# Reduce voltage to many lower output with resistors

I have a stable dc 12 volt input 2 Amps and i want to make 6 different exports out of it like a power suply.

I would like to make the six reduced outputs out of it : 12v as i get it from the input,9v,6v,5v,3v and 1.5v

I also like to use some easy and not complicated way such as resistors since my input is stable, so if i want to make the 12v into 9v out in the first output it would go something like 10ohm resistor as 1st and 30ohm resistor as second and my output will be 9v.

Then i just follow the procedure of the 2 resistors then to lower the 9v into 6v and so on to create the outputs i need.

If i am correct and it will work fine do i have to choose a type of specific resistors and ohms and does this affect my amps and volts or it doesnt matter at all since the Vs R1 and R2 and Vout maths are ok like the above ? I can even use for example R1=1ohm and R2=3ohm to get the 9v output but does this makes any difference ?

• What are you doing with the reduced voltages? Will they be driving a load? In general your method won't work because the resistance of the load will be in parallel with the resistors and you won't get the voltage you want. That's why voltage regulators are generally used to generate a "stiff" voltage reference. – dgreenheck Jan 20 '17 at 2:11
• i want to use it as small power supply – Nocs Jan 20 '17 at 2:16

No, this will not work.

Imagine if you have your 10 ohm and 30 ohm setup, and you hook a light bulb to the output that has an on resistance of 8 ohms.

The 8 ohms is then parallel with the 30 ohms and creates a 6.3 ohm resistance, and your supposed 9V power supply exhibits a voltage of 4.6V.

In other words, the load you put on the power supply will change the current drawn, and since the voltage drop across the top resistor will change according to current then your voltage will always change depending on the load.

You will need to use voltage regulation of some sort. Simple voltage regulators are available that are as easy to use as the resistors, though, so you should look up simple power supply designs using voltage regulators.

• thanks for the reply, yes that solved my questions about it but as far as i searched the voltage regulators usually drops also the amps so you get 9v but the amps are usually too low after it, is that correct or just missinformed ? – Nocs Jan 20 '17 at 2:16
• @nocs You'll need to find a voltage regulator that meets your needs. So if you only need 2A then a simple linear regulator will do the job just fine. If you want to increase the amps for the lower voltages then you'll need to look at switching regulators. This sounds like the beginning of another question, though, so I'd suggest making a new question with details about the power supply you have, and details about the outputs you'd like. – Adam Davis Jan 20 '17 at 2:19
• the outputs i want is the above, the input is stable 12v dc 2 amps and i want to use the outputs as small power supplies whenever needed, what do you suggest ? Keep in mind also that i want to avoid too much heat for such small reduces and my space is limited small – Nocs Jan 20 '17 at 2:21
• if i use the 2 resistors to get 9.7v and use a IN classic diode to go to 9v will this still affect according to your answer the current ? – Nocs Jan 20 '17 at 2:28
• @Nocs Yes, the voltage will still change. You could use several diodes in series to drop the current, but even diode voltage drop varies a little with current so the voltage will still change. I think you should get a 7809 regulator and make one 9V and one 12V output rather than trying to make everything all at once. Once you're comfortable with those two outputs and it's working, buy the other regulators for the other voltages. If the 7809 doesn't give you enough current at 9V (it's limited to 1A) then upgrade it to a 2A 9V regulator. Start small and grow in steps. – Adam Davis Jan 20 '17 at 2:39

Reducing voltage for power supplies using voltage dividers will not work - any current drawn from the voltage divider will reduce the output voltage, and the voltage divider resistors will waste power.

You should use voltage regulators instead. Depending on the current requirements at each voltage, it may be better to use switching regulators (DC-DC converters) rather than linear voltage regulators, as the linear regulators must dissipate the excess power as heat.

• thanks for the infos, i want to avoid to much heat as possible and i have also small space to make the reduce of voltage, what should i look for according you your opinion ? – Nocs Jan 20 '17 at 2:19
• +1 to you too Peter but i am still too low to vote :) thanks – Nocs Jan 20 '17 at 2:48