I got a full drawer of second hand wall warts which I'm planning to use for powering arduino projects (mostly 5V "pro mini" or "nano").
I'm using a voltmeter to check that they are still working, however most of them seem to output a far greater voltage than what is printed on the case.
For example the Arduino pro mini accept up to 12V so I've been looking at 9V wall warts, but they often give more than 12V.
Example (but typical reading):
- written specification: 9 V 300 mA
- Measured voltage: 12.37 V
- Measured voltage while powering a led (through a 300 ohms resistor): 11.2 V
I tried to add a led because I've read that measuring a voltage with no load may give a wrong result, but perhaps that's not enough to get a proper reading...
My questions are:
- Is there a way to measure the correct voltage that will be issued in "normal usage conditions"?
- Can I safely use a "9V wall wart" to power a "12V max" device if I measure more than 12V?
PS: So far I've use 6V wall warts which are giving about 10V, but I don't have any left so I'm trying to use 9V ones