Understanding voltages and amps...

[tvout]

Hi all,
I'm hoping someone can help me avoid blowing things up
I bought a small monitor thing which I know requires 12v but not known how many amps it needs. I have found many 12v power adaptors when checked with a multimeter read up to around 17v? Couldn't that damage the device?
I found with a universal adaptor that 12v setting gave about 15v and 7.5v gives about 11v so set it for 9v to get around 12v out for another device.
Also, I understand that too many amps is harmless (the device only takes what it needs) but too little could cause damage?
I have got a 12v adaptor which gives 1 amp output. I have plugged that in to an energy meter and it shows the following when I have the monitor plugged into it:
0.002 under Amps and next to that it has a 'power factor' of 0.49-0.53. I don't really understand what that means.
I want to use a 12v adaptor that is only 500mA output but can't tell if it would be enough for this device.
The monitor is normally used for car reversing cameras and would normally be connected direct to the power in a car (12-14v and maybe 4 amps?)

Please can someone with electronics knowledge advise on this one?

Many thanks in advance

[Jon T]

The power supplies you are using are what are known as unregulated power supplies, they depend on the load put on them by the device to bring the voltage down to the indicated value. A multimeter set to test for voltage will have minimal loading on the circuit, which is why you're seeing 17V. Equally the reverse is true, if you place a load on an unregulated power supply that requires more current than the PSU can supply, then the output voltage will go down.

Have you got a model number for the display unit you want to power?

As for universal adapters, If you need a certain voltage, set it to that output value. Don't try and second guess it.

[tvout]

Hi Jon T,
Many thanks for the reply, the same thing is on ebay (item number 320561537220) and shows that it takes <=3W. I did a calculation that suggested it would be up to 250mA?
I just checked one adaptor that showed 12v on it with a multimeter and it showed 20v output. How could I tell if a device is safe to use with an unregulated power supply? I just checked the 12v adaptor I'm currently using with it and the multimeter showed 12.5V on that one.

[Jon T]

Assuming that the device took a full 3 watts of power, power(W)=Volts(V)xAmps(A).

Re-arranging that a bit, you get A = W/V, so A = 3/12 which is 0.250A or 250 milliamps, so yes your calculation is correct!

Really if an adapter says 12V output on it, then that's usually the voltage it will supply with a load connected, provided you don't exceed it's maximum supply current. The only way you're really going to know for sure is if you can connect the mulimeter to the power supply whilst it under load.

[tvout]

Many thanks Jon T, I think I'll try and connect the multimeter to the DC jack whilst connected (if possible?) to see if it's taking just 12 volts. Cheers for your advice, I feel a lot more understanding of it all now.