Tachometer illumination too bright

[speedystar77]

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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1974 Baywindow Westfalia Bus
Tintop Campmobile
Production date: November 29, 1973
1700 cc engine with manual transmission
32/36 DFEV Weber Progressive

[Abscate]

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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.ssS!

[bsairhead]

If the tach has a replaceable lamp I would try to find a lesser wattage one.

[SGKent]

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.

[speedystar77]

[airschooled]

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
Robbie
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[telford dorr]

From the Gallery - adjustable dimmer, for an exact brightness match:

Note: does NOT work with LEDs.
_________________
'71 panel, now with FI
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Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[Abscate]

You can also put apiece ofwindowfilm over it.


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.ssS!