Is this design OK?(Automatic to back up battery switch Circuit)

[Allahyarzadeh]

If the adaptor 2 switches off the final output will be drove by other input.is there any problem with this circuit?and can it hold 2 Amper current.and give a good output.
the gate pin of thyristor is off because of the second input adaptor2 which is connected to ground from collector-emmiter. and if the adaptor 2 be off the gate would be connected to the other +12 volt input and thyristor willdrive the output.

 

[chuckey]

The transistor Q1 will be blown up as its base emitter junction has got 12V across it without a current limiting resistor. Also if adapt2 is above 2V or so the transistor will be on, it needs a zener diode in series with its base circuit. Once the SCR has fired some way of reducing its current two less then the holding current ( 0A?) to switch the SCR off.
Why not just use two diodes to OR the supplies?
Frank

 

[Allahyarzadeh]

thanks chuckey
the transistor was just an idea for NOT(consider it with good bias, is it oK now?)
about OR diodes,is it reliable and what's the layout exactley(just two simple diodes?! each adaptor gives 12V 2A if both diodes drive, won't give a bad output?)
can I use a power P-mosfet like IRF9540 which has less dropout for the supply,I mean I put it in the way of one +12 and give the gate to the other one?(with a resistor to the ground)
I also come up with this IC : ICL7673 and as datasheet says I can drive two out side transistors.can I use mosfet with this IC? for lower drops?

I want to use this for CCTV camera,as you know they're always on,and sometimes the adaptors stop working.I want to connect 2 adaptors that if one of them failed the other one work.

 

[betwixt]

Just use two diodes, none of that circuitry is needed at all. The diodes will allow current to flow from each adapter to the camera but not back into each other. As both produce 12V you actually get the same voltage but twice the current capacity, falling to normal capacity if one adapter fails.

Brian.

 

[Allahyarzadeh]

can I use a 2200 uf 25v at last stage or also it's not needed
diode was the first thing pop up in my mind, but after I checked many circuits,I didn't see anyone use them!,and I saw this IC in one of the back up battery circuits,so I doubt about using just two simple diodes for a pro design

 

[crutschow]

Diodes will reduce the adapter voltage by the forward drop of the diode, but there is usually enough voltage margin so that is not a problem. If you use large Schottky diodes, the drop can be reduced to less than a half volt.

The IC in the backup battery circuits may be just for controlling battery charging.

 

[Allahyarzadeh]

d'accord.
merci
but I don't want both of the inputs be active in one time,I'm using this so that I could have one active and another completely isolated from out put because the output voltage is driving some other circuits too(other than camera),which may harm the adaptor in long time usage.if both of them connect together then both of them will get harmed,I want to use the changer so that if one of them (which is in use)failed,then the other one come to circuit and work.and I have seen that, all of the adaptors(even best quality) will fail in long time(about 1 year).

 

[Tunelabguy]

d'accord.
merci
but I don't want both of the inputs be active in one time,I'm using this so that I could have one active and another completely isolated from out put because the output voltage is driving some other circuits too(other than camera),which may harm the adaptor in long time usage.if both of them connect together then both of them will get harmed,I want to use the changer so that if one of them (which is in use)failed,then the other one come to circuit and work.and I have seen that, all of the adaptors(even best quality) will fail in long time(about 1 year).

That is not how it works. When each adapter feeds the load through a diode, the two diodes will not both conduct. Only the diode with the higher supply voltage will conduct. The other will remain completely non-conducting. So if one adapter produces 12.03 volts and the other adapter produces 12.02 volts, only the first adapter will supply current. The second adapter will just be standing by. If the first adapter ever gets turned off or stops working, the second adapter will start supplying current through its diode.

 

[Allahyarzadeh]

thanks labguy, you're right.
is 1n5820 or 1n5824 ok for this purpose?
with VRRM =20v and If=3A.with Vf = 475mv.
is there any diode with lower drop forward voltage?

 

[crutschow]

A higher current rated Schottky (for example 10A) will have a lower forward drop at the current you will be operating.