Switch mystery

As you can see in the picture, the switch is set to the off position.

When I test the continuity, there is no beep.

However, when I test the outgoing voltage, it is 18 Volts D.c.

What gives?

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Reply to
AK
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On further testing, I found that the transformer holds a residual charge for a few minutes after it is unplugged.

Is it because of some capacitors in the transformer?

Reply to
AK

That switch looks like it is on the mains? AC input? Capacitors on the transformer secondary can't impose a DC voltage on the primary side.

What are you using to measure this DC voltage? Something hinky is going on and perhaps a high impendence meter is just responding to electrostatic charge in the area.

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default

Using a digital multitester.

I hooked up a 12 volt led bulb to the output.

When the switch was turned off, it glowed for about 20 seconds and then faded to no light.

Andy

Reply to
AK

disconnect the outgoing wire and check again.

--
  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

That's quite possible with input filters on the power line. They can remain charged after power is disconnected. An interesting experiment would be to take multiple readings after switching off a number of times and seeing what voltage you get each time.

Discharge the cap with the LED each time and you should see a different voltage and possibly a different polarity each time. The voltage should be dependent on where (at what point) on the input sine wave you break the connection.

Another pretty good possibility is a switching power supply. I discounted that idea originally because the switch looks ancient. (before SMPs became prevalent) Switchers often charge a cap from the mains and you could see some leakage from the diodes charging the cap. With a switcher the transformer is after a largish electrolytic cap that is charged directly off the mains. If that were the case, I'd expect the DC voltage to be dependent on the amount of leakage in some diodes over others and should be relatively the same each time you switch it off until the charge on the cap leaks off.

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default

n.

rge for a few minutes after it is unplugged.

faded to no light.

I found this.

This is an old-style unregulated "wall wart" adapter. The internal circuit is probably this, and nothing more:

The transformer is designed to fail in a safe manner (no external flames, n ot too much smoke and isolation between mains and output maintained) if it develops a shorted turn or is fatally overloaded.

The reverse leakage of the diodes in BR1 is quite low at room temperature a nd the electrolytic capacitor (perhaps 10,000uF rated at 16VDC) itself typi cally has not much leakage, so the charge can be maintained for minutes or more with nothing connected.

More modern switching supplies (as well as linear regulated adapters) typic ally drain down much faster.

If you're connecting such an adapter to a circuit of your own making, it's worth remembering this as the adapter can easily hold enough charge to dest roy something if it's connected incorrectly. With it pulled out of the wall socket, you can short the output with a wire and most of the charge will b e removed, but a bit of voltage will creep back due to the way capacitors b ehave (dielectric absorption effect).

Reply to
AK

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