I have a number of surplus "wall warts" from various electronic stuff that are supposed to be (they say on them) 12VDC. When I measrure the voltage on my VOM, they do indeed measure 12VDC, but also 24VAC on the AC scale. Why is this? Are they 1/2 wave rectifiers? Should I warm up my old Heathkit VTVM? Can I safely use these for circuits that call for a 12VDC input without damage to the circuit? Thanks in advance,
Tom Mann
Most cheap wal-warts are unregulated. I'm kind of surprised you see exactly 12v with no load - usually you get readings as high as 17v on the DC scale when no other load is connected. Your 24v reading on AC is probably the ripple - there is probably no filter capacitor inside the wal-wart.
This is fine for circuits that don't need fine regulation. To smooth the output you can add a 2200-4700uF electrolytic capacitor (watch the polarity). If you need regulated power, you can add a 78xx regulator for the desired voltage.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
As was said, usually unregulated with a electrolytic capacitor that is not large enough value to filter out the pulsations. All the wall warts I have stripped had full wave bridge rectifiers.
I would go with a 35 dc capacitors as voltage on the capacitor can rise above normal value if no load on the wall wart. Don't remember the math right now. Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.