Comms link power & polarity?

[Carlos]

Hi, I have just taken delivery of a comms link, what is the power requirement, and is it centre positive or negative?

[Martin]

Hi Carl..

Centre Positive... 175mA at 10V... Check out Jaaps Devices and Accessories page (here)..

And there is a re-published copy of his excellent 'Using the Comms Link' paper (here)..

Sincerely
Martin

[Carlos]

Thanks Martin,
10v is quite an odd size for a PSU, I wonder if a 12v under load would give that output?

Anyway I will have a search and share what I find.

[Martin]

Carl..

I used an after market PSU for many years and it gave 9 volts and successfully powered my LZ64 including using the Comms Link for transferring data and MAKEing Programme packs.

The PSU that is supplied with the PSION Mains Adaptor declares
Input 220/240V 50Hz 30mA
Output 10.4V DC 175mA

Sincerely Martin

[amenjet]

I could be wrong but I suspect that the 10.4 value comes from the designers wanting to power the Organiser from the PSU when it is plugged in and 'turn off' the 9V battery. This is all done with diodes so you'd need a voltage coming from the PSU that is a diode drop or so above the battery voltage. the battery voltage is nominally 9V and a diode drop is 0.6V. Wikipedia says that the voltage of a 9V battery is '7.2 to 9.6 volts' so 9.6 plus 0.6V gives you 10.2V, so 10.4 is definitely going to drive the organiser and turn the battery off.

You could use a higher voltage but that will lead to more dissipation in the regulator, so it's best to keep the voltage as low as possible. I don't know what the highest voltage you could safely apply from a PSU is. It depends on the regulator and it's mechanical arrangement (heat sinking, air flow) and ambient temperature etc. All quite complicated. I'd expect 10.8V to be foine, 12V probably ok, 18V to not be good, I could be wrong, though...

Andrew

[Lostgallifreyan]

I agree, it's down to heat buildup, in almost all cases, that defines the safe margins. I have never had trouble running on 12V but I always do it in conditions I find comfortable. If it got to 40 degrees C I might rethink that.

12V with an extra series diode can help, the voltage drop can help move some of the dissipation outside the Organiser, and the extra reverse polarity protection is never a bad thing. Notionally there is inefficiency in doing this, but an Organiser is a load so light that there's no easy gains trying to force more efficiency there! Instead, it's worth knowing that the increased efficiency of things like serial adapters and GPS units and sensors makes them ever more likely to work well with a Psion Organiser. The biggest obstacle to doing so is not the power demands, but that the Organiser usually is optimised to switch off power to external hardware when it's not accessing it, so some stuff will always want an external supply. A GPS will maintain an almanac when switched off, but it needs to run all the time to maintain a location fix... Small SMPU devices that can get 3.3V or 5V DC from a 10~14V DC source do exist, but aren't as cheap or easy to get (or use) as I'd like them to be.

In the context of using the EBL li-ion batteries, it may be worth aiming to run an Organiser off a 9V supply! That could spare the regulators, even on hot days. As far as I know, the internal PP3 needs to be countered by a higher voltage to prevent it from being drained when an external supply exists. These li-ion packs can stay steady at 8.4V or below, so a diode drop of 0.6V suggests that a 9V external source might be ideal in conjunction with these batteries, and the Comms Link will always be happy because its own circuit uses the 5V regulated source from the Organiser. Running the whole thing from 9V instead of 12 is a significant gain in efficiency and a wider safety margin for the internal regulator, but you'll probably need to use those 8.4V li-ions to do it. At that point, all we need is a modification to charge them in place...