# triple DC voltage switcher

Discussion in 'General Electronics Discussion' started by yves, Apr 4, 2011.

1. ### yves

1
0
Apr 4, 2011
Hi,
I am looking for someone to help realize the following circuit:
here is description in layman's terms

We have 1 input line (A) where voltage input can be 0 VDC or 5 to 36 VDC
We have an input voltage (DC) that can be 10 to 26 VDC to power the circuit
We have 1 output line (AA)

We would like that
1) when there is 0 to 5 VDC in the input line (A) that the output line (AA) = 0 VDC
2) when there is a voltage > 5DVC in the input line (A) that the output line (AA) voltage = the input voltage (DC)

Once this is solved our real need is for triple version of this circuit:
1 input voltage (DC) to power the PCB
3 input lines and 3 output lines (A) => (AA), (B) =>(BB) and (C) => (CC)

Practical use
The Voltage (DC) will be produced by truck’s batteries either 12 VDC or 24 VDC
The input voltage for lines A to C will be produced from parts of the same truck (ignition switch, alternator excitation terminal, other similar).

NB: this circuit will be working 365 days per year

Regards

Yves

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
The trivial solution is a diode. (due mostly to the fact that the input voltage is defined by you as never being between 0 and 5V)

You connect the diode with the anode to A and the cathode to AA.

When the voltage at A is higher than that at AA, current can flow from A to AA (but it can't flow the other way).

And yes, you can have as many of these connected as you like.

The disadvantage is that a diode drops between 0.6 and 1.2 volts (depending on current). Diodes also have a current limit that you don't want to exceed, so you may have more trouble finding a diode if the current exceeds 35A or so. For small values (i.e. under a couple amps, no problems)

What are you powering? Is it imperative that there is no voltage drop? Do you need to prevent current flowing the other way?

You could use a relay that gets switched on when the input voltage exceeds about 5V, but that won't prevent reverse currents from flowing. (Strictly speaking, your definition also allows a piece of wire to be used here)

Perhaps some more information (i.e. what is being powered) may be appropriate here.