ATTN elec. Gurus, lighting/dimming question

[jjunior887]

I just got a set of Omori gauges installed and I know it isn't possible to have them dim with the lighting controls(at least w/o tearing them apart and separating the grounds somehow) but I wanted to know if it would be possible to put a resistor or something on the power side of the lighting wire and dim them a bit that way.

It would be even better if a POT could be used to adjust it if you needed to for any reason. Would this be possible or am i asking too much? If so what size resistor or POT would i need to reduce it say 1-2 volts or so. Thanks-James

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[GLwagon]

I'm just guessing that they are comon grounded & the light is switched positive & you don't want to totally tear into the gauges... (I don't have Omori gauges)

If you want a simple resistor 1st we need a starting point...
how many watt lamps are they each?

you could go totally gheto... get a headlight switch/dash light dimmer out an old salvaged 70's-80's car or truck... figure out the couple of wires on the varistor & hide it up under the dash (if it doesn't get too hot...)
On second thought you probibly shouldn't do that.

[jjunior887]

I wish I knew the wattage, I guess I can see if there is somewhere that says on them but it's a pain to get them out of the pod now that they are in(even harder to get them back in) .

I think there are 2 bulbs in each gauge and there are 3 gauges all tapped to the same power wire for lighting(the plug behind the radio for the stock gauges, it's a constant 12V as far as I can tell) if that helps any.

Yes, common grounded and no tearing apart the gauges, I would rather just leave them alone if that is the only option. Thanks-James

[GLwagon]

Yes, Subaru uses the negitive to dimm & the positive is a full 12+ volts... at least that is switched.

The only things (avalible at RadioShack) that come to mind would be some Diodes in series (there is some loss in a diode not a lot though...)http://www.radioshack.com/product.as...Fid=276%2D1661

or if the wattage was low enough... (I can't find the high wattage ones... kina like speaker "L-pads" usualy good up to 25+ watts)
http://www.radioshack.com/product.as...5Fid=271%2D265

I am NOT saying go & buy this stuff & it will work...
I'm saying with out the gauges in my hand this is what comes to mind...

you would think there is a kit for this some where... Hmmmmm...

[mkultra69]

I taped into the plug in the back of the radio and my autometer dims....... what am I missing here???

[jjunior887]

Omori gauges use two ground wires that are connected togetehr inside teh gauge(this is teh problem). Autometer has a positive and a negative specifically for teh lighting where teh Omori shares it's ground for teh lighting with teh electronics in teh gauge.

The problem is you can't connect teh ground to teh lighting ground(teh one that varies with teh dimmer) because it does funky things to teh readings on teh gauge.

BTW GLwagon ,i can't seem to find anyhting on the wattage of the bulbs, any idea what an approximate guess would be, they should be about the same as the Greddy gauges if that helps any -James

[mkultra69]

oh. never seen omori gauges. well do you wanna dim tehm permanently? If so just slap a resistor in tehir.


sorry got to thinking, correct if Im wrong. the lighting circuit , and the gauge circuit are wired together in the gauge?

[jjunior887]

I can deal with it being permanent but I'm not sure what type or size of resistor to use. It would be better if i could use something like what GLwagon posted that is somewhat adjustable so I could dial it in to match perfectly and leave it alone. I don't plan on adjusting my dimmer much if that's what you mean.-James

[mkultra69]

I think tehir are 2 bulbs in each gauge and tehir are 3 gauges all tapped to teh same power wire for lighting(teh plug behind teh radio for teh stock gauges, it's a constant 12V as far as I can tell) if that helps any

so is there 1 wire for the lights & 1 wire for the gauge power??

[jjunior887]

Yes

[FatJoe]

I just bought some greddy gauges and one autometer and I was wondering how I could hook them up to the dimmer switch on the the headlight extension (the thing on the left side of the steering column) so that they dimmed and turned on along with my stock gauges, for a kinda OEM look. Thanks.

~Joe

[jjunior887]

bump

[Hope2BinWRC]

Do you want to adjust the dim of the Omori's cause they are too bright and don't match?

[jjunior887]

basically yes, i like to run my dash lights one notch below full and they are a bit bright to do this.

