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speedystar77 Samba Member
Joined: June 26, 2015 Posts: 76 Location: South Central Idaho
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Posted: Sun Jan 17, 2021 3:28 pm Post subject: Tachometer illumination too bright |
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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!
_________________ 1974 Baywindow Westfalia Bus
Tintop Campmobile
Production date: November 29, 1973
1700 cc engine with manual transmission
32/36 DFEV Weber Progressive |
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Abscate Samba Member
Joined: October 05, 2014 Posts: 22670 Location: NYC/Upstate/ROW
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Posted: Sun Jan 17, 2021 3:56 pm Post subject: Re: Tachometer illumination too bright |
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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. _________________ .ssS! |
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bsairhead Samba Member
Joined: October 08, 2008 Posts: 3580 Location: viroqua wi.
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Posted: Sun Jan 17, 2021 4:37 pm Post subject: Re: Tachometer illumination too bright |
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If the tach has a replaceable lamp I would try to find a lesser wattage one. |
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SGKent Samba Member
Joined: October 30, 2007 Posts: 41031 Location: Citrus Heights CA (Near Sacramento)
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Posted: Sun Jan 17, 2021 5:18 pm Post subject: Re: Tachometer illumination too bright |
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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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_________________ “Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin |
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speedystar77 Samba Member
Joined: June 26, 2015 Posts: 76 Location: South Central Idaho
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Posted: Sun Jan 17, 2021 5:45 pm Post subject: Re: Tachometer illumination too bright |
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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?
_________________ 1974 Baywindow Westfalia Bus
Tintop Campmobile
Production date: November 29, 1973
1700 cc engine with manual transmission
32/36 DFEV Weber Progressive |
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airschooled Air-Schooled
Joined: April 04, 2012 Posts: 12728 Location: on a bike ride somewhere
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Posted: Sun Jan 17, 2021 6:05 pm Post subject: Re: Tachometer illumination too bright |
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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 _________________ Learn how your vintage VW works. And why it doesn't!
One-on-one tech help for your Volkswagen:
www.airschooled.com |
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3555 Location: San Diego (Encinitas)
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Posted: Mon Jan 18, 2021 1:39 am Post subject: Re: Tachometer illumination too bright |
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From the Gallery - adjustable dimmer, for an exact brightness match:
Note: does NOT work with LEDs. _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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Abscate Samba Member
Joined: October 05, 2014 Posts: 22670 Location: NYC/Upstate/ROW
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Posted: Mon Jan 18, 2021 4:34 am Post subject: Re: Tachometer illumination too bright |
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You can also put apiece ofwindowfilm over it.
_________________ .ssS! |
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