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TR6 TR6 voltage to dashboard indicator lights

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J

Jere

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I am working on a 1976 TR6. In the process of slight modifications I am installing new smiths gauges. The speedometer and tach do not have the small indicator lights for beam; turn; ignition; and oil.

I would like to know the voltage and amps giong to those very small lamps. I'm wondering if I can utilize 1volt led lights?

I appreciate your help.
 
Voltage should be roughly battery voltage, meaning they need to handle anything from about 9 volts up to 15 or so. Original bulbs drew about 180 ma I believe (2.2 watts), but it might have been less. And of course your LEDs don't have to draw that much.

My limited experiments with somewhat similar lamps on the Stag said that even "ultra bright" LEDs were not as bright and even harder to see than the stock incandescent bulbs, when mounted behind the original filters. But the brightest available LEDs keep getting brighter, so there might be something suitable today.

Another point to consider is that most high-intensity LEDs have a fairly limited viewing angle. If you run them without a filter to spread the light, it might be a problem if they are not pointed roughly at your head (or the shortest/tallest person that might drive your car).

Obviously (I hope), you'll need dropping resistors for the LED lamps; although there are a few assemblies around that include the resistor inside.
 
I believe that the voltage regulator is supposed to put out a constant +10V to feed the instruments and dash lights. Don't know how well (reliable) the old mechanical voltage regulators worked. When I did the resto on my TR6 I replaced it with a solid state voltage regulator.
 
No,Gman. That voltage stabilizer really only affects the temperature and fuel gauges.
They should be 12 volt or as Randall puts it "battery voltage" bulbs, although by the time current passes thru the rheostat, I doubt if the bulbs get the full 12 volts.
 
Poolboy- Thanks for the clarification, that makes perfect sense.
 
Also, the stock voltage 'stabilizer' does NOT regulate to 10v. Instead, it switches between full battery voltage and zero, so that the average is 10v. The switching is quite slow; if you connect an ordinary bulb to it you can see the bulb flash on and off. (Which is actually a good first-level test to see if it's functioning.)

And the indicator lights are not dimmed by the rheostat, it's only for the gauge/dash illumination.
 
I'm in the same boat, as I am restoring my instruments. The "jewels" died as it came apart. So I need to come up with something to replace them. I wonder if that person who restores instruments, mentioned elsewhere on this forum, has a source.

I was also looking at "lightpipe" at www.newark.com as a possibility. I was also considering going the LED route.
 
For warning indicator lamps you can use LEDs. I don't think you'll have any issue with either "standard" or high-intensity LEDs. If you were talking about using LEDs to illuminate that gauge face... that's different. Randall is quite right that the LEDs are very focused compared to an incandescent bulb.

As mentioned above, the warning light circuits are at charging system potential. When you calculate the dropping resistor values for the LEDs, do not use 12V in your math, use 14V or 14.5V as that's where the voltage is with the charging system working.

Of the warning lamps you mentioned, you cannot use an LED directly for the charge warning light. The standard incandescent lamp is actually part of the circuit and allows a fair amount of current to flow to the alternator field coils when you first turn the ignition key to the run position. The LED will not allow enough current for the alternator to start charging.

There is a "work-around". Buy a 1/2 Watt or larger resistor rated for about 50 Ohms, wire this in series from the ignition switch to the small (warning light) terminal on the alternator. Once this is installed, wire your LED (with its dropping resistor) in parallel to the 50 Ohm resistor (one end at the ignition switch, the other at the alternator small terminal). The 50 Ohm resistor will allow enough current to flow for the alternator to start charging AND the LED will illuminate just like the traditional bulb.
 
Good point, Doug, I forgot about the alternator excitation. But I think the resistor should be larger than 1/2 watt; as with the key on but the engine not running, it will be dissipating over 2 watts; more if the battery is being charged from an external source. I'd use a 5 watt resistor just to be safe.

Adding the excitation resistor is a good idea anyway, as it will let the alternator keep working even if the lamp burns out or breaks.

And you'll still need a separate dropping resistor in series with the LED (which of course can be much lower power).
 
TR3driver said:
And the indicator lights are not dimmed by the rheostat, it's only for the gauge/dash illumination.
Click to expand...
Right. I guess I lost my train of thought. Sorry.
 
Randall, you're right, a larger Wattage resistor should probably be used. The current is "supposed" to only be flowing through the resistor for the amount of time it takes to start the engine (which shouldn't allow it to overheat). However, these are often Lucas alternators so if (by some odd chance) something were to fail, the warning lamp circuit could be carrying the current longer (like weeks until replacement parts arrive). A higher Wattage resistor certainly is justified.
 
Thanks to everyone who replied! It seems the simple answer would be to find a "12v" small light that would look good when installed in the dash below the new speedo and tach. then there is the question of finding four colors in the same style fixture. One thing always leads to another when you step outside of the original engineering.
 
dklawson said:
The current is "supposed" to only be flowing through the resistor for the amount of time it takes to start the engine
Click to expand...
I was thinking more of the case where the key is left on while troubleshooting, or even just to listen to the radio. I know, shouldn't happen (radio should be wired to accessory position so not necessary to leave ignition on), but I don't like designing in traps for the unwary.
I've even been known to limp down the road with a broken fan belt, which would cause the same thing.
 
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