# problem in selecting some parameters for power supply

#### ect_09

Joined May 6, 2012
180
Hi!
i see many circuits of power supply on google but all are diferent.but i want to know that how we limit the voltage and current by using single transistor.
for example i want to make variable power supply of 20v and .5A current.what should i do.
how i choose the transistor ,capacitor(how much voltage) ,and diode(some prefer 1N4001 and 1N4007....?), and transformer.

please help.i also want to make this project after clearing my concept.
Thanks!
waiting

#### wayneh

Joined Sep 9, 2010
17,153
... i want to know that how we limit the voltage and current by using single transistor.
You cannot control both current and voltage at the same time at the supply. This leaves no degrees of freedom. With some cleverness you can control voltage and limit current to some maximum level, or vice versa.

If you can describe what you really hope to accomplish, folks here will help you.

#### ect_09

Joined May 6, 2012
180
i want to make variable DC power supply of 20v and .5A

#### wayneh

Joined Sep 9, 2010
17,153
Variable what? Voltage? On or off? What do you want to power with this supply? Details matter. Please be as specific as you can.

If you want variable voltage in the range of 2-20V, you may want to start looking at a LM317 circuit. It's simple and the most popular way for a DIY to get started.

What is your power source - maybe a transformer? Do you have a rectifier yet?

#### ect_09

Joined May 6, 2012
180
power Source is AC 220v but i want to convert it into DC >>>rectification.
is there any circuit for 20v and .5A or PCB.
waiting!!

#### wayneh

Joined Sep 9, 2010
17,153
Again, these are not enough details. But I'll make some assumptions for you and you can correct them if they're wrong.
1. You want a simple, linear power supply and not a more complex switch-mode power supply (SMPS). By simple, I mean transformer + rectifier + filter capacitor.
2. You want to control the output voltage, and you can live with 2-20V (ie., not really 0V, meaning an LM317 is an option).
3. You need no more than 0.5A continuous current, at any voltage.
4. The degree of control you need is no more demanding than what the LM317 can provide.

If these are all true, than you need to find yourself a transformer rated to supply about 18v AC at at least 0.5A. That will rectify to 25V, and 23 after the diode drop in the rectifier. Another 2-3V will be dropped across the regulator, getting you close to your target of 20v.

Power-wise, the worst case scenario is when you try to run at 2v and max current. Your dropping about 23V across the regulator at 0.5A, requiring you to dissipate over 11W. I don't think you can do that and certainly you'll need a heat sink and a fan would help. If you really need to operate in that area, you'll need a better design. But one step at a time.