# Help with diode OR gate

Discussion in 'General Electronics Discussion' started by TOAD, Jan 12, 2017.

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Jan 12, 2017
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

2. ### BobK

7,682
1,688
Jan 5, 2010
I could make an assumption about what circuit you are talking about, but if I am wrong, my response would only confuse you,

Bob

3. ### Kabelsalat

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Jul 5, 2011
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.

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Jan 12, 2017
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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5. ### Kabelsalat

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

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,480
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Jan 21, 2010
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.

Bluejets likes this.

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Jan 12, 2017
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

8. ### Kabelsalat

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

9. ### Audioguru

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Sep 24, 2016
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.

10. ### Herschel Peeler

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