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good day, i'm working on with two voltage supply, one higher from the other(i.e. 12V and 9V), how can i let the higher voltage pass then block the other. is transistor switching can make it? i'd love reading your replies, please help me figure this out. thank you. :)

@marisan,

From your description it is not clear quite what you are trying to do:

" ...how can i let the higher voltage pass then block the other."

Do you want the 9V output to be on until the 12V output goes > 9V?

Do you want the 12V output to be off and the 9V output to be on until the 12V rail goes > 9V and then turn the 9V output off and the 12V output on?

Can you supply a more detailed specification of what you want the two voltage outputs to do?

Even a graph showing how you want them to vary versus time after switch on would help.

Better stil simulate it in CL using ideal components (the CL voltage or behavioural voltage sources and the voltage controlled switch) and then make it unlisted or public so that we can see what you want and then can suggest ways to do it with real components.

Other info required for a practical design:

What output current or load power do you need to supply?

What output protection do you need?

Fuse?

Current limit?

Foldback current limit?

sir, the 12V(dc), from the ac main supply; then the 9V is coming from a battery. I'm working on a power failure/resumption alarm(using 555) for a project.

if the main supp. failed the battery would supply the load (the led would glow) as an indicator that there is no power supply. on the other hand, if the power resumed the battery itself should not work(i mean, it would be blocked)

please ask me if there is more i have to clarify. thank you sir.

:)

Thank you sir :)

sir, i tried to simulate the circuit. when i turned the switch on, the LED2 lit up(slight glow). when i turned it off the LED2 shut off. if it is switched on or off, same goes with LED1 it doesn't lit up. supposedly the LED1 will light up if the switch is off, right sir?

is it possible if the switch is connected to the mains and not on the bat.? thank you very much...

I don't understand what you mean by:

"i tried to simulate the circuit."

The circuit I have posted is the simulation.

Run it and look carefully at the voltages around the circuit and the LED currents.

The sim starts with both the 12V supply at 0V (i.e. off) and the battery disconnected by opening SW1.

Dled1 is off.

Dled2 is off.

At T=10ms SW1 closes connecting the battery. D1 conducts and V(load) goes to about 8.3V.

D3 is reverse biased.

Q2 turns on and feeds about 1mA to Dled2.

Dled2 is on.

The 12V supply then climbs up to > V(batt). D1 conducts and D2 is reverse biased. V(load) climbs to about 11.3V.

D4 also conducts and turns on Q1. V(V12_ON) goes to almost 12V and feeds current to Dled1.

Dled1 is on.

Via D3, which is now forward biased, V(V12_ON) also pulls V(base) up to above V(V9) which turns Q2 and therefore Dled2, off.

Dled2 is off.

Dled1 LED is on only when 12V supplies the load.

Dled2 LED is on only when the 9V battery supplies the load.

If your LED is dim then you may need to use a lower current LED. You can reduce R4 and R6 to increase the LED brightness but you may have to reduce R3 and R7 by the same ratio that you reduce R4 and R6 (i.e. R3/R4 must be a constant and R7/R6 must be a constant).

Reducing the resistors increases the amount of power you waste just driving the LEDs.

If you don't need the LEDs then just use the basic Diode-ORing circuit of D1 and D2.

That's why there are boxes round the three sections of the circuit.

:)

I tried to simulate the circuit in another simulating software, I think I've done something wrong while simulating it. I'll try it on my workbench. thank you very much for your help sir. God bless.

sorry for taking so much of your time sir but can I ask for one more thing? I have a 12V(dc) and a 9v battery, I want to have a circuit that will protect my bat. from overcharging(automatically cutting off the supply if the bat. is fully charged).

thank you sir,

"I have a 12V(dc) and a 9v battery, I want to have a circuit that will protect my bat. from overcharging(automatically cutting off the supply if the bat. is fully charged)."

This is a different - and more complex - circuit from your original enquiry.

If you are asking for a design for this then you will need to search through CL - and maybe other sites and datsheets/apps notes/tutorials on sites such as TI, Fairchild, OnSemi and Linear Technology - for some ideas of similar circuits. If you turn up nothing from there then you are welcome to contact me through my website (see my profile).

You also need to specify much more clearly exactly what you want as in my earlier post:

https://www.circuitlab.com/forums/general-discussion/topic/958778ev/transistor-switching/#comment_5717

including what type of battery your wish to charge.

sir, I've come up with a power failure and resumption alarm circuit using a 555 (the output is a buzzer for the audio indicator and I used a bicolour LED to indicate whether there is a supply from the mains or none). a 12V(dc) is used to trigger the IC while a 9V battery is used as the supply. I'd like to use a rechargable battery to get rid of continuous purchasing another battery. if it is already drained. now what concerns me sir is that how can I make an added circuit wherein if the mains has supply I can use it to recharge my battery (drained from a previous power failure). plus making sure that my battery is protected from overcharging as well.

I want to show you my circuit sir, and if there is some more that you think I can still improve, I'd love to hear it from you.

Thank you very much.

sir, here is the circuit I've been talking about. Tatine and I are working on it for our project, you've been a great help. thank you very much sir :)

@marisan,

The circuit you are talking about is not clearly related to your original question.

It is now being described as "a power failure and resumption alarm circuit" which appears to require a rechargable battery in it.

@Tatine also posted a question about "a power failure and resumption alarm circuit" some time ago which as been commented on in this thread:

https://www.circuitlab.com/forums/power-electronics/topic/ennnr23b/transformerless-powersupply/

If you want other people to put their time into giving constructive help on a project then be prepared to put the effort in yourselves to clearly define, specify and describe exactly what you want or have been tasked to do before you ask what appears to the forum to be a set of random questions about a seemingly unrelated set of problems.

Please see comments:

https://www.circuitlab.com/circuit/4364cc/555/#comment_5765