autotransformer ballast?


When I was troubleshooting a 1000w 240v MH luminaire, with a capacitor in series with the lamp on the secondary of the ballast, I measured my lamp socket to see my output voltage (yes I used meter rated for over 1000v) and I was getting 285v. Previously I put a new lamp and it didn't light so I checked the cap and it was good. So am I understanding this right in that the ballast had to have shorted out partially and that is why I was still getting a voltage above line voltage? What are some of the symptoms for a luminaire when the cap goes bad? Does it just not fire up? Does the capacitor in series just create impedance to limit the lamp current?


Staff member
Durango, CO, 10 h 20 min without traffic from wing
wire pulling grunt
I don't know much but here is what I do know....

Yes, the capacitor is there to control current. I don't know what the output of a MH ballast should be but I am not going to put my meter between the pin and the shell to find out.

Screw in a new lamp. If it works you are good to go. If it does not, change the ballast and move on. Fixing one part and then having to come back in too short of a time to make the light work again is not a good way to make a customer happy.


Electron manager
NE Nebraska
If it is a probe start system that may very well be the correct voltage.

Pulse start systems have an ignitor that sends a high voltage at startup to start the arc in the lamp. Probe start operates at same voltage starting as running, but needs to start a smaller arc gap first before it will arc the main gap. I believe there is a temperature activated cut out in the lamp that opens the circuit to the starting probe once it reaches a certain temp, if this fails to close you will never get that lamp to start even though it is good otherwise.