Why Does My 9-12V Power Supply Output 16V?

[Pythagorean]

So I have a power supply, 9-12 V output and I have a component that takes 9-12 V.

When I plug the power supply into the wall and measure the potential across it's leads, I get 16V. I put a resistor across the leads and measure the diff across the resistor. Again, 16V. I look at several other power supplies on the same order (9-12 V). All 16 V.

What's going on here?

(these are not benchtop power supplies, but random manufacturer power supplies originally intended for other products)

 

[Pythagorean]

hrm... is it possible I'm plugged into 220 instead of 110? I'm using outlets in a university lab to test...

 

[pantaz]

Most likely they are linear power supplies. The output voltage tends to inversely follow load.

I remember reading an article recently... found it: http://digital-diy.com/General-Electronics/ac-dc-power-supplies-using-wall-warts.html

http://digital-diy.com/General-Electronics/ac-dc-power-supplies-using-wall-warts/Wall-Warts.html" has graphs comparing output of several brands of 12v linear and switching supplies, and waddyaknow, the three linear supplies have unloaded output of 15-17 volts.

 

[davenn]

you haven't said anything about the brand, type, and any marhing on the PSU's

many "plugpacks" for example are unregulated and the output voltage will always be higher by several Volts than that stated on the unit. They will come down to their stated voltage under the load drawing the specified current.

Many other plugpacks and psu's in general may be regulated. generally it will state on the unit if its a regulated supply or not

Dave

 

[Pythagorean]

thanks pantaz.

davenm, the point was to ensure that it is regulated (it's a sensitive device being hooked up to it, so it must be). I did use a load on it and still got 16V, but the load wasn't stated so I could have used to large (small?) of a load. I'm not at the lab now, so I don't know the brand names, but they were definitely built for a specific product like a cordless phone station or something.

We found a power supply that worked by now, but it's still a curiosity to me.

 

[davenn]

well by all information you have currently given I would say that they are unregulated

I personally haven't seen a regulated PSU that doesn't say "Regulated" on it :)

As pantaz and i have both said in previous replies, no load or very small load on unregulated supplies will allways have a higher output voltage than stated on the unit.

cheers
Dave

 

[Pythagorean]

Thanks for the input guys. Good tidbit to know about psus.

 

[JimAlexander]

thanks pantaz.

davenm, the point was to ensure that it is regulated (it's a sensitive device being hooked up to it, so it must be). I did use a load on it and still got 16V, but the load wasn't stated so I could have used to large (small?) of a load. I'm not at the lab now, so I don't know the brand names, but they were definitely built for a specific product like a cordless phone station or something.

If it was for something like a phone station or anything else with a battery, the voltage is deliberately high in order to drive a battery charger. Even with a relatively high load resistor the voltage values would still be high. Car batteries would be another example; they are always 12 volts or so, but the car's voltage regulators put out 14.7 to 16 volts depending on the particular electrical system. Regulated or unregulated, battery charging circuits and supplies have to aim at recharging up to their own half-power points.

J