Switch Mode Power Supply for the work bench

OK, for those of you that know me personally, this is a hard copy of the physical excitement I felt while performing my ‘Show and tell’. Haven’t got a clue what I’m talking about??? Let me start from the beginning.

Switch Mode Power Supply bare bones

One cold winters morning or afternoon (I’m not sure which so I’ll say winters day) I came across quite a few power supplies which offered several voltage outputs. 5V, 12.1V, -28V and 3.3V (wow, permission to rub my thighs missy). You see, As I slowly move away from the dark ages and 20 year old IC’s, I’m finding more and more that I need to use the lower voltage range for my circuits. I took it home and plugged it in (it worked a treat). With this partial success in mind I set out to make a switched mode power supply out of scraps around my work shop.

The first thing to decide was the outputs I wanted. What I settled for were the common outputs plus a variable output. These are:-

• 3.3V from one of the 3.3V outputs
• 5V from one of the 5.1V outputs
• Variable 1.25V to 18V from the -28V output

Now I must admit that I cheated abit with the 5V output. Since the actual output was 5.1V (only 0.1V over what I actually need), I decided to use the 5.1V directly to save circuitry. The 3.3V ouput was spot on so that just leaves the variable output which is taken from the -28V output.

Variable voltage regulator

Two issues here.

• The output is a fixed voltage
• The output is negative in relation to the other outputs (-28V)

The fist issue is simple to overcome. I placed a variable regulator on the output of the power supply to allow me to alter the output to the desired level. I used a LM317T regulator for the job but with a little tweek to the standard design. Instead of putting the -28V to Vin of the regulator, I put the 0V line of the power supply to Vin and the -28V to the ground side if the circuits biasing resistors (is that the correct term???). I did this because the most +Ve voltage should always go to Vin on the LM317T and 0V is more +Ve than -28.
But there’s a catch…

This arrangement makes it impossible to mix the grounds for the variable output and the other two outputs. If I use the wrong ground then the output from the regulated output will appear as a -Ve voltage (a small price to pay).

Adding mains power was simple enough.

Switch Mode Power Supply mains switch and filter

I had a spare filtered IEC socket lying around so I used that to get the raw 240V mains voltage to box. I then recycled a power switch from a previous hack and used it to switch the mains to the power supply.

Switch Mode Power Supply mains switch

Switch Mode Power Supply layout

A wiring loom was needed to get the outputs of the power supply to the outside world.

Switch Mode Power Supply wire loom

Finishing touches include an LED for ‘power on’ indication with it’s supply coming from the remaining 5.1V output via a resistor, four pieces of wood to support the frond and rear faces and four rubber feet (just because).

Switch Mode Power Supply fully connected up

And there you have it. My first Switched Mode Power Supply. May we grow old and create a multitude of projects together.