EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: alank2 on April 03, 2021, 01:30:51 am
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Switching power supply Mean Well RT85B - 12V no load, but when I connect it to the Tandy 1000 I am powering it with, the +12V goes up to 13V! I had the 5V trimmed to exactly 5V, but I had to turn it down to 4.75V to get the +12V to drop to +12.5V. Both are now with +/- 5%. Is it normal that a voltage would go up under load?
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Yes, the reason is that the 5V is the main output and it is regulated by a proper feedback loop (5V goes down under load, power supply detects this and turns itself up to maintain 5V). There is no control loop on the two 12V ouputs - they just go up and down as the 5V is turned up and down. So what happens is that when you load up the 5V the 12V gets turned up as the power supply starts working harder. The solution)if you need one) is to put a separate regulator on the 12V output or to load the 12V to bring it down.
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or... putting a dummy load on the 5V rails .. say about 1A .
Just to keep the regulator busy on point at the expense of
wasting this extra load.
Blame the poor design probably by supervising 5V and not 12V ?
Paul
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This is a matter of multiple output “cross-regulation”. With only one feedback loop, it cannot be entirely avoided but there are certain ways the designers could have made it better. Nonetheless, it doesn’t sound too bad.
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These supplies were designed for digital systems where the +5V needed to be controlled, and the 12 V outputs were not critical. Therefore, the feedback loop sensed only the +5V output.
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Thanks for the explanations everyone!
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I had a similar issue with a Mean Well QP-320F. In my case the 24v line jumped up to 25v.
I put a 6 amp general purpose rectifier diode (P600M) in series with the 24v output.
That drops the voltage within the spec of the load. The diode does get a little toasty but not enough that im worried.
The load is 3-4 amps so its within the safety margin.