110 V AC To 9 V dc Wal-Wart Question

Hello,

Not an EE, so please bear with me. (just a poor ME trying to learn something about the other world)

Just got a new handheld radio scanner, and it comes with the typical wall-wart which you plug into a 110 V AC wall socket and provides, according to the label, 9 V dc output (500 ma rating). Guess this is just a simple switching power supply with, perhaps, a full wave bridge on the output ?

The manual had the following warning:

- "Always connect the AC adaptor to the scanner before you connect it to AC power. When you finish, disconnect the adaptor from AC power before you disconnect it from the scanner.

- To power the scanner using an AC adaptor, connect the plug into the scanner's PWR DC 9V jack. Then connect the AC adaptor to a standard AC outlet."

Was wondering why the need for this sequence. All I can think of is that they want a load on the AC adapter to keep the voltage to what it should be.

I measured the no-load voltage with a digital VOM, and it was 13.1 V. Wow ! Is this typical for these things ?

So, perhaps this is the reason ? Or, might it have to do with a connect transient or plug-in transient ?

Bit, wouldn't one think that if the scanner on-off switch was off, it wouldn't matter ? This may not be a correct thought, as the unit's rechargeable batteries in the unit are recharged by the adapter even if the scanner switch is set to off, so there must be some kind of electrical bypass around the switch ?

Shouldn't the scanner have some kind of protection built in such that if one does not do this sequence, it is still safely protected. Or, the extra few cents that would be required to do so was deemed "cost effective" ?

Sure is easy for folks like me, or anyone I would think, to forget. Also, quite inconvenient to plug and unplug the adapter from a wall outlet each time under my desk.

How necessary do you think adhering to the sequence actually is ?

Good chance of blowing the unit if it is plugged into an already plugged-in wall wart with 13.1 V sitting on its output ?

Any thoughts on this would be most appreciated.

Thanks, Bob

The odds are that it is a simple transformer/rectifier/capacitor supply rather than a switched mode type. The former does have poor regulation, so the off-load voltage can rise well above full load and nominal value.

I would hope that the design would tolerate plugging in the powered-up up adapter - after all, it is possible for the plug to be accidently pulled out with the adapter powered and few people would go to the trouble of unplugging from the wall before putting the plug back in.

I would think that the chances of actually doing damage by doing so would be very slim. The number of returns would be simply too great, otherwise.

My reaction, however, with electricity costs rising all the time - would be to look to either put a socket where it was more convenient. Or remotely control the socket(s). It's a bit pointless leaving the thing plugged in, using electricity, unnecessarily.