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voltage detection circuit

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pjshah72

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Hi All,
I just joined this forum.
I am working on one of the projects and for that I need suggestions/help.

I have a situation where Voltage is coming in to my circuit and need to detect which one.
DC voltage is in the range of 20-30Vdc and also AC voltage in the range of 80-300Vac. I need to detect which one is coming in.

I need some kind of circuit that can tell me whether it is AC or DC voltage.

Any help will be greatly appreciated.

Thank you,
pj
 
Hi pj.

I assume you're rectifying the AC and filtering with a capacitor (e.g. the input stage of a power supply). If this is the case, you can use a window comparator to detect the DC between 10-40V, and a normal comparator to detect the AC above ~80V.

If my assumption was incorrect, then you can add a rectifier and capacitor such that it becomes correct and follow the above.
 
So you need two detectors which effectively give four states?
One that says DC is present (yes/no, 1/0. True/False)
The other that says AC is present (yes/no, 1/0. True/False)

00=nothing
10=DC only
01=AC only
11=Both.
 
Hi dougy83,
Why do I need to rectify my AC voltages?

Can I use window comparator to detect DC between 10-40V? and normal comparator for AC above 80V? If yes, could you please share link for both techniques?

fyi: - this circuit comes before power supply input terminal. This is what I am trying to do: - Once I can detect input power is AC or DC, my power supply should work on detected power. ps.jpg

Thank you for the response.
pj
 
Hi Mike,
I would like to know whether which on is present. That is AC present or DC present.

So you need two detectors which effectively give four states?
One that says DC is present (yes/no, 1/0. True/False)
The other that says AC is present (yes/no, 1/0. True/False)

00=nothing
10=DC only
01=AC only
11=Both.
 
Hi pj.

I assume you're rectifying the AC and filtering with a capacitor (e.g. the input stage of a power supply). If this is the case, you can use a window comparator to detect the DC between 10-40V, and a normal comparator to detect the AC above ~80V.

If my assumption was incorrect, then you can add a rectifier and capacitor such that it becomes correct and follow the above.


Dougy83 and Co

just to fill you in on some background, and what you may not be aware of so far in this thread
( he's posted this in another forum as well)

He wants mains supply and DC24V supply to be able to come into the same power input on the power supply and have a 24VDC output
YES the SAME input!! the same pair of wires

but not at the same time so he wants to be able to detect when its mains in or the 24VDC in
in the other forum he was basically told he cant have mains and DC using the same input even if switched ( safety reasons)
he could get mains going straight through to the 24VDC output if the switching failed

He claims others have told him it can be done but so far has been unable to provide any substantiation of that claim

Dave
 
Hi Dave,
First of all, I would like to clear one thing here, I am not wasting time of others as posting on various forums. I was just doing google about Voltage detection circuit and end-up with this forum. And that is why I thought to post SAME problem here.

In which forum I have mentioned this "in the other forum he was basically told he cant have mains and DC using the same input even if switched ( safety reasons)
he could get mains going straight through to the 24VDC output if the switching failed" ??

I have not used "CLAIM" word in other forums. As like you, few person told me that this kind of circuit could be possible (This is what I said on other forum.).

I apologize if you don't like my post on this forum. Thank you very much for explaining my problem in detail.

I am simply looking for hints/suggestions for my problem.

Thanks,
pj




Dougy83 and Co

just to fill you in on some background, and what you may not be aware of so far in this thread
( he's posted this in another forum as well)

He wants mains supply and DC24V supply to be able to come into the same power input on the power supply and have a 24VDC output
YES the SAME input!! the same pair of wires

but not at the same time so he wants to be able to detect when its mains in or the 24VDC in
in the other forum he was basically told he cant have mains and DC using the same input even if switched ( safety reasons)
he could get mains going straight through to the 24VDC output if the switching failed

He claims others have told him it can be done but so far has been unable to provide any substantiation of that claim

Dave
 
He wants mains supply and DC24V supply to be able to come into the same power input on the power supply and have a 24VDC output
YES the SAME input!! the same pair of wires

but not at the same time so he wants to be able to detect when its mains in or the 24VDC in
in the other forum he was basically told he cant have mains and DC using the same input even if switched ( safety reasons)
he could get mains going straight through to the 24VDC output if the switching failed

He claims others have told him it can be done but so far has been unable to provide any substantiation of that claim
Hi, Dave. Thanks for the heads up. Based on his drawing in this thread, he wants, as you've stated, a low voltage DC and mains voltage AC to be fed into his circuit. There is not really a safety issue so far. Provided that the DC/DC transformer in his block diagram is appropriately isolating, there is no greater chance of mains appearing on the output than there would be for a standard mains SMPS.

