Why is there a diode between two sources?

[Junior01]

I have a situation like on picture in attachment. I want to know, why I have a diode between two sources? What will happen without diode?

Thank you.

 

[phinds]

I have a situation like on picture in attachment. I want to know, why I have a diode between two sources? What will happen without diode?

Thank you.

What do you think will happen and why?

 

[Junior01]

What do you think will happen and why?

Probably something with sources, but I don't know what.

 

[phinds]

How far along are you in your studies of electrical circuits?

 

[Junior01]

How far along are you in your studies of electrical circuits?

Your questions aren't a point of this thread.

 

[berkeman]

Your questions aren't a point of this thread.

His questions are very appropriate. He is trying to get you to talk through the problem. That is a good way of helping people to learn a subject. If he just told you the answer, then the next time you have a similar question, you will have to go find somebody to tell you the answer. But if you get in the habit of doing some reading on your own and trying to talk through a problem, you will find that you are learning more and better.

So please tell us what you think the function of the diode is in this circuit. What does a diode do? Can you sketch the I-versus-V curve for a diode? What happens if you connect two voltage sources in parallel directly?

 

[Junior01]

His questions are very appropriate. He is trying to get you to talk through the problem. That is a good way of helping people to learn a subject. If he just told you the answer, then the next time you have a similar question, you will have to go find somebody to tell you the answer. But if you get in the habit of doing some reading on your own and trying to talk through a problem, you will find that you are learning more and better.

So please tell us what you think the function of the diode is in this circuit. What does a diode do? Can you sketch the I-versus-V curve for a diode? What happens if you connect two voltage sources in parallel directly?

I know that for example two generators can work in parallel if they are synchronised. But here I have two 24V DC system, and I don't know why they can not work in parallel.

 

[Junior01]

His questions are very appropriate. He is trying to get you to talk through the problem. That is a good way of helping people to learn a subject. If he just told you the answer, then the next time you have a similar question, you will have to go find somebody to tell you the answer. But if you get in the habit of doing some reading on your own and trying to talk through a problem, you will find that you are learning more and better.

So please tell us what you think the function of the diode is in this circuit.

The function of diode is probably to block current from second source to go "behind" (opposite direction).

 

[berkeman]

I know that for example two generators can work in parallel if they are synchronised. But here I have two 24V DC system, and I don't know why they can not work in parallel.

In general, DC power supplies are not designed to work in parallel. It is hard for them to have exactly the same output voltage, which means that one or the other will supply most of the load current. One solution is to "diode OR" their outputs...

 

[Junior01]

which means that one or the other will supply most of the load current

Why it is bad? Why it's important from which source consumers get more "energy"?

 

[berkeman]

Why it is bad? Why it's important from which source consumers get more "energy"?

Let's say you want to supply 10A at 12V, so you try to do it with two 5A, 12V DC power supplies. First of all, if they are switching power supplies, they can cause each other to go into oscillations (which is bad). if they are linear power supplies, the one with the 12.1V output will will go into current limit trying to supply the full 10A, which will cause its output to drop (crowbar) and the other supply will then try to supply 10A, so it will crowbar too.

You may get lucky if the two power supplies have almost exactly the same output voltage (unlikely), and if they have a moderate output impedance. But in the general case, DC power supplies can't just be paralleled without something like the diode OR connection.

When you see parallel battery connections, it is between batteries from the same manufacturer, same age, same charging history, etc. This causes them to have about the same output voltage versus time, and their inherent source resistance helps to balance them.

 

[berkeman]

https://www.eetimes.com/document.asp?doc_id=1273175

 

[Junior01]

Let's say you want to supply 10A at 12V, so you try to do it with two 5A, 12V DC power supplies. First of all, if they are switching power supplies, they can cause each other to go into oscillations (which is bad). if they are linear power supplies, the one with the 12.1V output will will go into current limit trying to supply the full 10A, which will cause its output to drop (crowbar) and the other supply will then try to supply 10A, so it will crowbar too.

