How can I understand an OR gate with diodes?

[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 ?

 

[jim hardy]

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

 

[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 ?

 

[jim hardy]

why don't we sum the two voltages resulted from two diodes ?

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?

 

[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.

 

[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 :)

 

[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.

 

[CWatters]

If he showed you a circuit like this...

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

 

[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".

 

[jim hardy]

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".

SN7438

 

[Jeff Rosenbury]

Ah, the good old days!

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

 

[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.

 

[Jeff Rosenbury]

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.

But now they're ROSH compliant wired or gates.

 

[sophiecentaur]

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 ?

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.

 

[jim hardy]

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

 

[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.

 

[Jeff Rosenbury]

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.

I wondered where that extra limb came from.

 

[Hassan Raafat]

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.

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