Variable power supply for under 3 volts

[george2525]

Hello,

I got a variable power supply but like a dickhead I didnt realise that it has a 3v lower limit. Im going to make a simple voltage divider type circuit to attach whenever I need under 3v.

Ive seen a few ways of approaching this - some using capacitors and some pots etc and was wondering if anyone could recommend the best method and some good components for accuracy.

This is very annoying but id be very grateful for some expertise. Im ok at basic stuff so can build easy to medium circuits but please dont overpower me with tech stuff as ill probably get a little lost.

Many thanks, G

 

[BobK]

A voltage divider is not the way to provide power. How low does it need to go? And LM317 adjustable regulator can go down to 1.25V.

Bob

 

[george2525]

well as low as possible. ideally I want to somehow make up for the fact that I dont have the 0-3v range. Im currently wanting to deliver a 1mA base current to a transistor and its really annoying that when i need to change it I have to mess with resistors and pots rather than just lower the supply voltage. maybe theres an easier fix but my idea seemed like the right one.

are you saying that theres no real easy way to do it properly?

thanks G

 

[BobK]

Controlling the base of a transistor via a forced voltage is a bad idea. If the transistor heats up, more current will flow, causing more heating...

Use a fixed voltage (say 3V) and a pot to control the current. Use a fixed resistor in series with the pot so it cannot go too high and destroy the transistor.

Bob

 

[george2525]

ok thanks a lot for the reply. sounds like a good idea

 

[KMoffett]

I would add a second, small -1.25v reference supply. Connect the commons of both the positive and negative supplies together, Then connect the bottom of the resistor/pot LM317 programming string to the -1.25v supply. That allows you to control the output down to 0v.

Ken

 

[george2525]

hmm thats interesting. you mean make the neg supply?

 

[BobK]

Yes, that is what he meant, and referencing the LM317 to that negative 1.25V supply will allow you to make a regulated voltage supply that goes all the way down to 0V. But I still say that a regulated voltage supply on the base of a transistor is NOT the way to do it. If you want a constant current supply, which is more appropriate, we can work on that.

This is exactly the same as a voltage supply vs a current supply for LEDs. The former is a bad idea and the latter is the ideal.

Bob

 

[KMoffett]

With a little Googling: https://i0.wp.com/www.electronics-lab.com/blog/wp-content/uploads/2008/10/lm317.gif
You can use a transformer with a center tapped secondary, a bridge rectifier and two filter capacitors to generate the positive and negative DC sources.
Ken

 

[george2525]

thanks everyone. i appreciate it. it doesnt have to be permenant so i decided to chain some pots together. one for big R's and one for fine tuning. seems ok for now. ill be looking at the link though. thanks for the responses