Wall plug type power supplies

[BigE4u]

Is it typically normal for the set voltage to read higher when read thru a DMM?

Example... +12v reads as +15v

 

[Hero999]

Yes, the off load voltage is often over 25% than the rated voltage. Try adding a load to the adaptor and the voltage should drop significantly.

 

[BigE4u]

So your saying when "loaded", the voltage should drop back to or close to the rated voltage?

 

[audioguru]

So your saying when "loaded", the voltage should drop back to or close to the rated voltage?

Yes.
They are cheap. The wire in the windings is not enough so they design the voltage to drop down to the voltage rating at full load current. Then the voltage is much higher without a load.

You can make or buy a regulated voltage power supply where its voltage is almost the same with no load to full load.

 

[BigE4u]

Yea... i was being cheap about it, but at $10 for a +12v@1.5a heavy as heck wall plug type power supply, it should do nicely for my electronic projects testing.

EDIT: I want to start using PICs soon as well, so i already have a LM7805 handy for just such an occassion.

 

[audioguru]

At an electronics surplus store they had hundreds of 5V/2A name-brand switching power supplies for less than $2.00 each. I got a few and they work perfectly. They are tiny and are very light weight. Their voltage is regulated at 5.0VDC. They were made for dial-up modems.

 

[BigE4u]

I take it the +5v switching power supply you mentioned can be made in a smaller package due to using a tiny stepdown transformer with a regulator circuit? Versus the non regulated, higher output current having to use a much larger stepdown transformer and output smoothing circuit...

 

[audioguru]

A transformer that operates at 100kHz (in a switching power supply) is much smaller than one operating at 50Hz or 60Hz. Then the circuit can have more output current and still be much smaller.

The voltage regulator is part of the switching circuit.

 

[BigE4u]

Like most power supplies thats included with most cell phones for recharging. My current one for my cell is rated for 5.1v@800ma and its regulated. I checked with a DMM and its dead on.

 

[mvs sarma]

Some dial up modems supply a step-down transformer only with 12V or so Isolated AC output in the shape of wallwart and do the balance (rectification and regulations) inside the modem itself.

While using the cellphone chargers better to have further filtering arrangement on the user PCB for reducing noise level.

 

[Diver300]

The switch mode power supplies, the light weight ones, have to have regulation built in. That is why they have the correct output voltage.