Psion 5mx pocket computer - UK Vintage Radio Repair and Restoration Discussion Forum

majex45 · 5th Mar 2024, 3:48 pm

Split from this thread:-

https://www.vintage-radio.net/forum/...d.php?t=209011

Different (and easy?) one for all you geniuses.
I have and use all the time a Psion 5mx pocket computer (more always required as they are somewhat unreliable).
It uses (nominal) 3v from 2 x AA batteries or a 3v mains adapter. It eats a set of batteries every week or so. It would be nice to be able to use a longer life battery. I have some large Li battery packs used for charging mobile phones (5v out for 1 to 3AH).
The Pson uses an average of 64 mA so, say 100 Ma Max (it needs to update a Compactflash card every now and then).
I need a very simple circuit to drive the Psion from the Li packs.
I was thinking of a series resistor and a 3v zenner.
I think the zenner only needs about 10mA to stabilise.
So: 2V across the resistor at ~100mA = 20 ohms.
So a 20 ohm resistor and a 3V 500 mW zenner. Would this work?
What happens under no load when the whole current goes through the zenner?

SiriusHardware · 5th Mar 2024, 6:32 pm

Hello majex45 - your zener regulator idea is really only one to use where you have power to burn, for example a constant mains derived supply, because the zener will sink a significant amount of current whether the Psion is running or not.

The other problem is that the series resistor required for the zener to not burn out will also limit the current available to the Psion when it does want to draw a significant amount of current - do they have backlights for the screen, those models?

You would be better off with a 'buck converter' regulator, you can get those all over the place, just Google '5v to 3v3 buck converter PCB' and you will be shown a good selection of small DC to DC converter PCBs designed to do exactly the job you want to do - when looking at them make sure you are aware of the maximum output current they can supply and make sure it is enough to power the Psion at times of maximum demand. Generally speaking, small self contained SMPSUs like that are much more power efficient regulators than a resistor and zener would be.

As regards charging from a USB phone charger pack or similar, there is currently a discussion about using those as a general power source elsewhere on the forum - one potential problem is that they are a bit too smart for their own good so they tend to power off when no significant load is running from them, which may not be the behaviour you want.

factory · 5th Mar 2024, 7:04 pm

Alternately a LDO regulator could be used, or several diodes in series to drop the voltage would be even simpler.

David

SiriusHardware · 5th Mar 2024, 7:22 pm

Diodes, I didn't even think of that.

Mark1960 · 5th Mar 2024, 7:33 pm

I wouldn’t trust diodes to drop from 5v to 3.3v. At low current drain the 5v would be present at the output. LDO would be less efficient than a SMPSU, but might be more reliable.

ortek_service · 6th Mar 2024, 12:48 am

You wouldn't need much current through most power diodes to drop at least 0.5V - even the 10M input resistance of a DMM would probably be low enough to get that amount of drop, but if concerned then could maybe put a 10K etc resistor on the '3V' output side to ground.
The diodes voltage drop will vary a bit with current, but id 1N400x 1Amp etc ones were used, then it shouldn't vary too much between 0 and 100mA and within range of typical battery voltages over their lifetime.

Three standard silicon rectifier diodes in would probably be required, as the Psion 5mx should still work if slightly-below 3V. And may get away with only two, if their forward voltage (even at lower currents) is closer to 1V on these compared to small-signal ones.

However, IIRC, the VBUS spec. for USB is 4.4 to 5.5V and can vary with load-current / cable length. so a simple dropper won't always be that good (even though I think diode droppers used to be used from 5V to 3.3V for CPU on some older PC Motherboards).
So a linear LDO or SMPSU would be better (although efficiency of the SMPSU may not be that much better when input is quite close to output voltage)

I do note that the external mains PSU's for the Psion5mx appear to all be 6V 1A output - Not the 3V originally stated? e.g. https://psionex.co.uk/en/product/pda...-supplies.html
So using the power input connector for that, rather than having to attach to the battery contacts, may be better.
I presume this may also recharge the batteries in the unit and may hopefully have an internal step down SMPSU. In which case, it might even work down to below 5V, so could maybe try feeding that from the USB Power Banks (which I presume the OP meant, as a straight-forward Li Battery 'Pack' may only be approx 3.7V, without the added SMPSU to generate the 5V USB supply voltage output.

majex45 · 6th Mar 2024, 2:33 pm

As the man says: Reach for your meter before your soldering iron.
I have just tested the output voltage of the Psion mains adapter and it outputs 6.2V.
I'll try to run the Psion from the battery pack direct and see if it works.
Thank you everyone for all your help. It is most appreciated.

