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who is good with electricty and resistors ? UPDATED : WITH PIC PAGE 2

sweetk30

sweetk30

Back to play nice .
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its been soooo many years i forgot what i use to know on this stuff .

12volt gauge light YOU CANT change the bulb and i need to dim it down a little bit .

what size resistor should i add inline to the feed for it ?

12volt and just 3watt draw rating for the system . i am thinking 1/3 down brightness and 1/2 down brightness so i have a choice .

thanks for any help .

i dont need my tach blinding me anymore in its new spot on the dash compared to the old spot .
 
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You will need to try out different size resistors to get the brightness you desire since brightness is not directly proportional to watts used. You would be better off using a 100ohm variable resistor to find the brightness you desire and measure its resistance.

So at 12v using 3watts it would draw 1/4 amp (P=VI) and the resistance of the filament in the bulb would be 48 ohms(V=IR). But the filament resistance will drop a little when it has less current flowing through it.
If you want 9v across the bulb then you could try 15 ohm 1/2 watt resistor (I’ll go through the calculations if you’d like), but like I said, resistance of the filament will change and brightness is not linear. Next standard resistor values close to 15 ohms are 12 and 18 ohms.
 
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Just used that link you provided. Only way to get it to work correctly is to enter the final current value after you add the resistor (0.2A), which you don’t know unless you manually go through the calculations. :doah:
 
Just build a PWM controller....

Do you have an ammeter to measure what it draws now? How about a bulb part number to know what the power should be? The amount of resistance you need varies a lot depending on the bulb power. Don't forget that a resistor can get hot, so power rating and mounting should be considered. Bulb resistance varies with current and brightness is not linear with applied voltage, so without a lot of math, trial and error is your friend. I suggest math just to get in the ballpark and have the right parts on hand. Another option is to buy a pack of diodes and start chaining them in series with the bulb until the brightness is dropped enough for your liking (each one lowers the applied voltage about 0.7V). An advantage to multiple resistors or diodes is spreading the heat out across them.

iv-graph-of-filament-lamp.png


EDIT: after talking about building a PWM controller I realized you can buy them super cheap now. If you search on eBay for DC bulb dimmer the choices start under $3.
 
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You could hook just the tach's light wire to the "LPS" tap in the fuse box,then the bulb can be dimmed by the headlamp switch,along with the other dash lights--if you only want to dim the tach alone,then you'll need to experiment with resistors..a ballast resistor or one off a blower motor might work,but I'd be careful where it mounts because they can get very hot..
 
Blue85 said:
Just build a PWM controller....

Do you have an ammeter to measure what it draws now? How about a bulb part number to know what the power should be? The amount of resistance you need varies a lot depending on the bulb power. Don't forget that a resistor can get hot, so power rating and mounting should be considered. Bulb resistance varies with current and brightness is not linear with applied voltage, so without a lot of math, trial and error is your friend. I suggest math just to get in the ballpark and have the right parts on hand. Another option is to buy a pack of diodes and start chaining them in series with the bulb until the brightness is dropped enough for your liking (each one lowers the applied voltage about 0.7V). An advantage to multiple resistors or diodes is spreading the heat out across them.

iv-graph-of-filament-lamp.png


EDIT: after talking about building a PWM controller I realized you can buy them super cheap now. If you search on eBay for DC bulb dimmer the choices start under $3.
Click to expand...


no replacment bulb option as this is built in to the unit up front . https://www.autometer.com/2-3-4-tach-6-000-rpm-mini-black.html

has to be seperate from the rest as its brighter than the rest and is a bit overpowering .

heck i just found i forgot i had a old headlight switch . i could run it threw it as a seperate unit and just zip tie it up under the dash . . :dunno::thinking:
 
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I didn't say to replace the bulb. I just said to measure the current so you can pick a resistor. That's the advantage to the diode drop method - the voltage and brightness reduction are similar over a wide range of current draw. If you want it about 1/2 as bright, put 5 diodes in series, for a 3.5V reduction. The other advantage is that it can still be placed on the dash light control, so it dims with the other dash lights.

(OK, the current matters for power considerations. Let's assume the bulb is 2W or less, so the power in the diode is under 1/8W. If that's not true, you need to have parallel and series diodes to share the power or use fewer large power diodes, which have more voltage drop).

If you don't care about the tach dimming with the other dash lights, you can use an adjustable regulator like this: https://www.amazon.com/DZS-Elec-Adj..._rd_t=40701&psc=1&refRID=6VD8QT58ERK59SHPPRWH. Probably cheaper than a power rheostat. You could also use an old school LM317 regulator board.
 
dude that like way more complicated than i need .

i just wanted 1 or 2 resistors to put inline before the dash light tie in job .

reason i showed the link and said its built in is i dont know what size bulb it is . i could try and measure draw threw my dvom .
 
You said it’s 3W at 12V, which means 0.25A (P=IV). But if you want precision I guess you could cut a wire somewhere and insert your ammeter.
 
its not wired in this truck yet . setting it all up and remembered from old truck it was brighter than the rest .

i will see what i get .
 
sweetk30 said:
12.86 dc volt supply for testing
200ma range scale i get 66.7 on the read out display
Click to expand...
That doesn’t sound like an incandescent buld, more like a couple of LEDs. If that’s the case then a resistor won’t work and you’ll need a PWM.
So can we back up for a minute. Exactly what gauge are we trying to control (make/model/part #). You said it was 3W which is 250mA and now you’re measuring 66.7mA - why the discrepancy?
 
i was A$$uming that bulb was like any other in the dash system like a 161/168/194 . but as you see i am not the best at this this week .

i linked it before but here ya go again . it has 1 tiny little bulb inside at the 12:00clock / noon position up front . no way to change it as its ll glued together .
https://www.autometer.com/2-3-4-tach-6-000-rpm-mini-black.html

i used my jumper pack good size one not china cheepy junker . used it to power just the light portion only on just this gauge .
 
Missed that post somehow. :doah:
Yep, says incandescent bulb (no LED).
So you’re thinking of adding the resistor/diodes to the white wire that come off the headlight dimmer switch? If so, what you could do is use a 9V battery to test out how much it cuts down on the brightness. Just connect the ground of the 9V battery to the ground of the car battery and +9V to the white wire.
 
Two bulbs in series will glow dimmer than one...second bulb acts as a resistor..maybe wire in another bulb to the power feed and hide it somewhere where it wont be seen,or start anything on fire ?..
 
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