| Author |
Message |
Jack Conrad (Jackconrad)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 25, 2005 - 9:37 am: | |
I want to install a rheostat to dim the gauge lights in my new instrument panel. I tried using the OEM rheostat, but the least movement simple shuts off the lights. It was for 5 bulbs at 24 VDC. I now have 14 bulbs at 12 VDC. I have not had time to check for current draw per bulb (I am on duty today at the Fire Station). All bulbs are #53. Thanks in advance, Jack |
Gary Stadler (Boogiethecat)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 25, 2005 - 12:30 pm: | |
Hi Jack,
A #53 bulb draws .12 amps, so 14 of them will total out at just under two amps on 12-14 volts. Your 24 volt rehostat is probably much too high in resistance as you have seen.
If you want to use a rheostat, I'd say probably one in the 20 ohm range would work, and it would have to be rated at 10 watts or more to not get too hot.
Electronic Surplus has one that might be just about right for seven bucks although it is 2.4" diameter...might be a bit big.
Another possibility is to go to a junkyard and nab a little electronic unit out of a VW or other small import car. They use a little transistorized dimmer that works pretty well over a wide range of load, and they sell for $5 around here. The only hitch with those is that they use (+) as lamp-common, and switch the ground side of the lamp to dim it... meaning all your lamps can't be hooked to ground as their common terminal...they will all have to be isolated and hooked to (+)12. In a lot of dash lighting installations this is just not practical...
Hope that gives you some good options... |
Frank Allen
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 25, 2005 - 1:46 pm: | |
I got a reostate off a freightliner truck and it works great
Frank Allen
4106 |
Richard Bowyer (Drivingmisslazy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 25, 2005 - 6:40 pm: | |
Napa sells a dimmer rheostat for heater fans and the 8 inch defroster fans like came on most busses and similiar applications. These typically run pretty hot and they are rated at 25 watts wire wound. I do not remember the resistance but 100 ohms seems about right. You should get one and try it with the option to take it back for a lower ohmage unit. If the resistance is too high you will get only off and on with little in between.
Richard |
Tim Strommen (Tim_strommen)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 25, 2005 - 9:03 pm: | |
Cole Hersee has several Digital PWM dimmers for the heavy duty truck market. It doesn't care what current is being drawn (like fixed resistance ohm-based rheostats do...). These can be found/ordered at just about any truck or aftermarket-lighting supplier
C-H P/Ns (all with black knobs):
74600-02 4Amps (Max) @ 14.7VDC (Neg GND)
74600-09 4Amps (Max) @ 14.7VDC (Neg GND)
74601-01 7Amps (Max) @ 14.7VDC (Neg GND)
74603 4Amps (Max) @ 14.7VDC (Neg GND)
Cheers!
Tim |
James Maxwell (Jmaxwell)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 25, 2005 - 9:28 pm: | |
I'm confused here Tim. If it doesn't care what current (translates amps if I remember correctly) why does it come in various MAX amp ratings? |
Gary Stadler (Boogiethecat)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 26, 2005 - 2:25 am: | |
Tim, these are GREAT! Thanks for finding that source.
James, the current ratings are just the "most" the unit can power without overheating or otherwise damaging it. But it will work perfectly with loads from zero up to and including the maximum rating.
So if you are only using a few light bulbs that together draw say, 1 amp, you could use any of them just fine. But if you wanted to dim a headlamp that draws 6 amps, the only one that would work would be the 7 amps unit. Make sense?
These are a COOL way to go...pun intended!
Check thru their site for other neat stuff too!
cheerio |
Tim Strommen (Tim_strommen)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, July 27, 2005 - 11:12 pm: | |
Yes,
Gary is correct. The current is a limit but unlike a rheostat, you can burn out a bulb or two and keep your full dimming range (which allows parallel wiring for redundancy as opposed to losing your whole dash when one device/bulb/LED fails). In fact it isn't hard to make a digital PWM dimmer that can go to a higher current. One simply must pick a MOSFET or IGBT which can handle it (they make N-Channel MOSFETs that can handle up to 330 Watts, which could easily operate your fans, halogen lighing circuits, pumps solenoid valves, etc... Plus you can double or multiply them in parallel X ammount of times for increased current.) Want to build a N-Channel MOSFET based high side switch (clickless-relay/relay replacement)? Check these DIG12-06-250S out, this is what I'm using to control the cooling fan's MOSFETs (about 6 MOSFETs total) on my rig. you could also use these with low current column type automotive headlight switches to keep the dash current and voltage low, and directly opperate high current/voltage devices (isolator uses the same current/voltage as a high-brightness LED ~3.4VDC @ 50mA).
The feature I like about MOSFETs the most is that their conductance is inversley proportional to their resistance. What this means is if you parallel two or more MOSFETS they will naturally balance out the current draw between themselves (you don't have to get macthed pairs like with bipolar transistors...)
Cheers!
Tim |