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Help with diode OR gate

TOAD

Jan 12, 2017
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Hello, after some help. I want to make an or gate using two diodes and a resistor. When i thought of it i didnt know the circut would need the resistor. Why does it need the resistor between the output and ground and whats the formula for working out what size resistor id need?
Thanks :)
 

BobK

Jan 5, 2010
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I could make an assumption about what circuit you are talking about, but if I am wrong, my response would only confuse you,

Bob
 

Kabelsalat

Jul 5, 2011
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If you mean two diodes in parallell followed by a resistor, the minimum and max resistance depends on the feeding source before and the reciving circuit after. Also the direction of the diode might reverse if the opposite side of the resistor swap between ground and Vcc. However that would only work if the feeding source and circuit after is designed with this in mind.
 

TOAD

Jan 12, 2017
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This is what im going to make. I just dont know why i need the resistor or how to work out what size to use. The circuts going to be 12 volt and 6 amps
 

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Kabelsalat

Jul 5, 2011
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For 6 amps I would strongly recommend using relays instead of diodes.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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It looks like you're just isolating two power sources. In that case there is no need for a resistor to ground.

What are you powering from this?

The first time I read your post I didn't understand. I read it as you wanting "a diode or a gate". Now I read it as "a diode OR gate"

Ship sails today.
 

TOAD

Jan 12, 2017
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Haha sorry i want the day run lights on my car to come on when i unlock the car so i want to take a feed from the footwell light (which lights when i unlock) and have my day run lights on when the ignition is on OR when the footwell light is on. I thought of 2 diodes, looked into it and didnt know why all the ones i found had the resistor
 

Kabelsalat

Jul 5, 2011
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you don't want to use diodes on car light because the light will be noticeable weaker.
 

Audioguru

Sep 24, 2016
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you don't want to use diodes on car light because the light will be noticeable weaker.
Daytime running lights are supposed to be dimmed a little, unlike Chrysler and Jeep (and lately Kia and VW) who use high beams at full blast to blind oncoming drivers when it is cloudy.
 

Herschel Peeler

Feb 21, 2016
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You don't need the resistor. You are not doing logic. Just diodes. I suggest a Schottky for less loss.
 
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