Silly question I know..

woollymammoth

Just started checking my old computers and started with the ac/dc power adaptors.

The 9 volt output is showing 12 to 13 volts

The 12 volt output is showing 13 to 14 volts

So are they ok or do I need new ones ?


Thanks

pmsr

If they are the original PSUs they are probably linear, not switched, so that is to be expected. You must measure them under load.

/Pedro

woollymammoth

By under load, do you mean plugged into the equipment, is that risky ?

Edit Yes they are the originals

1024MAK

The "9 volt" original Sinclair ZX Spectrum mains power supply / adaptor is an unregulated type. With the output not connected to anything ("no load" / "off load"), the voltage that is developed across the large electrolytic "smoothing" capacitor will charge up to the peak output voltage of the transformer (unlike when it is supplying a load).

AC voltage and the rectified voltage in this type of PSU is normally measured as RMS voltage (because the power in a resistor at this voltage is equivalent to the same level of power from a battery) not the peak-to-peak voltage, which is the top and bottom of a mains AC sine wave.

In addition, because the transformer is not supplying the normal load current, it's output will be between 10% to 20% higher than it's rated voltage output.

Is it safe? If the PSU's are original, and the output voltage is 14V or less off load, it should be. If you have a 12V 5W or a 10W car / motorcycle lamp, connect it to the PSU output and if the voltage is now 11.9V or less, it is safe :-).

Mark

woollymammoth

Thanks for that Mark, I will hunt something up to test with ( did not think of that )

AL