# How can I understand an OR gate with diodes?

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1. Feb 21, 2016

### Hassan Raafat

can i understand at OR gate with diodes , why o/p not equal the sum of two voltages (5+5=10v) , it's confusing me ?

2. Feb 21, 2016

### jim hardy

I have no idea what it is that you have imagined.

3. Feb 21, 2016

### Hassan Raafat

in OR gates as we have high(5v) on both two diodes why the result equal 4.3 as practical approximation ?
why don't we sum the two voltages resulted from two diodes ?

4. Feb 21, 2016

### jim hardy

we only sum voltages when they are connected in series.

Are yours not in parallel at the cathodes?

Have you a diagram from which you are working?

5. Feb 21, 2016

### Merlin3189

Why should we sum them? Or, why would we?
The diodes simply connect the output to the 5V line (via a 0.7V drop)
Once the connection is made, further connections don't do anything new.
Since we have only 0 and 5V lines, all voltages in the (passive) circuit will lie between these two values.
If the top diodes were removed, the output would drop to 0V.
If 1, 2, 3 or 4 diodes connect to the 5V line the output is simply 5V less a diode drop.

6. Feb 21, 2016

### Hassan Raafat

so i consider if one works the o/p would be high non-caring case of other diodes , that right ?
finally thanks my professor :)

7. Feb 21, 2016

### Merlin3189

Yes. Any diodes connected to a low voltage are reverse biased, do not conduct, so have no effect.
The resistor connects the output to low, but if any diodes are connected to high voltage, they conduct and raise the output to 4.3 V.

8. Feb 23, 2016

### CWatters

If he showed you a circuit like this...

The + sign is the logical operator for "OR" not "add the voltages".

9. Feb 29, 2016

### Jeff Rosenbury

Using the way back machine, I remember circuits with open collectors and simply wiring two signals together to get a virtual or gate. They were called "wired ors".

10. Feb 29, 2016

### jim hardy

SN7438

11. Feb 29, 2016

### Jeff Rosenbury

Ah, the good old days!

A time when nothing worked right and inhaling lead based solder fumes was every EEs life quest.

12. Feb 29, 2016

### CWatters

Still used today.... I think the Raspberry Pi has an I2C bus and (if the lead in the solder I use hasn't affected my brain) I think that uses an open collector/wired OR system.

13. Feb 29, 2016

### Jeff Rosenbury

But now they're ROSH compliant wired or gates.

14. Feb 29, 2016

### sophiecentaur

I know it's bit late for me to join in but it would have been a good idea to supply a diagram with this, initial question. Once C Watters supplied one, the answer was clear. Circuit problems are usually much more easily solved with the actual circuit sitting in front of you.
PS Now you've gone and rattled the cages of the 'Conservationists'! They will be sleeping tonight with visions of glowing radio valves and carbon resistors.

15. Feb 29, 2016

### jim hardy

I love the smell of Kester44 in the morning...

16. Feb 29, 2016

### meBigGuy

No body was inhaling lead fumes (1750C to vaporize lead). It was flux fumes. You still inhale those. One develops a technique of exhaling to blow the flux away as they solder.

From wikipedia
The lead in solder will not typically get hot enough to vaporize and become airborne. However, you will end up with lead on your hands after handling solder, so it is necessary to wash your hands with soap and water afterwards. Lead is toxic and can cause a variety of health problems. Children and pregnant women should avoid all contact with lead due to the severe developmental problems it causes.

17. Feb 29, 2016

### Jeff Rosenbury

I wondered where that extra limb came from.

18. Mar 1, 2016

### Hassan Raafat

thanks for your time :) , i've understood it .