There may be regulations disallowing such a configuration (i.e. both LV DC and mains AC through the same connector) - I don't know.
 
Can't you just use a simple presence-of-mains detector? If that indicates 'yes', then handle the input as AC. If it indicates 'no' then handle the input as DC or zero.
 
Can't you just use a simple presence-of-mains detector? If that indicates 'yes', then handle the input as AC. If it indicates 'no' then handle the input as DC or zero.

and thats the saftey problem alec

if it fails to detect properly you may have mains voltage being fed to the the 24VDC output
and how does whatever is supplying 24VDC to this PSU handle the mains voltage across it, there's going to be some tricky switching there as well

Dougy and alec tho the block diag above shows 2 separate inputs one DC and one AC that is not what his stated requirements were which I will copy and paste here.......

Hi Dave,
You misunderstood my explanation. I am not looking alternate option if my mains fails.

As I have mentioned in my previous post, I am looking for design of P.S. circuit which has only "one input" and that should take any incoming power, AC or DC.I am still confused as many people have already told me I could design "Single Input" power supply but you are saying that I cannot design the same. Why?
I agreed with you that there should be some switching or sensing circuit to differentiate AC and DC. But many folks have already told me, no sensing or switching circuit is required.

I am totally confused at this point. (Because I am getting inputs from both the sides, positive and negative.)

Thanks for looking into my post,
pj

AC and DC on the one pair of input wires

Dave
 
Last edited:
Dave, I understand that he wishes to use a single input for either an AC or DC source. From his diagram, the input voltage cannot get to the output without passing through at least one transformer. If the final transformer is appropriately isolating, there is no greater chance of mains appearing on the output than there would be for a standard mains SMPS.
 
pj,


I have not used "CLAIM" word in other forums. As like you, few person told me that this kind of circuit could be possible (This is what I said on other forum.).

I apologize if you don't like my post on this forum. Thank you very much for explaining my problem in detail.


its got nothing to do with whether I like the post or not. Its got everything to do with making sure you or someone who uses this planned construction doesnt earn the "Darwin Award". No-one wants to see you or someone else get toasted.

But many folks have already told me, no sensing or switching circuit is required.


Thats your claim ... and all I ask is that you show us what possible examples they are proposing so that we can see if they are feasable and safe so that collectively, people here can help you through the construction :)

Dave
 
Thats your claim ... and all I ask is that you show us what possible examples they are proposing so that we can see if they are feasable and safe so that collectively people here can help you through the construction :)
The suggestions may have been as simple as feeding the DC through the filter/rectifier/bulk caps designed for the AC SMPS... which will have issues, including poor efficiency and being generally crap.
 
Having read this thread. I can't help but wonder. Instead of getting caught up on AC must be detected on a DC line. Perhaps back up one step further and tell us what it is you are trying to do. There may be other methods that may be looked at or discussed.

A full disclosure of the intent of the design would perhaps be helpful here. Show us what you have now as a design and why the need for the mentioned specifications.

One would not approach a chef and say you can only use two tools to make a meal and it must be cooked over a camp fire, that would stifle the creative capabilities of the chef.

Just a thought.
 
I have only one question to ask here. Let us forget about what I have already mentioned about entire design of power supply project.

Only one thing that I need to ask here: - Is there any method/way to detect or sense AC voltage and DC voltage? (Do not consider Power Supply design for this question.)

Thank you,
pj
 
I already explained one way above. Once you've fed the input through a rectifier and capacitor, measure the voltage to decide weather it's in your specified ranges for DC or AC. A comparator can be used, or a zener diode and transistor or any number of methods...
 
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