Low voltage power supply switching...

[Nightowl171]

I'm a real noob to electronics (although I did have an electronics hobby kit back in 1972) as my paths of employment took me away from the hobby, but I'm retired now, so I've got some time on my hands - and you know what they say about idle hands..!

Anyway, I'm happy to provide more details if necessary, but essentially I have a machine control board that normally runs via a (UK) mains-powered 24V 350W PSU, and the machine has the facility to record certain information to a 'resume' file, if that power should fail. However, the power 'drain' for my machine is too quick so the file isn't written.

The control board can also run with a 5V (USB) power supply (but not to do any work, just when configuring it) and I was hoping to use this as a way of keeping the board powered long enough to save the file, but both shouldn't provide power at the same time, so I thought of using a pair of relays and a microcontroller, to switch from one supply to the other, if the 24V fails - and it sounded so simple.

Here's a sketch of my thoughts (it's not really a circuit diagram) and I'm hoping the principle is sound, but I'm not quite sure how to get the microcontroller to recognise the failure of the 24V supply, as it can only tolerate 3.3-5VDC. I'm thinking perhaps an optocoupler?

Any suggestions would be appreciated, but (I think) I'm fairly happy about the code for the microcontroller.

Thank you.

 

[rjenkinsgb]

The Duet3 6HC has got a dedicated 5V power input connector, the three pin "Ext 5V" header just behind the 24V input screw terminals.

You can connect a separate 5V supply to that (and externally connect it to the Pi) but you must remove the 5V jumpers from the middle of the board - "Internal 5V En" and both SBC power jumpers.

If you want, you can used the "pson" signal on the Ext 5V connector to control a relay or SSR to switch the 24V supply on & off (M80 & M81 control it).

I use that setup on one of my machines.
(Trying to switch between two power sources for the 5V could damage the board).

 

[Nightowl171]

Thanks for your reply, rjenkinsgb

I did actually get a response from the Duet forums, which said almost exactly the same - the exception being to leave just the Internal 5V EN jumper installed.

What are your thoughts on that, please?

 

[Nightowl171]

If you want, you can used the "pson" signal on the Ext 5V connector to control a relay or SSR to switch the 24V supply on & off (M80 & M81 control it).

I'm not sure if I would want to go down this route, at least for the moment. I don't use an SBC, preferring to use a web browser on my Surface Pro.

In fact, would it be a good idea to supply (trickle charge) the PowerBank from my Surface Pro, so I'd still have that communication link, as the powered network hub would also stop working?

 

[Nightowl171]

You can connect a separate 5V supply to that (and externally connect it to the Pi) but you must remove the 5V jumpers from the middle of the board - "Internal 5V En" and both SBC power jumpers.

I'm sorry if I'm being a bit slow, but I've just reread your message and you say I could connect the Pi(co) to the 5V PowerBank. If i can just connect the PowerBank via the microUSB port and trickle charge it from the Surface Pro, I wouldn't need the Pico installed at all, or have I missed something?

Thank you

 

[rjenkinsgb]

I'm sorry if I'm being a bit slow, but I've just reread your message and you say I could connect the Pi(co)

Sorry, I originally misread your info and thought you were using a full Raspberry Pi as a controller for the Duet - that's how I have both mine configured..