05-01-2025, 09:17 PM
(This post was last modified: 05-01-2025, 09:18 PM by Mike Watterson.)
Half mains is still "live". Unless you could connect audio in/out without a transformer (and easily add video I/O, it's effectively live.
I had a so called UPS for external battery end of last year and when I earthed -ve of battery there was an almighty bang when I plugged in the IEC mains input plug. A bridge rectifier connected mains output socket to monitor / display PCB connected also to the 24V DC. So the two minus diodes of the bridge connected direct to internal "0V" rail and the two plus diodes to a potential divider to the CPU to turn an annunciator on LCD on/off. Not actually measured. Hence bang. In practice if the panel was powered there was 230V on output. In normal UPS mode the output socket was connected direct to mains in.
The SMD mains voltage rectifier blew off the PCB.
So I "fixed" it by simply tying part way on the 300V DC divider (multiple SMD resistors) to the +24V in on the Display panel.
My theory is that it started life purely as 24V to 1.2 kW 230 VAC inverter and they added a battery charger and relay to connect mains in to output. Never resubmitted for test, even if the various approvals ever real.
Basically the external battery -V terminal was -300V and the +V one 26 V less, direct via the rectifier bridge. Designed to be used with 2 x 12V car batteries.a
I had a so called UPS for external battery end of last year and when I earthed -ve of battery there was an almighty bang when I plugged in the IEC mains input plug. A bridge rectifier connected mains output socket to monitor / display PCB connected also to the 24V DC. So the two minus diodes of the bridge connected direct to internal "0V" rail and the two plus diodes to a potential divider to the CPU to turn an annunciator on LCD on/off. Not actually measured. Hence bang. In practice if the panel was powered there was 230V on output. In normal UPS mode the output socket was connected direct to mains in.
The SMD mains voltage rectifier blew off the PCB.
So I "fixed" it by simply tying part way on the 300V DC divider (multiple SMD resistors) to the +24V in on the Display panel.
My theory is that it started life purely as 24V to 1.2 kW 230 VAC inverter and they added a battery charger and relay to connect mains in to output. Never resubmitted for test, even if the various approvals ever real.
Basically the external battery -V terminal was -300V and the +V one 26 V less, direct via the rectifier bridge. Designed to be used with 2 x 12V car batteries.a
