modern PSUs

[Chris Holmes]

Hi,

The card machine (pdq? , pdc?) at a friends bookshop died (no light on the PSU).

I tried a known good fuse in the plug. No joy.

I tried connecting a couple of redundant PSUs I had, but they didn?t light up the device.

I know some laptop PSUs only put out a decent amount of watts when they get a signal from a compatible laptop. And the laptops won?t talk to other PSUs.

I wondered if maybe the Card machine (and / or the PSU I tried) was the same.

I had always assumed that this behaviour was an attempt on the part of the manufacturers to force you to buy their PSUs.

But I wonder if it?s some kind of smart charging system, whereby the laptop can tell the PSU when it does / doesn?t need lots of watts?

[Martin Brown]

I presume you checked for matching voltage, polarity and wattage/current before plugging it in?

Or are they USB charging devices - they come in a multitude of weird standards some of which can be locked to device signature.

Obvious thing to try is measuring the output voltage of the PSU.

Most of them are still pretty dumb bricks with any smart protection integral to the laptop and/or its battery.

One reason for it is to put high power into the bettery to get it up to a useful level of charge and then a less brutal rate to top it up.

[John Rumm]

Its more a case of ensuring backward compatibility.

Systems that use USB C for charging, will often go through a negotiation to enable the charger to switch to a 20V output. It can't present that by default or it would damage any 5V devices connected to it.

[Chris Holmes]

The original supply was marked 8V 3A.

The nearest I had was 9V which was rated at a much lower amperage.

When that didn?t work and with nothing to lose, I tried a 12V 3A.

That didn?t light it up either.

I did check the output of the laptop style (12V) PSU, but the trouble with these appears to be that they put out a small amount of watts at their rated volts, but don?t send a decent amount of wattage until either they get a ?Yes, I am the laptop you are looking for? signal, or a ?I?d like you to charge me now please? signal (either of which means no useable watts if trying to power something else).