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Tachometer illumination too bright
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speedystar77
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PostPosted: Sun Jan 17, 2021 3:28 pm    Post subject: Tachometer illumination too bright Reply with quote

I am hoping to add a tach like this to my '74 Baywindow and have read the illumination is significantly brighter than the existing instruments.

Anyone have any experience or advice in adding a resistor in line with the bulb circuit?

I read somewhere to use ceramic 20ohms ?

Not sure what wattage?

I'm an electrical neophyte-any help would be appreciated.
Thanks!
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Abscate Premium Member
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PostPosted: Sun Jan 17, 2021 3:56 pm    Post subject: Re: Tachometer illumination too bright Reply with quote

Ill SWAG this as the lights are probably running at about 2 Watts power on that gauge.

I - current in Amps
V = voltage in Watts
P = power, or Energy per unit time

Ohms Law Voltage drop = CUrrent x Resistance , V = I x R

Power Law Power = I x V = Ix I x R

If the gauge light is 2 Watts then...

Current I = P/v = 2 Watts/ 12 Volts or 0.16 Amps

DC resistance is R = V/I = 72 Ohms

So to cut the current in half, you could match the resistance of the Gauge. I thnk this would be too dim and the 20 Ohm estimate is probably closer. If you want to match your other gauges I would buy a bunch of resistors and experiment.
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bsairhead
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PostPosted: Sun Jan 17, 2021 4:37 pm    Post subject: Re: Tachometer illumination too bright Reply with quote

If the tach has a replaceable lamp I would try to find a lesser wattage one.
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SGKent Premium Member
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PostPosted: Sun Jan 17, 2021 5:18 pm    Post subject: Re: Tachometer illumination too bright Reply with quote

I had the same issue. Telford gave me a diagram to make a transistorized dimmer circuit that trims the Tach to the same level of brightness. Works really well.

SGKent wrote:
here is a dimmer box I made from a schematic Telford provided. It is around here somewhere. It dims the VDO tach, and 4 gauges so they are the same brightness as the factory lights and since it controls the current for the VDO units the dimmer switch doesn't carry the load. It is designed so that the lights on the VDO go up and down with the dimmer switch so the adjustment knob on the box is only to fine tune them one time to the other lights.

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speedystar77
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PostPosted: Sun Jan 17, 2021 5:45 pm    Post subject: Re: Tachometer illumination too bright Reply with quote

SGKent wrote:
I had the same issue. Telford gave me a diagram to make a transistorized dimmer circuit that trims the Tach to the same level of brightness. Works really well.

SGKent wrote:
here is a dimmer box I made from a schematic Telford provided. It is around here somewhere. It dims the VDO tach, and 4 gauges so they are the same brightness as the factory lights and since it controls the current for the VDO units the dimmer switch doesn't carry the load. It is designed so that the lights on the VDO go up and down with the dimmer switch so the adjustment knob on the box is only to fine tune them one time to the other lights.

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Beautiful set up!
I like the idea of using a reostat to dial in the correct resistance. It is really not much more cost than a resistor.

One like this might work?
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Production date: November 29, 1973
1700 cc engine with manual transmission
32/36 DFEV Weber Progressive
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airschooled Premium Member
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PostPosted: Sun Jan 17, 2021 6:05 pm    Post subject: Re: Tachometer illumination too bright Reply with quote

I use a 20ohm ceramic in-line and the tach is just a hair brighter than the stock gauges at full output. Colin wires the stock gauges to full brightness and uses the headlight switch rheostat to dim the tach.

If you run the potentiometer, get it looking perfect and measure the resistance please Smile
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telford dorr
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PostPosted: Mon Jan 18, 2021 1:39 am    Post subject: Re: Tachometer illumination too bright Reply with quote

From the Gallery - adjustable dimmer, for an exact brightness match:
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Note: does NOT work with LEDs.
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PostPosted: Mon Jan 18, 2021 4:34 am    Post subject: Re: Tachometer illumination too bright Reply with quote

You can also put apiece ofwindowfilm over it.

Laughing
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