Power output of A500 Power Supply

[snoopy77]

Have read comments before that A500 power supplies are either 25 watts or 35 watts.

I have a "heavy" A500 power supply from about 1990,

On the back it doesn't state a power output but the input as shown in the photo is stated as "60W",

So I'm curious, is the power output actually 35watts, 60 watts or 25 watts, what do you think?




btw the part number is stated as 312503-05 (a higher part number than the two variants on Ian Stedman's site)

 

[8 Bit Dreams]

It's 60W cause 5v and 12v rails are counted together.
Most important is 5v rail, yours is 4.3A, just recap it, do a voltage measure DIRECTLY at CPU on running system 4.8v is bad do adjustment, should be 4.95v ideally

 

[Bryce]

It's 60W cause 5v and 12v rails are counted together.
Most important is 5v rail, yours is 4.3A, just recap it, do a voltage measure DIRECTLY at CPU on running system 4.8v is bad do adjustment, should be 4.95v ideally

Not quite...
The 60W refers to how much power it pulls from the socket. To this you need to add all the losses (heat etc) inside the PSU. The actual output power can be easily measured with simple maths: P(power in watts) = I (Amps) x V (Voltage). So in your case:

5V Rail: P = 4.3 x 5 = 21.5 Watts
12V Rail: P = 1 x 12 = 12 Watts
-12V Rail: P = 0.1 x 12 = 1.2 Watts
Total: 34.7 Watts.

This means that the PSU is dissapating 25.3 Watts of wasted energy as heat/noise/eddy currents etc and the efficiency of the PSU is a rather poor 57.8%
However, it's probably not that bad as the 60W spec is a max requirement, not what it actually needs in reality.

Bryce.

 

[snoopy77]

Thanks guys!

 

[slaz]

... and just a FYI - I understand some of the later ones are SMPSU design (switch-mode) - the light weight ones.

These should be a good deal more efficient - i.e. the power input would be much closer to the power output. So a PSU rated at 35W output would have power input more like 40W. Less heat also - but possibly more failure-prone if they use cheapish components.

 

[Bryce]

A well designed SMPSU can be more than 90% efficient, but I don't know if the Amiga ones were that good. They didn't use very cheap components and they are generally well designed and have enough room to keep cool, so they should be pretty reliable too.

Bryce.