Wall-warts generally have minimal filter caps - just enough to "get by" the specifications.

The other components are also minimal. If you add on a large cap, you may blow the bridge, which will turn the wall wart into a doorstop.

If you're going to add another filter cap to a wall wart, make sure you limit the current going into the cap to below the rating of the wall wart.

 

Wall-warts generally have minimal filter caps - just enough to "get by" the specifications.

The other components are also minimal. If you add on a large cap, you may blow the bridge, which will turn the wall wart into a doorstop.

If you're going to add another filter cap to a wall wart, make sure you limit the current going into the cap to below the rating of the wall wart.

If I did limit it. I would just need a current limiting resistor? Would it need to be switched out after the supply was up and running?

 

Been doing some reading. I guess I could use a thermistor for this?

 

Did you think of a light bulb?

There's a brief surge current when they first turn on, but they are really quite remarkable as far as current limiters. The more current passes through them, the higher their resistance.

 

Did you think of a light bulb?

In one of the most famous amateur electronics texts of all time

Foundations of Wireless and Electronics

by Scroggie


In the section on He describes how to set up an electronics workshop and how to use light bulbs as safety devices for connecting equiment to the mains.

The light bulbs need to be the old fashioned tungsten types though. Modern energy saving varieties will not do.

 

Would that not also limit the current on my supply? But of course my wall wart (depending on which one I select) can put out 4.5A. So I guess I can fuse it after the bulb with a 1 amp fuse (limits of my supply).

True?

 

This discussion reminded me of HP's first product. Bill Hewlett had designed an oscillator that had a light bulb in the circuit. In the garage they built the HP 200A audio oscillator, after Disney bought a few the business was born. Check out the HP web site under"Virtual Museum" for some interesting details.

 

Bill another follow up question on this post. If you already had the filter caps, would you add some larger caps right on the output of the wall wart? Or just go as is?

Depends on the ripple, but you can't go wrong adding more capacitance. If you go that route I'd also add a coil and make a PI filter. A good coil will help limit the surge current too.

 

Wall-warts generally have minimal filter caps - just enough to "get by" the specifications.

The other components are also minimal. If you add on a large cap, you may blow the bridge, which will turn the wall wart into a doorstop.

If you're going to add another filter cap to a wall wart, make sure you limit the current going into the cap to below the rating of the wall wart.

I don't think this is much of a problem with most wall warts. One can get a quick feel for whether or not it will be by measuring the DC resistance of the primary, if it's not a "switcher". If there's just an ordinary transformer inside, the primary resistance will be a strong determinant of the maximum short circuit current, and therefore also the maximum surge current. In reality, the resistance of the secondary will be involved, too, so the short circuit current will be even lower than just the primary resistance would indicate.

For example, my highest power ordinary transformer wall wart has a primary resistance of 83 ohms, so with the peak grid voltage of 170 volts, the maximum half cycle surge would be less than 170/83 ~ 2 amps. This wall wart has an output voltage of about 12 volts, so the turns ratio is perhaps 1:10+, so the secondary current will be multiplied by 10. The rectifiers in these wall warts that I've opened up are 1N400X types, with a surge capability of 30 amps, so a 20 amp surge, which is unlikely, won't hurt anything. The lower power units will have even less of a problem.

So, measure the primary resistance (the resistance at the prongs).

Depends on the ripple, but you can't go wrong adding more capacitance. If you go that route I'd also add a coil and make a PI filter. A good coil will help limit the surge current too.

Surge current is definitely not a problem with the "switcher" wall warts that I checked out.

I measured the primary current and output voltage at turn-on with and without an extra 1200 μF on the output.

I've attached a couple of images showing primary current (orange) and output voltage (purple).

The first image is without added capacitance. The first orange spike is where I connected the grid voltage, and there is about a 1 second delay before the unit turns on (soft start?).

The second image is the same circumstances but with an additional 1200 μF connected to the output. I see no surge in primary current; just the same startup delay.