# 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

## 1 Answer

You are trying to create an adjustable voltage power supply? I'm sorry to say that this design will not work. As you read, the output impedance (also called output resistance) will be very high.

There is a certain amount of current that goes through the voltage divider resistors in your circuit (ignoring the output), which you can calculate using Ohm's Law. The current that you want to supply through the output must be much less than the current through the resistive divider.

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.

• If I understand, let say my circuit here has 300mA of total current in it, my output load should be a lot less that that ? Knowing that my power supply is rated for 18A, it will be a complete waste ? – Karnalta Jun 13 at 13:40
• You would need very large resistors to handle the power. Something like this digikey.com/product-detail/en/ohmite/810F50RE/810F50RE-ND/…. Not to mention difficulties finding adjustable versions. – Reinstate Monica Jun 13 at 13:49
• All that being said, I'm not trying to discourage you from creating a circuit to do what you want (drop voltage from 30 V to an adjustable desired voltage). It would be a great project, but it will need to be a different design. – Reinstate Monica Jun 13 at 13:50
• An Emitter Follower may be a simple fix or an Op Amp buffer. Also plan on taking the time to learn to use a good simulator. tinyurl.com/y35qj3gy – Tony Stewart Sunnyskyguy EE75 Jun 13 at 14:18
• tinyurl.com/yxaqjsmx another version you may edit 0 to 5V with the right Op Amp but needs stable 30V tinyurl.com/yy5uv45t – Tony Stewart Sunnyskyguy EE75 Jun 13 at 14:34