Dash light rheostat or other dimming device assistance requested

[crazybry79]

I have a cabin cruiser (boat) that I have just built a new dashboard for and installed all new gauges. Previously there was no dimmer switch for the dash lights. At night the old gauges were blinding while cruising. The new gauges are brighter yet, so I'm sure they'll be even worse.

Inside each gauge is a 12v 194 bulb (same style you'll find in an automotive dash or side courtesy light). These are regular non LED bbulbs.

My question is, can I just install a rheostat in the power supply wire? Or is it more complex than that?

 

[crazybry79]

Here is what I'm working with. There are 8 gauges that will be illuminated.

 

[KrisBlueNZ]

Hello there! Welcome to Electronics Point

Thanks for identifying the bulb type. All the "194" bulbs I've found are rated for 14V and 0.27A (amps). When they're connected in parallel, the currents add together. So eight of them will draw 2.2A at 14V.

If you connect a rheostat, the rheostat will have to dissipate a significant amount of power, i.e. heat. This dissipation will be worst when the bulbs are running at around half voltage. There will be 7V across the rheostat and the bulbs will be drawing about 1.1A so the rheostat will be dissipating (7 * 1.1) = 7.7 watts. There are wirewound rheostats available that will do this. See http://www.digikey.com/product-detail/en/RES50RE/RES50RE-ND

That one is rated for 12.5 watts dissipation and costs USD 28. It has a resistance of 50 ohms, which will give a reasonable range of adjustment. It might give you more dimming than you need, in which case you can get a 25 ohm one: http://www.digikey.com/product-detail/en/RES25RE/RES25RE-ND

Digikey also have 25 watt rheostats. These may be more reliable because they won't get so hot. See http://www.digikey.com/product-detail/en/RHS50RE/RHS50RE-ND (50 ohm, USD 32) and http://www.digikey.com/product-detail/en/RHS25RE/RHS25RE-ND (25 ohm, USD 32).

Connect it in series with the positive supply to all of the light bulbs. Use the middle terminal and the end terminal that the rotating contact is next to when the spindle is turned fully clockwise.

A rheostat is definitely the simplest solution, but there are other options - an adjustable voltage regulator, either linear or switching type. If the rheostat does the job, I would go with that, because a boat is not the best environment for electronic circuitry.