AC / AC adapter question (Hornby G8025W IP40)

Thread Starter

brianmk

Joined Dec 23, 2016
59
I have a Hornby G8025W IP40 230V AC to 16V 400mA AC adaptor.
The output measures about 19.5V AC with no load.

What I don't understand is that if I measure the output resistance using a DMM or analogue multimeter, it reads an open circuit.
That suggests it cannot be a simple step-down transformer inside.
If it were just a transformer, you would expect to measure the resistance of the secondary winding i.e. a few ohms.
The output appears to be fully isolated from the mains input.
Does anyone know what's inside one of these adapters?
Could it be a transformer but with a capacitor (or two) in series with the output?
There are no screws, so I cannot open it up to find out what's inside without breaking it.
I am baffled.
 

Thread Starter

brianmk

Joined Dec 23, 2016
59
I simply connect a resistance meter between the center and outer ring of the output plug (with the adapter unplugged from the mains of course!)
These are the same connections that I use to measure the ac output voltage.
I've tried two different meters - a DMM and an analogue multimeter.

I'm starting to wonder if there is some sort of internal short circuit protection that disconnects the output when it's powered off.
The adapter is intended to be used for scalextric, so frequent short circuits would be expected.
I've tried measuring the output impedance using an AC (LCR) bridge.
That also suggests the output is disconnected when there is no power.
 

Thread Starter

brianmk

Joined Dec 23, 2016
59
I think I have figured it out.
If I put my ear close to the adapter, I can hear the click of relay contacts when I switch off the mains input.
As I suspected, there must be a short circuit protection relay inside.

I was actually trying to directly measure the internal resistance of the transformer secondary so I could figure out the expected output voltage with different loads. Instead, I will do it by measuring the voltage difference when unloaded and when loaded with a known resistance.
 

MisterBill2

Joined Jan 23, 2018
18,176
If you are able to measure both current and voltage out at the same time, as with two meters, the one point is all you would need, although 2 or 3 would potentially be a bit more accurate. And good luck with the project.
 
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