Aidan wrote:
It's a BMS outstation, a computer type device intended to monitor &
controls heating & stuff.

On the pcb, it has an encapsulated transformer 0-24V pri/0-18V sec,
adjacent to the power input. The power is from a 24V ac mains
transformer. Also 4 electrolytic capacitors, but nothing I recognise as
an rectifier. There's a small battery on the pcb, but it's just to
maintain the RAM & programmed settings. The device fails if the 24 Vac
goes off. I'd like to monitor inputs and keep the alarms working. The
control valves are also 24 V ac, but I wouldn't intend maintaining the
power supply to these.

You could probably achieve what I want with PLCs but I'd have to learn
how to programme those, so that's relegated to plan B!


Going through an inverter will be much less efficient than direct DC
injection.
There MUST be a rectifier; almost certainly the secondary of the traffo
will be connected straight to it (maybe via a fuse) so trace the pcb
tracks. If you can't find it just disconnect the secondary of the traffo
and connect your DC feed to wherever the secondary previously went - the
rectifier may run a little warmer but this is very unlikely to be a
problem - and if it DOES get too hot you'll be able to find it

If you want this as a back-up system , rather than replacing the mains
supply, you'll need to find the rectifier and connect the new DC supply
to its output via a diode.

Dave