[TR] Converting TR6 rheostat to LED use

[Ron Olds]

Everyone,

I have been following this chain and have to admit that not being an electrical engineer I am getting lost and confused.  I have a good opportunity now to convert my gauge lights to LEDs and have the following basic questions.

1.  Can I simply replace the original bulbs with LEDs? I am not concerned about ever dimming them.

2. Is it ok to leave the dimmer switch in the circuit if I leave it in the position for full power at all times?

3. Will a 12v LED work with the ~13.6 volts in a 12V circuit or do I need to uses an LED with a higher rating than 12V?

Thanks for the help. 

Ron Olds
72 TR6
50 MGTD
-----Original Message-----
From: Triumphs [mailto:triumphs-bounces at autox.team.net] On Behalf Of Randall
Sent: Thursday, February 22, 2018 17:40
To: 'John Innis' <jdinnis at gmail.com>
Cc: 'Truiumphs' <triumphs at autox.team.net>
Subject: Re: [TR] Converting TR6 rheostat to LED use

> The other issue with the LM317 is
> that it is not very efficient.  All the gains you get from going to 
> LEDs are essentially going to be lost as heat from the regulator.

Power consumption with the LM317 will always be lower than without it; so you're not "losing" any power.  The power dissipated in the 317 will be matched by less power dissipated in the LEDs and ballast resistors.

Besides, we're not talking about enough power to worry about on a car.  That whole chain of LEDs only draws 6 or 7 watts at 12v.  If you lose a watt
(0.08 amps @ 12v), who cares?

The main disadvantage (if you want to call it that) is that the LM317 has a minimum forward voltage drop of 1.25 volts; while the PWM module should be under 0.5 volts.  So even at "max" brightness, the LEDs will be a tiny bit dimmer with the LM317 feeding them.  Unless of course you rig a switch to bypass it for full brightness :)

-- Randall


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