Dual dc power supply switch over led circuit.

[Daniedp]

Guys need a circuit to do the following:
- supply 12v dc from power supply to load
- supply 9v dc from backup battery to load
-12v supply led indicator must be on when 12v is supplied
-9v supply led indicator must come when battery backup kicks in and 12v supply led must be off.
12v supply return then battery backup led must be off and the 12V supply led is back on.

 

[Harald Kapp]

Welcome to EP.

• Could you accept the voltage drop from decoupling diodes (~0.7 V) so the load would see 11.3 V instead of 12 V and 8.3 V instead of 9 V? Then diodes can be used to do the switchover. A simple transistor circuit can control the LED for 9 V backup power.
• Could you accept power loss for a few milliseconds (~10 ms) during switchover from 12 V to 9 V and back? A relay could then be used to switch the power sources as well as the indicator LEDs.
• What are your power requirements (current) for the switch-over circuit?

 

[bertus]

Hello,

When using the diode arrangement as @Harald Kapp said, will introduce a voltage drop.
The drop will be a bit lower when using shottky diodes in stead of standard diodes.
(about 0.3 versus 0.7 Volts)

Bertus

 

[Daniedp]

Yes do you guys have a circuit diagram perhaps. I am a builder not a designer of circuits.
The main supply would be a 12v dc power support anything from 500ma to 1 amp and powering 2 red leds 1 yellow and 1 green But not all at once only 2 red leds.
The battery backup is 9v battery that must carry the load as above but again not all at once and only 2 red leds.
I have a circuit diagram that I want to incorporate it into but can't post it somehow.

 

[Harald Kapp]

Yes

Yes what?
What about answers to the question in post #2?

can't post it somehow.

Reduce to y 300 kB.

 

[Daniedp]

Welcome to EP.

• Could you accept the voltage drop from decoupling diodes (~0.7 V) so the load would see 11.3 V instead of 12 V and 8.3 V instead of 9 V? Then diodes can be used to do the switchover. A simple transistor circuit can control the LED for 9 V backup power.
• Could you accept power loss for a few milliseconds (~10 ms) during switchover from 12 V to 9 V and back? A relay could then be used to switch the power sources as well as the indicator LEDs.
• What are your power requirements (current) for the switch-over circuit?

 

[Daniedp]

This a circuit I want to incorporate the 2 switch over leds to show which supply is on as per my previous question. I am only a hobbyist and limited knowledge of design including working out values etc.

 

[Harald Kapp]

The green 12 V indicator already does as required.
For a red 9 V indicator a simple transistior circuit can be used:

When 12 V is present, the base emitter voltage of Q1 is < 0.6 V, the red LED (D4) is off.
When 12 V is not present, R1+R4 draw the base f Q1 to ground, thus allowing base current into Q1 and consequently D4 (red LED) will be on.

 

[Daniedp]

Thanks a million.
I see Q1 is pnp what type? I only have bc547, bc337, bc327 and bc557.
What exactly is D1 and D4 is it leds? Can I use normal Red leds or is this a particular type?

 

[bertus]

Hello,

The BC327 and BC557 are PNP transistors.
The BC557 will do to drive the led.

Bertus

 

[Daniedp]

Thanks a million

 

[Daniedp]

How can I incorporate the circuit into the circuit I posted?

 

[Harald Kapp]

My circuit - your circuit
------------------------------
D2 - D1
D3 - D2
V1 - 12 V
V2 - 9 V

Swap R1 and the green LED to match the circuit R1 / D1 in my schematic (same values, swapped positions). Then add R3, R4, Q1 and D4 as shown in my schematic.

 

[Daniedp]

The green 12 V indicator already does as required.
For a red 9 V indicator a simple transistior circuit can be used:
View attachment 52751
When 12 V is present, the base emitter voltage of Q1 is < 0.6 V, the red LED (D4) is off.
When 12 V is not present, R1+R4 draw the base f Q1 to ground, thus allowing base current into Q1 and consequently D4 (red LED) will be on.

Harald wil this workbas per your schematic?

 

[Harald Kapp]

Yes, that should do it.

 

[Daniedp]

Yes, that should do it.

Thanks