Basic 12v input with 5v output with monitor

[frenaud]

I'm looking for a basic circuit/chip that would perform the following:

-12v input (20 Amps)
-5v output (1 Amp)
-There will be a second 12v input that needs to be monitored: when it falls to 0v, there has to be some sort of signal sent out to a simple 555 timer to start counting down.

So the circuit has 2 different 12v coming in and a 5v coming out. When the 2nd 12v is off, it needs to start a timer.

Any idea where to start?

 

[goldsmith]

Hi Frenaud
Are you familiar with ohms law ? and basics of circuit analyzing ? if yes what do you mean by this :

-12v input (20 Amps)
-5v output (1 Amp)

A notice : load will mention the required current if you are not making a current source ! so why you have mentioned 12 volts 20A as input ????? Is the word 20A required ? you want make an step down converter which needs lower input currents ! ( if it is switch mode )
Best Wishes
Goldsmith

 

[tpetar]

I'm looking for a basic circuit/chip that would perform the following:

-12v input (20 Amps)
-5v output (1 Amp)
-There will be a second 12v input that needs to be monitored: when it falls to 0v, there has to be some sort of signal sent out to a simple 555 timer to start counting down.

So the circuit has 2 different 12v coming in and a 5v coming out. When the 2nd 12v is off, it needs to start a timer.

Any idea where to start?

Yes, I agree with my friend Goldsmith, what is 20A ? If you need 5V on output you can simple use 7805 or stronger 78S05 regulator. What do you consider on that second monitored 12V rail ? When falls to 0V, is this battery ? That is no problem to be done, when voltage falldown bellow some level then some circuit starts to count. You didnt specify timer design, is it 7-sed led or LCD or LED in row,.... ?

 

[frenaud]

Apologies if I was not clear. When I said "input", I meant that the power source for the circuit will come from a 12v source with a maximum of 20 amps. The 555 timer has no display itself. It is already designed and simply needs a signal to tell it to start when the second 12v rail (indeed a battery) hits 0v.

A simple dc-dc step down converter would be sufficient for 12v to 5v on the first rail.

 

[BradtheRad]

A simple dc-dc step down converter would be sufficient for 12v to 5v on the first rail.

You can use a voltage regulator using a transistor and a zener diode. It will maintain 5V on any load.

It is a resistive drop. Thus if your load draws 1 A, then the transistor must dissipate 7 W.

Per schematic:

 

[tpetar]

You should know that battery cant (should no) go to 0V, if lead acid with 6 cells (12V nominal) 11,9V is lower level (10,5V for deep cycle).

 

[goldsmith]

Hi My friend , dear Tpetar .
I think you hit to the point ! about zero volts of a battery ! i think the author of this thread has some problems with correct understanding some initial basics . and i think solutions for this this aim are :
A simple 78XX or a simple LM2576 or the good circuit of Bradtherad .
Best Regards and Best Wishes
Goldsmith :grin:

 

[zsolt1]

hi,
as i understand your problem , you want to start a timer when your battery discharges below a threshold value (indeed it should not be around 0 V) . The solution is simple, you need an electric or an electronic circuit to connect to the battery that will detect that specific threshold value you want (i hope you do not want that to be 0V :grin: ) . This kind of circuits are called 'minimal voltage relays' ... about making 5V from 12V is not a problem.... if so & need help , just ask

 

[frenaud]

Yes, I think an LM7805 would do the job nicely. I forgot about those useful little chips!

The second 12v comes from a battery, but I want to detect if the battery is removed, hence the 0v. So in this case a MVR circuit is a no-go.

 

[BradtheRad]

The 555 IC will start a timing cycle when you pull pin 2 low.

This means you must install a pull-down resistor at pin 2, because removing the battery will not pull pin 2 low.

Presumable with the battery present, it will keep pin 2 high enough to inhibit the timing cycle.

 

[frenaud]

Excellent, thank you everyone for all your inputs!