I've been testing numerous wall transformers and I've noticed that quite a few DC wall warts can have anywhere from 10-52 volts AC.
I'm assuming a bad diode or two inside and a bad cap?
Thanks, just wondering why.
I'm assuming a bad diode or two inside and a bad cap?
Thanks, just wondering why.
Give us some examples.
Say.. a 12 VAC wall wart may put out 16 vac peak.
Also, if these are switch mode wall warts, your DMM may simply be picking up on the high frequency noise.
Say.. a 12 VAC wall wart may put out 16 vac peak.
Also, if these are switch mode wall warts, your DMM may simply be picking up on the high frequency noise.
most of these are DC 9-16 volts (I have AC wall warts and I check them for DC)
I am using a cheap digital volt meter since I tend to forget to make sure they are not on ohms when checking voltage.
I usually hook the meter up to the wall wart, plug it into a power strip power it on, check the DC voltage, then check for AC voltage, then switch back to DC and turn the power off to see how long it takes to discharge.
Occasionally I see no AC on DC wall warts, a lot will have 10-16 vac and occasionally I'll see a few that are in the 50' or 60 vac.
The last one that had 56vac had 4 diodes and a good size capacitor, I didn't bother checking those since I assumed it was trash.
I have not opened one that has 0 volts AC to see what is in it (voltage regulator) I always thought those transformers were built properly.
I am using a cheap digital volt meter since I tend to forget to make sure they are not on ohms when checking voltage.
I usually hook the meter up to the wall wart, plug it into a power strip power it on, check the DC voltage, then check for AC voltage, then switch back to DC and turn the power off to see how long it takes to discharge.
Occasionally I see no AC on DC wall warts, a lot will have 10-16 vac and occasionally I'll see a few that are in the 50' or 60 vac.
The last one that had 56vac had 4 diodes and a good size capacitor, I didn't bother checking those since I assumed it was trash.
I have not opened one that has 0 volts AC to see what is in it (voltage regulator) I always thought those transformers were built properly.
I'm also guessing a meter error. Do you have something to load down the supply? Maybe a 12V light bulb. Once loaded, it would be interesting to see what AC your meter finds.
I.E. the AC voltage may be there, but the current is so tiny it doesn't matter.
I.E. the AC voltage may be there, but the current is so tiny it doesn't matter.
Put a DC blocking cap in series with the DMM lead.
That will make sure the "AC" function of your DMM will not be "confused" by DC.
Another thought: What does the DMM read for AC on a 9V battery?
That will make sure the "AC" function of your DMM will not be "confused" by DC.
Another thought: What does the DMM read for AC on a 9V battery?
I know that anything older then about 6 years will be the old style iron transformer with a discrete bridge and probably a 1000 uf cap if you are lucky. The caps would always die being is such close proximity to the transformer which where usually always undersized and got extremely hot. Also the discrete diodes often went open circuit as well. So even brand new the dc that these things put out where often 30 or 40 percent over their loaded voltage. Often they were made for a specific piece of equipment with a certain load. The last time I saw an old pure ac style wall wart was for an old dial up modem, using the ac output to create the pos and neg rail internally.
Then came the first series of switch mode wall warts characterized by their low weight. Often though they used very average discrete smps circuits with zero filtering on the ac side. No low esr caps and no feedback. So typical of cheap Chinese made smps today.
Look inside a modern well designed wall wart though and you will see everything surface mount parts apart from the transformer and caps.
I like the modern wall worts for projects, I usually remove them from their case and use them internally in projects with a proper emi filter and upgrade the main ac res cap and the output caps with decent low esr high temp caps.
The thing to note though is that there is always a low value cap between one of the active lines, earth and the zero line on the output. This often allows a tiny trickle of high frequency high voltage emi through to the output of the supply. It's a very low current of course but its one of the reasons I never liked using smps in audio circuits. You can remove that cap of course but you still get emi and rfi from the interwinding capacitance in the transformer unless it has a copper interwinding shield. Which isn't that common.
I find the decent low current wall warts make ideal standby circuit supplies though. The ubiquitous Nokia charger is a good example of this. Their super low standby current easily beats any standard iron core transformer for standby current consumption.
Then came the first series of switch mode wall warts characterized by their low weight. Often though they used very average discrete smps circuits with zero filtering on the ac side. No low esr caps and no feedback. So typical of cheap Chinese made smps today.
Look inside a modern well designed wall wart though and you will see everything surface mount parts apart from the transformer and caps.
I like the modern wall worts for projects, I usually remove them from their case and use them internally in projects with a proper emi filter and upgrade the main ac res cap and the output caps with decent low esr high temp caps.
The thing to note though is that there is always a low value cap between one of the active lines, earth and the zero line on the output. This often allows a tiny trickle of high frequency high voltage emi through to the output of the supply. It's a very low current of course but its one of the reasons I never liked using smps in audio circuits. You can remove that cap of course but you still get emi and rfi from the interwinding capacitance in the transformer unless it has a copper interwinding shield. Which isn't that common.
I find the decent low current wall warts make ideal standby circuit supplies though. The ubiquitous Nokia charger is a good example of this. Their super low standby current easily beats any standard iron core transformer for standby current consumption.
no ac on a battery and even under a load (tried 6 ohms boy did that get hot and a 8k load) there is still AC present on some transformers.
I think Rainwulf explained why. Since I do not need those transformers I'll take a hammer to them and into the recycling bin they go.
I think Rainwulf explained why. Since I do not need those transformers I'll take a hammer to them and into the recycling bin they go.
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