I like to use ordinary 12VDC wall warts for most of my projects. Such as this one, which is capable of delivering up to 2.5 amps:
What I like to do is to open it, extract its main board, and incorporate it into my designs. That way things not only look more elegant, but one ends up without having to plug the cumbersome thing into a power outlet, avoiding the nuisance of its getting in the way of neighboring receptacles.
This thing is, however, quite prone to transients. So what I normally do is add an inductor at its output, and a big fat cap, as shown in the following schematic.
When I started adding said components to my circuits, glitches like sporadic resets and unstable input signals from encoders and sensors almost went away. So I decided to add yet another 4,700 µF cap in parallel to the previous one to see if things improved even further. And sure enough, they did. (As a side note, the average circuit draws about 3/4 of an amp at most).
Well, a few days ago, I used this same circuit in two boards that were simultaneously connected to a single 2.5A wall wart. I figured things would be perfectly safe, since together they wouldn't be drawing more than 1.5A at most. I was wrong.
When I powered up the wall wart, both circuits turned on for a couple of seconds, and then went back off again... then on again... off again... etc... Strangely enough, if I connected one board first, waited a couple of seconds, and then connected the second one, things would work just fine!
After a little thinking, here's what I concluded. Each board has one 50 µH inductors and two 4,700 µF caps in it. So the wall wart was trying to charge four fat caps totaling 18,800 µF when it was powered up ... My guess is that the wall wart has overcurrent protection in it, and the poor thing was struggling too much to charge those caps up during startup. And I confirmed this when I powered the circuits from a bench top power supply capable of delivering up to 30 amps without a hitch.
And now for my question: What would be the best way to have the wall wart charge those caps during startup without affecting the overall output voltage?
What I like to do is to open it, extract its main board, and incorporate it into my designs. That way things not only look more elegant, but one ends up without having to plug the cumbersome thing into a power outlet, avoiding the nuisance of its getting in the way of neighboring receptacles.
This thing is, however, quite prone to transients. So what I normally do is add an inductor at its output, and a big fat cap, as shown in the following schematic.
When I started adding said components to my circuits, glitches like sporadic resets and unstable input signals from encoders and sensors almost went away. So I decided to add yet another 4,700 µF cap in parallel to the previous one to see if things improved even further. And sure enough, they did. (As a side note, the average circuit draws about 3/4 of an amp at most).
Well, a few days ago, I used this same circuit in two boards that were simultaneously connected to a single 2.5A wall wart. I figured things would be perfectly safe, since together they wouldn't be drawing more than 1.5A at most. I was wrong.
When I powered up the wall wart, both circuits turned on for a couple of seconds, and then went back off again... then on again... off again... etc... Strangely enough, if I connected one board first, waited a couple of seconds, and then connected the second one, things would work just fine!
After a little thinking, here's what I concluded. Each board has one 50 µH inductors and two 4,700 µF caps in it. So the wall wart was trying to charge four fat caps totaling 18,800 µF when it was powered up ... My guess is that the wall wart has overcurrent protection in it, and the poor thing was struggling too much to charge those caps up during startup. And I confirmed this when I powered the circuits from a bench top power supply capable of delivering up to 30 amps without a hitch.
And now for my question: What would be the best way to have the wall wart charge those caps during startup without affecting the overall output voltage?
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