[jjunior887]

Come on guys there has to be someone out there that figured this out already. If someone can give me a rough estimate on what to look for I can try a few things and see if any of it works.

I think the Rheostat that glwagon posted might be a bit small for the wattage these run but I don't know because i can't find the wattage of the bulbs, anyone have any suggestions? What would happen if I put it in and itr was too small would it just fry the rheostat or would I be in for a fire or something? If it just blows the rheostat I might consider it but i don't want to f up my gauges over trying to figure this out. I guess it would help if I knew what the f I was doing -James

[IA Performance]

I too am in the same boat as jjunior887. I have Omori gauges that are a tad to bright when compared to my stockers. I want to put a diode or resistor in line of the power illumination wire to cut down on the current. What size to buy?

The wiring layout of the Omori gauges is as follows:

1 black = ground
1 black = ground
1 red = gauge power
1 white = gauge illumination
1 brown = is external output, i.e. warning light, etc.

In the end all I want are the 3 of them to be dimmer all the time.

[GLwagon]

Originally posted by East Coast Scooby
The wiring layout of the Omori gauges is as follows:

1 black = ground
1 black = ground
1 red = gauge power
1 white = gauge illumination
1 brown = is external output, i.e. warning light, etc.

In the end all I want are the 3 of them to be dimmer all the time.

??? why the 2 "black" listings... ??? what does each do ?

The dimming would be done from the "white" wire...(unless one of the black wires actually was the light???)
If some one could get an amperage reading on the "white" wire then I would have a clue as to know how beefy the components would need to be...
As discussed if the wattage was known... bla bla bla...

[jjunior887]

as i said before there are 2 black wires, both being ground but they are connected inside the gauge for some reason. When you hook up one to the dimming wire for the stock gauges and the other to ground they won't dim with your gauges. I didn't test it but I would assume it wouldn't allow you to dim the dash lights either since it basically grounds both sides when you ground one and it woulkdn't allow the voltage to fluctuate with the dimmer switch. I'll see if i can figure out what kind of wattage they are pulling, any suggestions on how? I'm not very good with a multimeter just to warn you -James

[jjunior887]

Tell me if this sounds right, like I said I'm an idiot with this multimeter. I measured the amperage( I think ) between the wire I have running to my three gauges and the connector behind the radio that provides the power for my lighting, it looks like it was about 1/2 amp or so if that sounds right(I think I was dong it right).

I checked the one that changes voltage and it went from about 1V(dash lights fullt lit) to about 11 V( lights completely dimmed) When I hooked up the gauges to the 1-11V wire it would be perfect when the rest of the gauges were dimmed all the way if that helps any.

I guess what I'm saying is the gauges look perfect(or very close to it with about 1 less volt than what they are getting now which is about 12V with the engine off). If I need to do anything differently let me know and does 1/2 amp sound about right?

I really have no clue or if it's possible with what I have to measure the wattage. It looks like if I did this right This should work. Thanks for all your help Glwagon(sorry don't know your name )-James

[GLwagon]

Ok...
When you take an amperage reading with a non-clamp on meter you set the dial to the highest range (unless auto sensing) in AMPS "A" or "I"...
The leads should have no polarity in this mode & color isn't an issue...
The meter goes in series with one of the conductors (like you had described)... So at this point it should go:
POWER WIRE----meter lead----MULTI METER----meter lead----GAUGE WIRE (for light).
This is should take care of the positive from the car & white from the gauge. The blacks are already hooked up somewhere else...
Now the needle or LCD should be reading something... If the reading is very low then you can adjust the "range" to a lower more sensitive one... The light on the gauge should be lit up too.

Some basics... If you have 12volts at 0.5 amps = 6 watts (V x A = W)
If you got a reading around 0.5amps for one gauge then x3=1.5amps or 18watts total...

At this point it is established that the nice wirewound 3-watt rheostat is NOT going to work.

Now from what I rember... the voltage drop across a diode might be enough to notice...
If it were me, I'd buy one pack of these --->Diodes. I'm not going to recomend the 3amp diode packs just to be on the safe side...
They are polarity sensitive (basically an electron one way valve)
Start with one between the white wire from the gauge & the positive from the dash... IIRC put the silver end of the diode towards the dash (If it doesn't work flip it around, silver towards gauge) if not dim enough add another in series:
WHITE WIRE----diode----diode----POWER WIRE... The silver will have to go the same direction on each one... I wouldn't think you would have to add more than 3-4 at most...
Solder is your friend.
You should be able to hook all 3 gauges together no problem...
? make sense ?
Let me know if & how well it works...

