Hi,
I measured few of the primary coil of DC power supply chargers, none of them conduct, this is against my cognition,
In my knowledge, the primary coil even the secondary coil should conduct and got a resistance reading, am I right?
Thanks
Adam

 

No. Those are switching power supplies. There are rectifiers before it ever gets to a transformer any other coil/inductor of any type. A transformer or linear power supply would have the primary coil your thinking of.

 

No. Those are switching power supplies. There are rectifiers before it ever gets to a transformer any other coil/inductor of any type. A transformer or linear power supply would have the primary coil your thinking of.

Thanks.
The thing is some normal power supplies' (non switching power supplies) primary coil doesn't conduct neither. I even opened them and saw the primary coil direct connect to the plug.
Best

 

The thing is some normal power supplies' (non switching power supplies) primary coil doesn't conduct neither. I even opened them and saw the primary coil direct connect to the plug.

That strongly suggests those supplies are not working.

 

Thanks.
The thing is some normal power supplies' (non switching power supplies) primary coil doesn't conduct neither. I even opened them and saw the primary coil direct connect to the plug.
Best

Show us a picture of what you're talking about please. I've opened many of those SMPS's (Switch Mode Power Supply) and have seen transformers in there. It may be possible you're seeing something and mis-interpreting it. It's also possible that some of the transformer coils you've measured were somehow open circuited. Dual primary can be configured by a PCB for 120 or 240 VAC. If there's a jumper between the two center leads it's probably for 240 VAC. If each lead marked with a • is joined together and the other two leads are joined together then it's a 120 VAC transformer (primary side). Secondary sides can also be individually wound coils. A center tapped transformer (secondary side) will only have three wires. And there are many many more transformer configurations out there. Ask and I will post a schematic diagram of a transformer I took out of a stereo. You'd be surprised at the number of different voltages that can be obtained by different sets of secondary coils.

 

And there may be a common mode choke at the input of an SMPS which may look like transformer but the power input will still read open circuit.

 

That strongly suggests those supplies are not working.

Thanks.
Yes, you are right, the normal changer I tested doesn't work.
Best

 

Hi,
I measured few of the primary coil of DC power supply chargers, none of them conduct, this is against my cognition,
In my knowledge, the primary coil even the secondary coil should conduct and got a resistance reading, am I right?
Thanks
Adam
View attachment 214523View attachment 214524

Your knowledge is correct, but the devices shown have neither primary coils nor secondary coils because they are switching-mode power supplies. Their functioning is fundamentally different.