Rheostat resistance and supply voltage

[Michael Carter]

The Grimes cockpit map light I bought is wired to the 12VDC buss only for convenience as I didn't have a 28VDC bulb to wire it to the 24VDC buss.

My question is, because of the resistance of the rheostat used for the dimmer, is this causing the lamp to be dimmer on 12VDC than it would be on 24VDC? Seems to me it would.

The lamp works fine, but I don't think it's as bright as it should be even when turned all the way up. I'm wondering if the rheostat has a higher resistance than would be required for a 12 volt circuit.

[andarlite]

Is there any difference in brightness if you bypass the dimmer rheostat and just connect the light directly to 12V?

Rgards,
Henry

[Michael Carter]

Yes, but I haven't performed the same test at 24VDC.

[andarlite]

Then I would tend to agree with your assumption.

regards,
Henry

[fweinrebe]

The brightness is dependant on the wattage of the lamp. You could have a low wattage 12V bulb where the 24V might have a higher wattage and therefore appears brighter. A 24W 24V bulb draws exactly the same amount of current (1 Amp) as a 12W 12V bulb (P=V x I).

Another factor is that when the rheostat is set to the maximum brightness, is there any resistance on the rheostat? Normally at the full setting it is 2 metal parts in contact with each other, so there should be no resistance.

Try a higher wattage 12V lamp.