Low cost bench power supply

[kek]

Power supply review and mod

Here's an inexpensive but surprisingly decent linear power supply based on the lm317 linear power regulator chip. The power supply provides a range of voltages between 3V to 12V, all with up to 2 amps of current. The selectable voltage output are 3V, 4.5V, 6V, 7.5V, 9V, 12V. I modified the 4.5V selection to provide 3.3V and the 6V selection to provide 5V (info on this mod following).

I like the fact that I can tell what the voltage output will be before I turn on the power supply. I also like that the power on LED is a dim red LED, no need to be blinded every time I want to use this power supply.

While investigating the possibility of modifying the selected output voltages I noticed that the design of the power supply maximized the amount of current available for all the voltage ranges by selecting an appropriate voltage tap off of the power transformer. By having a lower voltage for the lower voltage outputs, the power (heat) dissipated by the regulator chip is reduced.

Mod details

The formula for selecting an output voltage is very straight forward. R2 is the resistor between Vout and ADJ. R2 = 68 ohms in this power supply. R1 is the resistor between ADJ and 0V. The formula for desired voltage output is approximately:

Vout = 1.25V x  (1 + R2/R1)

Or rearranged to find R1:

R1 = (Vout/1.25V - 1) x R2
R1 = (3.3V/1.25V - 1) x 68 ohms = 111.52 ohms

The existing value for R1 (4.5V output) is 176 ohms. I had the option of removing this resistor and replacing it with a 110 ohm resistor but I simply added a 300 ohm resistor in parallel to achieve the required value.

To adjust the 6V output to 5V I put a 1k ohm resistor in parallel with the existing 258 ohm resistor.

I noticed the selector switch is a little noisy when switching, this won't really affect my use of this power supply as I'll mostly just be using the power switch to turn the thing on and off between uses. I can simply change voltage setting when the power supply is turned off.

Not a bad little unit for the $30 CAN that I paid for it. A search on the net found that this unit can be had for as little as $20.

[kek]

Should have included the model info!

MW122A

Not sure who makes this but a web search on the net with the model no. brings up plenty of hits.

[ColinA]

Hey great timing. I was recently looking at getting a small bench PS. Was looking at the BK Precision 1514 but the lack of a power switch was turning me away (could mod on in, but why). Plus that looks cheapers..

[Mechatrommer]

lm317 v. reg is simple and easy (hence fun) to do, i'm going to do it myself later. but dont know if the resistors drift, the voltage maybe off.

[david77]

This thing has been around for decades. It is of relatively decent quality, I think. There's much worse
stuff out there.

AFAIK these guys make it http://www.minwa.com/product.asp#

[OhmEye]

Vout = 1.25V x  (1 + R2/R1)

I think there's a typo there, R1 and R2 are reversed. This works:

Vout = 1.25V x (1 + R1/R2)

Thanks for the info, I picked one of these up for US$16 and swapped out values for 3V and 4.5V to get 3.3V and 5V respectively. I hand selected two 56 ohm 10% resistors in series to get the 111.5 ohm value, and a 150 ohm + 56 ohm in series to get the 204 ohm value.

[kek]

Thanks for the correction OhmEye

What I should have said was...

The formula for selecting an output voltage is very straight forward. R1 is the resistor between Vout and ADJ. R1 = 68 ohms in this power supply. R2 is the resistor between ADJ and 0V. The formula for desired voltage output is approximately:

Vout = 1.25V x  (1 + R2/R1)

Or rearranged to find R2:

R2 = (Vout/1.25V - 1) x R1
R2 = (3.3V/1.25V - 1) x 68 ohms = 111.52 ohms

(I got the R1 and R2 mixed up in the original description, the formula now works)