[jjunior887]

That's exactly how i hooked up the meter and it is auto sensing so it was reading 1/2 a watt for all three gauges together. Basically I wired all the White wires together from all three gauges(1 each) and soldered them to a larger gauge wire which runs to the power for the lights.

This may have not been the best way to do it but it works and I havn't seen any problems yet. If you can think of why this is a bad idea let me know and I will change it so I don't cause any problems in the future.

Very nice instructions for an electical idiot like myself, I'll try it out and let everyone know how it goes. Thanks alot GLwagon-James

[ride5000]

Tell me if this sounds right, like I said I'm an idiot with this multimeter. I measured the amperage( I think ) between the wire I have running to my three gauges and the connector behind the radio that provides the power for my lighting, it looks like it was about 1/2 amp or so if that sounds right(I think I was dong it right).

hey james, i'm going to help you out here.

that 0.5 amps will come in handy. keep reading...

I checked the one that changes voltage and it went from about 1V(dash lights fullt lit) to about 11 V( lights completely dimmed) When I hooked up the gauges to the 1-11V wire it would be perfect when the rest of the gauges were dimmed all the way if that helps any.

yes, it certainly does help. when the gauges are dimmed all the way, the +13vdc to them stays solid, but the ground reference has been moved up to 11vdc. that creates a potential difference of 2vdc across the dash lights, making them dim.

you state the brightness of the aftermarket gauges is perfect at the 11v mark. that means you've got to drop 13-11 or 2vdc across some component to make the perfect intensity.

this is where the 0.5a comes in. v=ir, which is ohms law, relating voltage, current, and resistance. let's see what variables we have... we have current, i, which is 0.5a. we have voltage, v, which is 2vdc. so we have 2vdc = 0.5a * r. that means r is equal to 2/0.5, or 4. the units of resistance are ohms. therefore a resistance of 4 ohms should create the exact intensity you desire.

now, we're not exactly done yet... we've got to make sure that the POWER dissipated in your resistance is not greater than about half of what the resistor is rated for. for this we use a different formula, p=iv. p is power in watts, i is current, and v is voltage. in our case, i = 0.5, v is 2, so p = 1w. therefore you need to use a 4 ohm resistor rated at least 2w. this is not much power and should be easy to obtain.

remember you can hook multiple resistors together to get different values.. for example, two 8 ohm resistors in PARALLEL will net you 4 ohms. two 2 ohm resistors in SERIES will do the same. in these configurations, the power dissipated will be evenly divided amongst the resistors, so you can get away with smaller power rated parts.

it just so happens that radio shack stocks 10w 8 ohm resistors for a few bucks each. it's overkill but they will remain nice and cool and you would never have to worry about them again. put them in parallel, attach one end to the steady 13vdc lighting circuit in the car, and attach the other end to the +ve side of the lighting connections in your aftermarket gauges.

if you want to get really fancy, we can talk about making it adjustable. first, try this out to verify the current measurements are correct and your ideal luminous intensity is on the target.

hth
ken

[jjunior887]

Wow, thanks a lot Ken, that's a really elaborate post, the funny thing about it is I understood most if not all of it. I may try both of these methods(the one ride5000 posted as well as the one GLwagon posted) just to see which one works better(or if they are the same). BTW adjustable would be pretty nice too and it might be a nice little project for me to work on.

I'll let you guys know what I find out. I wish pictures would do justice but I'm afraid you won't be able to tell the difference so I guess you'll have to take my word for it. Wish me luck-James

[GLwagon]

I didn't bother with the sand cast resistor route because I "assumed" it would be an oddball resistor needed... my bad...
The parts aren't overly costly so try either or both...

my other bad... edit my above posts for the white wire NOT the red one...

[turboICE]

Ahh just smoke the gauge face!

Actually good thread, since I have Omori coming. I will likely be back here wanting to use the stock dimming function rather than a permanent setting.