SMPS Earthing Issue

Good Day,

Im struggling with a weird SMPS issue.

I have the following component connection layout, includes a AC->DC 24V SMPS that feeds a DC-DC 5V SMPS:


My 5V SMPS "shuts off" as soon as I plug in the 24V supply to the Modem, I believe the issue is that the Modem internally is tying GNDa to Earth (E). I can confirm that without the modem plugged in the same behaviour is found if I join a wire from Earth (E) to GNDa.

I've taken some measurements on the circuit shown above with only the Modem disconnected:
Voltage between (GNDa) and (+24V) = 24.17V
Voltage between (GNDb) and (+5V) = 5.6V

Voltage between (GNDa) and (+5V) = 9.75V
Voltage between (GNDb) and (+24V) = 20V

Voltage between (E) and (GNDa) = -4.2V
Voltage between (E) and (GNDb) = -0.045V

Voltage between (GNDa) and (GNDb) = 4.15V


And some measurements with the Modem connected OR Earth (E) connected to (GNDa): the 5V output shuts off.
Voltage between (GNDa) and (+24V) = 24.19V
Voltage between (GNDb) and (+5V) = 0V

Voltage between (GNDa) and (+5V) = 0V
Voltage between (GNDb) and (+24V) = 24.16V

Voltage between (E) and (GNDa) = 0V
Voltage between (E) and (GNDb) = 0V

Voltage between (GNDa) and (GNDb) = 0V

I would really appreciate an info as to what is going on here and what can be done to prevent the 5V SMPS from shutting down?

Kind Regards,
StudentSA

The 5V PSU module appears to have an odd configuration where negative in and negative out are offset somehow.

I'd get a different module than has negative common through.

Edit - from the data sheet, input and output are supposed to be fully isolated. Is the Pi connected to anything else?
You should be able to hardwire both grounds to each other (and earth), to avoid random voltage offsets.

Thanks for the quick reply. I've been scratching my head for sometime with this.
I have actually tried disconnecting the Raspberry Pi i.e. No device on the 5V line. even in this state joining (E) to (GNDa) shuts-off the +5V SMPS.

Edit- With GNDa, GNDb and Earth wired together I still find the 5V SMPS shuts off.

That sounds like it has an internal fault; it should not be affected.

Disconnect tour earth ground from the 5v dc/dc converter. Life will be good.

To close this thread, rjenkinsgb was spot on, I managed to wire all GND's and Earth together after swapping out the 5V SMPS with a new unit. I think running it with offset voltages must have caused an internal fault on my original unit.

Thank for the help

A success story, good to hear.

JimB