automatic switch betwen two power supplies for one circuit

hello
soppuse we have two different electrical power supply sources for one circuit
i want the first source to supply the circuit and at the same time
prevent(disconnect) the second source(battery) of the mentioned circuit
but if the first source failed or disconnected form the circuit
the second source, supply the circuit and if the first source
connected, again prevent second one and itself supply the circuit
i guess this can be done by one transistor
i want use your suggestions,expriences and guid
thanks

Re: automatic switch betwen two power supplies for one circu

Helllo ktec1,

If you could tolerate the forward voltage drop in a diode (and I think you can, because you are planning to use a battery to power your circuit) you can simply use a diode in series with each power supply. Switching between power sources will be automatic.

Hope this helps and best regards.

Re: automatic switch betwen two power supplies for one circu

hi jorgito
thanks for your suggestion
i want to explain more about the circuit
one of my power sources is a 7805 regulator and other one
is four ni-ca 1.2 volts batteries in series
i never want these sources to supply the circuit at the same time
(i dont want batteries to be discharged when the city power isnt failed)
if i use two series diods with two sources above
how can i be sure that both sources dont work at the same time?
thanks for suggestions

Re: automatic switch betwen two power supplies for one circu

HI. There is a very simple solution for your problem.
Use a 3 pole relay.

That make it very easy to switch to battery when the other powersupply goes down.

A transistor will not be any better than a diode solution because when your main power sypply fails, you have no places to get the emitter forward voltage from other than from battery itself - and you'll loose 0,7 volts.

Two posible solution other than this:
- Use an p-Channel JFET. When zero voltage at gate, it will conduct. But I don't know if they are suitable for this (resistance between drain-source when Ugate=0)
- Use a opto isolator combined with a transistor. Using a regular bjt requires an current to keep it conduct, and may reduce battery time.

Re: automatic switch betwen two power supplies for one circu

thanks
the main part of the circuit is a microcontroller and i dont want it to
face sudden voltage changes during relay switching
but if you have a successful expriences in such a case and during
a lot of time your uc worked well please tell me and if you used diods,zener or...
in your circuit
i will try your second seggestion and will work on it
and soon will tell you
thanks again

Yes, forgot to mention about voltage drop - but that's quite obvious.
One solution for that may be to place a "big" cap in parallel with your circuit.

Yet there will always be some different between the voltage sources, so there will always be a little change in voltage when switching between them.

A transistor may also provide a little "jump" in the voltage when switching, only it happens faster though. I guess it's better having a cap over the circuit any way, and using another cap between the transistors emitter/base in order to get it conducting gradualy and smooth.
Problem you might face here is that it will also turn off smooth, and then you will have a short lived situation where both the power supply and the battery is conected in parallel - just what you want to avoid.
You might think out a relatively simple solution for that, a diode/cap/resistor coupling to get it slow on, but fast off.

You could also put a drossel in series between the switch and the circuit in aditional.

Re: automatic switch betwen two power supplies for one circu

Hello,

Did you look at the autoswitching power muxs ?
https://focus.ti.com/lit/ds/symlink/tps2111a.pdf

Re: automatic switch betwen two power supplies for one circu

dear yazou
although your suggestion is very good but i wouldnot get that part
during next few days in my country
dear jorgito thanks for your suggestions too
i have to work on jfets and mosfets more
to have a solution

Re: automatic switch betwen two power supplies for one circu

What you need is also called "ideal diode", with this circuit you can reduce the power loss and voltage drop due to the classical diode approach, look at the attached paper from Linear Tech it might be useful for you (see e.g. figure 3).

Re: automatic switch betwen two power supplies for one circu

hi friends
nearly i have solved my probem with two transistors
although in the circuit i have 0.5 volt drop but
i myself need this drop because of four series batteries
that output voltage when full charged may reach 5.7 or 5.8
what we need is only two transistor and some resistors
what i before searching and it costs less than 0.05 $ and in cents dont know ??
thanks

Re: automatic switch betwen two power supplies for one circu

ktec1 said:

hi friends
nearly i have solved my probem with two transistors
although in the circuit i have 0.5 volt drop but
i myself need this drop because of four series batteries
that output voltage when full charged may reach 5.7 or 5.8
what we need is only two transistor and some resistors
what i before searching and it costs less than 0.05 $ and in cents dont know ??
thanks

Click to expand...

Dear Sir,

I am currently designing the same circuit. I have 3.3V battery and 5V power supply. So when my 5V supply fails, it should connect to the battery. please provide me circuit for the same.

Thanks in advance.

Regards
Keyur Parekh.

Hello,

Can anybody help me to solve my problem.

I have two power supplies, one is 3V battery and other is 5V power supply. So i want when 5V power supply cut than circuit should automatically switch on battery.
can anybody provide me circuit diagram for that. how can i achieve this ???????
is it possible using TL431. please give me schematic diagram for that.........

Re: automatic switch betwen two power supplies for one circu

Hi jorgito, you answered to this post with a circuit using diodes. I have two power sources: solar cell and a wind generator and one load. How can I connect the powers to the load and use either source when needed. It is a power project and I'm trying to make it easier. Can u help me? Remember one of the sources will not be functional and the other will power the load.