resistor and voltage didivion

[hydrojohn]

Hi, I need to divide my voltage on a power supply. If I have a 3.4v out put and a 5v out put what resistors do I need to halve these two? Thanks in advance.

 

[FvM]

Divide the voltage to achieve what? You can use a wide resistance range as long as the divider output is unloaded. But the load resistance becomes part of the divider, respectively the load current causes a voltage drop according to the divider output resistance.

Which load shall be connected to the voltage divider?

 

[hydrojohn]

I need to halve the voltage of the 3.4v out put and the 5 v out put to run a single cell electrolyzer. Optimum voltage for a single cell is between 1.7 and 2.2 volts. It asks about 20 amps.

 

[FvM]

Quite simple, it's impossible with a voltage divider. A series connected variable power resistor (rheostat) could be used. These days variable constant current switch mode power supplies are used for electrolysis.

 

[danadakk]

Some buck possibilities -

https://www.ti.com/power-management/non-isolated-dc-dc-switching-regulators/step-down-buck/products.html#p238min=1;3.4&p238max=3.4;100&p634min=-3;1.7&p634max=1.7;90&p451max=20;30&p212max=58.53658536585367;100

Regards, Dana.

 

[hydrojohn]

Some buck possibilities -

https://www.ti.com/power-management/non-isolated-dc-dc-switching-regulators/step-down-buck/products.html#p238min=1;3.4&p238max=3.4;100&p634min=-3;1.7&p634max=1.7;90&p451max=20;30&p212max=58.53658536585367;100

Regards, Dana.

Hi Dana just had a thought, would it not be easier to just take a 12 battery and use a series voltage dropping resistor to get 2.2 volts? Regards john

 

[danadakk]

If you don't need a regulated accurate V or care about power loss or
load does not change yes a R would do it.


Regards, Dana.

 

[barry]

Hi Dana just had a thought, would it not be easier to just take a 12 battery and use a series voltage dropping resistor to get 2.2 volts? Regards john

You would need a 200 Watt resistor. Quite possibly the worst possible solution.

 

[wwfeldman]

if you do a little searching at digikey, or mouser, or newark,
you will probably find an appropriate switching power supply for a for less than the cost of one 200 W resistor
it will also not get as hot, will not waste as much energy, and will take up less space.

 

[danadakk]

Do you need

5V >> 2.5V @ 20A &
3.4 >> 1.7V @ 20A ?

(5 - 2.5) / 20A = .125 ohms P = I^2 x R = 400 x .125 = 50W Resistor
(3.4 - 1.7)/20A = .085 ohms P = I^2 x R = 400 x .085 = 34W Resistor

Buck regs that can do it (5 > 2.5) -

https://www.ti.com/power-management/non-isolated-dc-dc-switching-regulators/step-down-buck/products.html#p238min=1;3.4&p238max=3.4;100&p634min=-3;1.7&p634max=1.7;90&p451max=20;30&p212max=58.53658536585367;100

Regards, Dana.

 

[hydrojohn]

Do you need

5V >> 2.5V @ 20A &
3.4 >> 1.7V @ 20A ?

(5 - 2.5) / 20A = .125 ohms P = I^2 x R = 400 x .125 = 50W Resistor
(3.4 - 1.7)/20A = .085 ohms P = I^2 x R = 400 x .085 = 34W Resistor

Buck regs that can do it (5 > 2.5) -

https://www.ti.com/power-management/non-isolated-dc-dc-switching-regulators/step-down-buck/products.html#p238min=1;3.4&p238max=3.4;100&p634min=-3;1.7&p634max=1.7;90&p451max=20;30&p212max=58.53658536585367;100

Regards, Dana.

Yeah I had come to the same conclusion re 12v battery. I have access to a 5 volt ps, going to try the buck regulator approach. Thanks again John