I want to use a Marshall monitor I own with my Steadicam but the SDI module on the back is not working. We had a little SDI to HDMI converter knocking about and I thought I’d use that. I tested and the set up with the converter plugged into the wall and it worked fine so I split the power cable coming from the V mount battery on my Steadicam, plugged the Converter and the monitor in, plugged the SDI into the converter and the HDMI from the converter to the monitor and smoke came out the top of the monitor!

I unplugged everything and checked the monitor, it’s working fine thank god. Checked the converter and it’s also working fine, checked the wires and they are all fine. I also checked how much power was coming down the HDMI, it’s about 5V plugged into the wall and the same plugged into the battery.
I don’t know how much should be coming down the HDMI but it worked just fine when the converter had a separate power supply so I’m thinking it must be shorting somewhere in there.

Electrics is not my strongest area so I thought I’d come here and see if anyone has any experience or ideas what’s going on. Is 5V about right for a HDMI feed? Does this mean that I can’t use my Marshall monitor with these little converter without a separate battery?

Which converter do you have? There are a lot of "little" ones. What is the power spec on the Marshall monitor? In fact a good place to start is to list up all the components you're using and their power spec that should be on them somewhere and than make a little sketch of what's attached to what.

It's possible to have power coming via the HDMI line depending on what equipment you have attached where. The idea was that the HDMI line can provide low power to some circuitry in a regular TV when the TV is in "sleep" mode. At least that's how I understand it. IF (big if) something is putting a voltage on the HDMI line and the monitor doesn't expect a voltage on the line, strange things could happen.

When did the smoke start? If it started to smoke as soon as you plugged in the HDMI cable, this is a clue as to where the issue is. I forget which HDMI pin can carry power.