5th Mar 2024, 3:48 pm	#1
majex45 Hexode Join Date: Jul 2011 Location: Hastings, East Sussex, UK. Posts: 307	Psion 5mx pocket computer Split from this thread:- https://www.vintage-radio.net/forum/...d.php?t=209011 Different (and easy?) one for all you geniuses. I have and use all the time a Psion 5mx pocket computer (more always required as they are somewhat unreliable). It uses (nominal) 3v from 2 x AA batteries or a 3v mains adapter. It eats a set of batteries every week or so. It would be nice to be able to use a longer life battery. I have some large Li battery packs used for charging mobile phones (5v out for 1 to 3AH). The Pson uses an average of 64 mA so, say 100 Ma Max (it needs to update a Compactflash card every now and then). I need a very simple circuit to drive the Psion from the Li packs. I was thinking of a series resistor and a 3v zenner. I think the zenner only needs about 10mA to stabilise. So: 2V across the resistor at ~100mA = 20 ohms. So a 20 ohm resistor and a 3V 500 mW zenner. Would this work? What happens under no load when the whole current goes through the zenner?

5th Mar 2024, 6:32 pm	#2
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,598	Re: Psion 5mx pocket computer Hello majex45 - your zener regulator idea is really only one to use where you have power to burn, for example a constant mains derived supply, because the zener will sink a significant amount of current whether the Psion is running or not. The other problem is that the series resistor required for the zener to not burn out will also limit the current available to the Psion when it does want to draw a significant amount of current - do they have backlights for the screen, those models? You would be better off with a 'buck converter' regulator, you can get those all over the place, just Google '5v to 3v3 buck converter PCB' and you will be shown a good selection of small DC to DC converter PCBs designed to do exactly the job you want to do - when looking at them make sure you are aware of the maximum output current they can supply and make sure it is enough to power the Psion at times of maximum demand. Generally speaking, small self contained SMPSUs like that are much more power efficient regulators than a resistor and zener would be. As regards charging from a USB phone charger pack or similar, there is currently a discussion about using those as a general power source elsewhere on the forum - one potential problem is that they are a bit too smart for their own good so they tend to power off when no significant load is running from them, which may not be the behaviour you want.

5th Mar 2024, 7:04 pm	#3
factory Octode Join Date: Jul 2009 Location: Cheshire, UK. Posts: 1,740	Re: Psion 5mx pocket computer Alternately a LDO regulator could be used, or several diodes in series to drop the voltage would be even simpler. David

5th Mar 2024, 7:22 pm	#4
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,598	Re: Psion 5mx pocket computer Diodes, I didn't even think of that.

5th Mar 2024, 7:33 pm	#5
Mark1960 Octode Join Date: Mar 2020 Location: Kitchener, Ontario, Canada Posts: 1,298	Re: Psion 5mx pocket computer I wouldn’t trust diodes to drop from 5v to 3.3v. At low current drain the 5v would be present at the output. LDO would be less efficient than a SMPSU, but might be more reliable.

6th Mar 2024, 12:48 am	#6
ortek_service Octode Join Date: May 2018 Location: Northampton, Northamptonshire, UK. Posts: 1,444	Re: Psion 5mx pocket computer You wouldn't need much current through most power diodes to drop at least 0.5V - even the 10M input resistance of a DMM would probably be low enough to get that amount of drop, but if concerned then could maybe put a 10K etc resistor on the '3V' output side to ground. The diodes voltage drop will vary a bit with current, but id 1N400x 1Amp etc ones were used, then it shouldn't vary too much between 0 and 100mA and within range of typical battery voltages over their lifetime. Three standard silicon rectifier diodes in would probably be required, as the Psion 5mx should still work if slightly-below 3V. And may get away with only two, if their forward voltage (even at lower currents) is closer to 1V on these compared to small-signal ones. However, IIRC, the VBUS spec. for USB is 4.4 to 5.5V and can vary with load-current / cable length. so a simple dropper won't always be that good (even though I think diode droppers used to be used from 5V to 3.3V for CPU on some older PC Motherboards). So a linear LDO or SMPSU would be better (although efficiency of the SMPSU may not be that much better when input is quite close to output voltage) I do note that the external mains PSU's for the Psion5mx appear to all be 6V 1A output - Not the 3V originally stated? e.g. https://psionex.co.uk/en/product/pda...-supplies.html So using the power input connector for that, rather than having to attach to the battery contacts, may be better. I presume this may also recharge the batteries in the unit and may hopefully have an internal step down SMPSU. In which case, it might even work down to below 5V, so could maybe try feeding that from the USB Power Banks (which I presume the OP meant, as a straight-forward Li Battery 'Pack' may only be approx 3.7V, without the added SMPSU to generate the 5V USB supply voltage output.

6th Mar 2024, 2:33 pm	#7
majex45 Hexode Join Date: Jul 2011 Location: Hastings, East Sussex, UK. Posts: 307	Re: Psion 5mx pocket computer As the man says: Reach for your meter before your soldering iron. I have just tested the output voltage of the Psion mains adapter and it outputs 6.2V. I'll try to run the Psion from the battery pack direct and see if it works. Thank you everyone for all your help. It is most appreciated.