# Resistors scaling in voltage divider?

For my bench power supply, I am willing to make a voltage divider with two potentiometers (for coarse and fine tuning). I think I got the schematic right but what I am not sure about is the influence of resistors scaling.

Here is my schematic sample: I think my schematic is working in theory, but it would also work with R1 and R2 at 500k. Or R3 at 100k and R4 at 1M, and so on... In theory, the resistors scaling always give the same result (at my knowledge level -> beginner).

From my search results, it only seems to influence the output impedance of my PSU but I don't understand what it means practically?

• When you refer to "my PSU", are you referring to this resistor-divider circuit? – brhans Jun 13 at 13:30
• Not just that circuit, in fact I have a 30v constant power supply (not build by myself) and I will add this circuit to it to control voltage. – Karnalta Jun 13 at 13:35
• The problem with using that as a "PSU" is that as soon as you connect any load to it which draws any current, the output voltage will change. Imagine your potentiometers are adjusted for a particular voltage at Output. Now imagine you have a 1k load connected from your Output to ground, then re-run the calculations for what the voltage will be at Output (without re-adjusting the pots). Now change the load resistance to 100R and re-run the calculations again ... Resistor-dividers are not suitable for use as power-supplies. – brhans Jun 13 at 13:53
• You should never use resistors to "change the voltage" of your power supply. Your resistors will either catch fire or your voltage will drop to 0. You can have a voltage divider using a POT to control your output, but not directly like that. What exactly do you want to make? Specify your supplies Voltage and Current (or Watts) and what output voltage range you want at what maximum current. – hekete Jun 13 at 14:30
• I think the problem is, making an adjustable power supply is actually kinda tricky. A much easier project would be to make a supply that just has a couple of fixed voltage outputs. So you could have a 3.3V, 5V, 12V and that's probably enough for most projects. You could always come back to making an adjustable one later, but it's going to need some way of showing what its set to and you probably want to be able to use it in constant current mode too. That means an LCD and micro controller. – hekete Jun 13 at 14:48

For example, if you have a simpler voltage divider (to divide by 2) of 10k and 10k, the current from a 30V supply will be $$\I = \frac{30\text{ V}}{20\text{ k}\Omega} = 1.5 \text{ mA}\$$. It will not be able to supply anything close to 1.5 mA without the voltage dropping toward 0 V.