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    Connecting a low current LED to 150-300VDC source

    Hi. I'm new here, be gentle :)

    I have a circuit where I need a 12VDC current limited LED to light up when a voltage between 150 to 300VDC (2-4 A) turns on.

    I have a 12VDC power source that I could use to power the LED, but I basically need a good way to control it. I thought about using a relay that supports voltages up to around 300VDC on the control signal and a low current on the switched signal but I can't seem to find any that matches.

    Oh, and all of these things are external mounted devices.. No PCB components.

    Does anyone have any good ideas on how to do this?

    Thanks,

    Tomas

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    Re: Connecting a low current LED to 150-300VDC source

    You may use a low input current optocoupler and a bipolar transistor. See the attached diagram.

    http://www.vishay.com/docs/83722/sfh628.pdf
    http://www.fairchildsemi.com/an/AN/AN-3002.pdf

    Click image for larger version. 

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    Re: Connecting a low current LED to 150-300VDC source

    Is it possible you can power the LED from the high V supply? Or does the LED have to be remote?

    You can use a current-limiting resistor to drop the excess volts.

    300V through a 30k ohms resistor will allow 10mA through the LED. The resistor must dissipate 3W and it will get hot.


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    Re: Connecting a low current LED to 150-300VDC source

    Quote Originally Posted by mister_rf View Post
    You may use a low input current optocoupler and a bipolar transistor. See the attached diagram.

    http://www.vishay.com/docs/83722/sfh628.pdf
    http://www.fairchildsemi.com/an/AN/AN-3002.pdf

    Click image for larger version. 

Name:	Opto 1.png 
Views:	10 
Size:	29.0 KB 
ID:	72508
    Thank you mister_rf. In an attempt to keep time and spent money to a minimum, I want to avoid developing my own PCB for just this cause. But you are right, I am looking for something like this, but I need it or something similar to this as a complete mechanical part.

    I was, among others, looking at this solid state relay. http://uk.rs-online.com/web/p/solid-...elays/7360885/

    Unfortunately it goes only up to 280VDC for the control signal, and the relay has a minimum current of 20mA which is just around what the LED uses. I could of course buy one and test if it works anyway, it probably would. But the fact that it only goes up to 280VDC makes it a bit tough to use.

    Quote Originally Posted by BradtheRad View Post
    Is it possible you can power the LED from the high V supply? Or does the LED have to be remote?

    You can use a current-limiting resistor to drop the excess volts.

    300V through a 30k ohms resistor will allow 10mA through the LED. The resistor must dissipate 3W and it will get hot.
    Thank you for your answer BradtheRad :) Yes I can use the high voltage supply. I have actually used this solution earlier on a similar setup. I was hoping this time that I could avoid the high voltage at the LED, but if there is no better solution this will be the way to go.
    Last edited by Tomas04; 13th April 2012 at 11:07.



  5. #5
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    Re: Connecting a low current LED to 150-300VDC source

    Quote Originally Posted by Tomas04 View Post
    .........................
    I was, among others, looking at this solid state relay. http://uk.rs-online.com/web/p/solid-...elays/7360885/

    Unfortunately it goes only up to 280VDC for the control signal, and the relay has a minimum current of 20mA which is just around what the LED uses. I could of course buy one and test if it works anyway, it probably would. But the fact that it only goes up to 280VDC makes it a bit tough to use.
    .....................
    The control signal is a current of 4mA @ 240V in. So all you need to do is add a resistor in series that drops the voltage from 300V to 240V @ 4mA. The resistor value would be 60V/4mA = 15kΩ. The maximum resistor dissipation is 60V * 4mA = 240mW so use a 1/2 or 1 watt resistor.

    I'm not sure why they have a minimum load current but, if it's a problem, just add a resistor in parallel with the LED to increase the current slightly.
    Zapper
    Curmudgeon Elektroniker



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    Re: Connecting a low current LED to 150-300VDC source

    There are relays whose coils operate on mains AC voltage. They draw tiny current. I'm pretty sure one could handle DC. If you need to limit current then a resistor will do the job (as post #5 describes).

    As for the switch, just about any of them should have no problem handling a load of 20 mA at 12V.



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