question on power supply design

[p72]

Hi All,

I am planning to work on power supply design very soon. I wanted to make sure my ideas going to work or not.

One of our products running on both AC (85-265VAC) and DC (24VDC) power supply. We have those as separate supply at this point.

I am planning to design common power supply circuit which can take both AC and DC power supply. Is it possible? If yes, I am looking for help on the same.
The output of power supply should be 24V and 5V in both situations.

I will come up with all other parameters like load, current requirements etc.
Before I move ahead I want to know is it possible to design such a circuit that can sense AC or DC and behaves according that?

Thank you for looking in it!!!!!!
p72

 

[tusemo]

You want, one input point and two output points ?

 

[p72]

You want, one input point and two output points ?

Hi,
I do not get your point but my input would be AC or DC any one of them. I am looking circuit that can differenciate input (wethere it is AC or DC) and after that point rest power supply circuit act as normal.

Now number of outputs will be two. 5V and 24v, whether its AC or DC......
Thank you for quick response!!!!!!
p72

 

[BradtheRad]

A full-wave diode bridge is all you need in order to produce DC from any incoming supply, whether it is AC or +DC or -DC.

However you state there could be 24V incoming DC, and you want 24V output. You would get 1.2 V drop through the diode bridge. This may be unsatisfactory.

You may instead want to use a relay, to steer the power supply to the proper conditioning circuitry.

 

[p72]

A full-wave diode bridge is all you need in order to produce DC from any incoming supply, whether it is AC or +DC or -DC.

However you state there could be 24V incoming DC, and you want 24V output. You would get 1.2 V drop through the diode bridge. This may be unsatisfactory.

You may instead want to use a relay, to steer the power supply to the proper conditioning circuitry.

Hi BradthRad,
Thank you for the response!!
My problem is here: -

Let assume first case: - we have AC (85-265)VAC as Input on primary side and in second case if we have DC (24VDC) as input on primary side then how to differentiate that?

I hope it clears a bit.
Thank you,
p72

 

[rahdirs]

Is your circuit going to be something like the one below.
Do you have both those sources simultaneously/sometimes there is only AC source,no DC source.Sometimes only Dc source & no AC source.

Do you have both of them simultaneously on ???

 

[p72]

rahdirs,
I really appreciated your suggestions. In my circuit, there will be either AC or DC at a time. Not the same time.
My concern is how can an instrument will detect AC supply or DC supply?

Is there any circuit idea available?
Only reason behind this, when user connect AC power in to DC socket by mistake (or vice versa), circuit could handle any input supply. And save the rest power supply circuit.

Thank you,
p72

 

[rahdirs]

Oh,so you want to differentiate AC & DC,which might come one at a time from a single source.

• You can do something like this,use a capacitor to block DC,then connect a step-down transformer & a bridge rectifier.
• To block AC,use an inductor & connect it to bridge rectifier or as AC is blocked you can directly use it to avoid bridge rectifier loss of 1.4 V.
• You might have a slight problem that an inductor connected in series to a signal input is used to create high impedance not really a block,so a small AC could pass through as Ac mains frequency is generally low(50 Hz) but it will usually be low of some mVas in schematic.But I don't think a couple of mV will be a problem for you.

If it is a problem,perhaps you could use a capacitor between the signal an neutral or both an inductor & Capacitor, like in a LC filter. It depends on the requirements of your circuit. A LC filter might be the best option. There is also the Pi filter, having one first capacitor in paralel, then the inductor in series, then again another capacitor.

Out of curiosity,what kind of source is that,that it might give AC/DC???

 

[p72]

Hi rahdirs,
Thank you for the advise. I apologized for late reply but busy with the personal stuff.

I have not said that AC and DC are coming from same source. Here is an explanation in simple words: - basically you can run any instrument through AC power or DC power. Now I know that one of the instruments (for what I am designing this circuit) is accepting both AC and DC but as a separate supply. We build that according to customer's requirement. If user needs AC, we build it and if user needs DC, we do DC.

My thinking is to design common power supply circuit which can accept AC as well as DC power supply. So that if customer order anyone (AC or DC), does not matter to us (if we have this circuit.). Second advantage is when we do manufacturing of this kind of instrument, sometimes manufacturer make mistakes and they plug AC in to DC supply and after that circuit will be blown/damaged.

I am working on it to get other required parameters for the same.
Thank you for your time!!!!!!
p72

 

[rahdirs]

So you are trying to design a power supply unit that can give out either AC/DC according to user's requirements.

This is what i understand,you have a PSU with a switch indicating whether to supply AC or DC.This i think you will get from mains(220 V, 50Hz).
I am still not sure how you are going to protect an instrument,if you connect a DC operating device into AC.

 

[p72]

Now you hit the exact point. When customer plug AC instead of DC, it is obvious that instrument/supply will damage BUT this what I am thinking to build "some circuit" which can take both AC or DC as an input of instrument and will not damage rest of instrument.

 

[rahdirs]

I don't understand how you are going to determine that customer has plugged in at the wrong supply.

I think the adapters are capable of protecting the instruments from such errors.

But say a customer connects in at the wrong supply.Say you need to supply DC,but he connected in AC,how are you going to determine that he connected it at the wrong supply.

 

[p72]

Hi,
I think you are not getting the entire scene behind my point. To protect an instrument is not the cause of this design. That is extra advantage of it.

I do not want to determine whether customer has connected correctly or not.
My point is to make "UNIVERSAL" power supply circuit, which means no matter what user can connect (AC or DC), it will always give me 24V and 5V output from it.

Let see if anyone else here make their suggestion or not. I know it is wired thought but wanted to make sure that it is possible or not.

Thank you,
p72

 

[BradtheRad]

Theoretically you could rectify the incoming supply, whatever it is (whether mains AC, or +24 VDC, or -24 VDC).

The output will be DC, in a range between +22.5 V and 370 V.

This would then go to a converter (preferably containing a transformer to provide isolation, eg., flyback or other), which is regulated to provide 24 VDC and 5 VDC.

 

[p72]

Hi BradtheRad,
At this point, I am only thinking on Input side of power supply. I think I should re-frame my question like this.

"How to distinguishes/sense AC(85-265) voltage and DC(20-28) voltage signal?" Once I know the method, I can use it before Transformer stage and after that transformer behaves according to incoming voltage that will sync up with secondary side of transformer.

Thank you,
p72

 

[rahdirs]

If you just want to distinguish/separate AC from DC,i already explained it in post #8 to use capacitor to block DC & inductor(or a pi-filter & other methods exp. in post#8) to provide high impedance to AC.

That point where the sources are short in fig. in post #8 is where you will connect your input.As it may either be AC/DC you will get it seperated & you can do further processing that you wish.

 

[picgak]

Hi All,

I am planning to work on power supply design very soon. I wanted to make sure my ideas going to work or not.

One of our products running on both AC (85-265VAC) and DC (24VDC) power supply. We have those as separate supply at this point.

I am planning to design common power supply circuit which can take both AC and DC power supply. Is it possible? If yes, I am looking for help on the same.
The output of power supply should be 24V and 5V in both situations.

I will come up with all other parameters like load, current requirements etc.
Before I move ahead I want to know is it possible to design such a circuit that can sense AC or DC and behaves according that?

Thank you for looking in it!!!!!!
p72

Hi,
If a .7v doesnt matter you can use a floating DC power supply and a Diode for separation