You may get lucky if the two power supplies have almost exactly the same output voltage (unlikely), and if they have a moderate output impedance. But in the general case, DC power supplies can't just be paralleled without something like the diode OR connection.

When you see parallel battery connections, it is between batteries from the same manufacturer, same age, same charging history, etc. This causes them to have about the same output voltage versus time, and their inherent source resistance helps to balance them.

Thank you so much @berkeman . I'm just a young men without much experience in EE, but I want to learn.
Maybe I just misunderstood @phinds questions. If so, my apologies.

 

[Junior01]

https://www.eetimes.com/document.asp?doc_id=1273175

Thank you, but in my situation, on picture, I have consumers "behind diode" and "in front of diode" so consumers on right side of picture can draw more current from one of the sources and sources can go in oscillations, so in this situation I still don't see a point of diode...

 

[CWatters]

Why are the two sides linked at all?

Who determined which way the diode should point?

 

[Junior01]

Why are the two sides linked at all?

Who determined which way the diode should point?

I can not make different arangement. So, I want to know why is diode on that position, although consumers on the right.

 

[Tom.G]

It sounds like you are asking a more basic question than the answers you are getting.

First, do you know what a diode does? Let's hear your understanding so we know where to start.

By the way, I'll be asking a lot of questions. Hopefully they will lead to you figuring out what the circuit does.

Tom

 

[Junior01]

First, do you know what a diode does?
Tom

I think that diode is there to block current from second charger to go "on the left side".

 

[CWatters]

Perhaps. But why is it necessary to do that yet it's not necessary to prevent current from the left hand charger going to the right?

It's why I asked who determined which way the diode should point earlier.

I can make a case for a circuit with two diodes. It's hard to make a case for just one without more information.

 

[Tom.G]

I think that diode is there to block current from second charger to go "on the left side".

I agree, so far. Now how about when the power fails and both chargers are off?

 

[Junior01]

I agree, so far. Now how about when the power fails and both chargers are off?

The consumers will be powered via batteries... so, again diode is here to block the current, this time from batteries.

 

[Tom.G]

Yes.

Lets assume for the moment that the Consumers are perhaps computers; with the left one controlling some industrial plant, and the right one controlling some safety systems in the plant. In an emergency maybe the right one also sends panic text messages to all the plant engineers.

When something bad happens in the plant, the left computer tries to shut things down. Sometimes this can be done quickly, other times the shutdown takes longer than the batteries can last. With a rapid shutdown, both batteries will still have a useful charge in them, so the right side, the safety system, can use both batteries by drawing current thru the diode if needed.

Now with a lengthy shutdown, the left battery may be drained. What about the the safety system on the right side? Does it shut down? Why?

 

[CWatters]

Junior01: Can you tell us more about the situation? Is it a system you are repairing? Part of a truck? Caravan? What exactly are the "consumers"?

 

[Junior01]

What about the the safety system on the right side? Does it shut down? Why?

In such situation it's possible that computers on the left side need more energy, so the left battery will be drained, while those computers can't draw current from right battery due diode. The right battery stay "safe" to supply consumers on the right side.

 

[davenn]

In such situation it's possible that computers on the left side need more energy, so the left battery will be drained, while those computers can't draw current from right battery due diode. The right battery stay "safe" to supply consumers on the right side.

reading through this thread, it seem very muddled

so you still haven't answered the Q as to why this system is even linked at all ??
from all your comments so far it seems pointless.
So what are you not telling us ?
where did this circuit come from ?
is it an actual working system or just something you have drawn up ?

please answer all questions
It is really difficult for people to give you good answers when you are so vague about the setupDave

 

[Tom.G]

In such situation it's possible that computers on the left side need more energy, so the left battery will be drained, while those computers can't draw current from right battery due diode. The right battery stay "safe" to supply consumers on the right side.

Absolutely correct. I think you just answered the question you first asked!

Although implemented somewhat differently, the consumers on the left could be the lighting and other support functions in an office or apartment building, and the consumers on the right could be the emergency lighting so you can find the exits.