program predicts. If that's not the case, then it may be more accurate to change the load in PSUD2 from a constant current load to a resistive load. It's up to you which type you put at the output of your simulated power supply.
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Old 5th October 2004, 08:56 AM   #4
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You have right. It warns me every time I have current taps for loads.
But what can I do if I have more than one stage? Just to ignore it?

Quote:
I think it's caused by the simulator assuming that the load is a constant current even when the voltage is ramping up from zero at initial turn-on, which is not true.
hmmm,
So, ignore it!


Thank you guys!I was starting to
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Old 5th October 2004, 09:14 AM   #5
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Try "time delaying" your simulation. Instead of running it from 0-1 second, try running from 0.5 to 1.5 or 1 to 2 seconds.
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Old 5th October 2004, 11:06 AM   #6
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I think you'll find that time-delaying the simulation won't make any difference. I typically run my simulations with a reporting delay of 45 to 60 seconds, to allow enough time for all capacitors to charge up but I still get the same message. The simulator always starts at time = 0 seconds.

The fact is, of course, that tubes will not draw full current until the voltage rises to allow them to do so, therefore a constant current tap representing the load is not realistic during "warm up", so
the message is meaningless. However, you might have no alternative with it in PSUD2: you can specify a resistive load at the end of the chain but intermediate loads can only be represented by a current tap.
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Old 5th October 2004, 12:10 PM   #7
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Use the CC step function - start at half current, and after a period, step up to your final load.
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Old 5th October 2004, 02:50 PM   #8
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Pedroskova's idea is the best.
You can also, see if it's stable!Or rings!
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Old 6th October 2004, 12:02 PM   #9
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The current sink message can happen for one of a few things:

You haven't calculated for adequate power trafo current (rare).

You have placed an excessivley large resistance either in an RC stage /or/ a final resistive load, or you've placed too much resistance in a component, like an inductor, or capacitor.

You are simulating a very very low voltage, < 100vDC or a very low current power supply design (bias comes to mind).

You have specified too-high a capacitance on a voltage-doubler design.

I haven't come across one like this, but it could happen and that is you're simulating a tube diode rectifier circuit without specifying that you want to do a soft-start with low first-cap capacitance, < 16uF.

I may be wrong.

And as I remember fdegrove once saying, "Just try it in real life anyway" (Or something along those lines)

Cheers!
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