LM317T and Relay (Need Help)

Ok so I'm using an LM317T to controll the current to some Red Star-O's

I'm trying to figure out an easy and cheap way to vary the brightness when I activate a switch.

I'm thinking there should be an easy way to do this with a relay but I don't have ay experience with them.

Basicaly I have 12V Always going to the lights, when I hit a particular switch a seperate 12V line becomes available. (I want the lights to get brighter when I activate this switch)

Is there a way I can vary the resistance (add more resistors in parallel) going to the LM317T with a relay wired to the switch?

If not, is there a way I can switch from driving them with one LM317T to another LM317T with higher current output through a relay?

Probably a dumb question but I have not yet needed a relay for anythign so I don't know. If Possible part numbers from ratshack would be awesome! Smaller the relay the better!

I think the circuit gurus need to see your circuit before offering more detailed help.

Next, a relay is usually used so that a small(er) control current can switch the 'main' current.

You can use a LM317T in either a constant voltage OR a constant current configuration, so saying that you "have 12V always going to the lights" isn't quite descriptive as to what the circuit is doing.

You can have a circuit using a LM317T and a switch so that you can have two output levels, you'll just switch in or out a second resistance R_adjust_2 in parallel with R_adjust_1 to change the value of R_adjust that the circuit is using. No relay needed, just a plain switch and a second resistor (or resistor bank if you need to tweak the value).

Remember that the LM317T is internally limited to 1.5A MAX output, so if your 'bright' setting needs more juice than that, a simple LM317T circuit won't work.

How much current do you want to 'feed' the LEDs? How many?

Is your 'original' 12v source capable of delivering the needed current or is it somewhat limited, hence, the need to parallel another 12v source?

If you need more than 1.5amps, you can parallel another 317, a power transistor or use a 338 instead.

This sounds like an automotive turn signal:thinking:I'm listening for the answer too.

MoonRise said:

I think the circuit gurus need to see your circuit before offering more detailed help.

Next, a relay is usually used so that a small(er) control current can switch the 'main' current.

You can use a LM317T in either a constant voltage OR a constant current configuration, so saying that you "have 12V always going to the lights" isn't quite descriptive as to what the circuit is doing.

You can have a circuit using a LM317T and a switch so that you can have two output levels, you'll just switch in or out a second resistance R_adjust_2 in parallel with R_adjust_1 to change the value of R_adjust that the circuit is using. No relay needed, just a plain switch and a second resistor (or resistor bank if you need to tweak the value).

Remember that the LM317T is internally limited to 1.5A MAX output, so if your 'bright' setting needs more juice than that, a simple LM317T circuit won't work.

Click to expand...

Afte talkign to some CPFers last night I found i can add aditional Current LM317T's in current limiting config with diods, however, after some thought I came with the same idea you described.

I'll just wire the switch between the Vadj (additional resistor) and the Vout line so when I trip the switch the resistance is lowered and teh current setting will increase.

with a 10 ohm and a 6 ohm resistor (probably 2 12 ohm resistors in parallel or whatever I cna find at ratshack to make a 6 ohm lol) I can achieve my 125ma and 333ma which switch is tripped.

Thanks for the reply you jus totaly confirmed what I was thinking. For some reason I was set on using the switch with a seperate 12V line. I think because the switch in stock location does this to trip the second filiment in the brake light. So I just need to compleatly change what the switch is doing!


Yep this is for a brake light!