Well,

I've been fiddling today and have made myself another bench PSU.

I was looking for a 28V supply the other day, so I've knocked up something to give me a 28V, 12V and variable output.

The 28V line is a simple LM317 - there are plenty of calculators for that voltage regulator on line.

The 12V line is simply a 7812 regulator, I think I need to add a big heatsync through because the power dissipation at 500 mA will be 22 - 12 = 10V * 0.5 = 5W so its going to run quite hot.

The variable supply gives an adjustable output between about 2.5V and 15V; it's based on my favorite the old lm723:

The first pass transistor is a BD139 and then the main one is a TIP3055.

The current limiting is dictated by the voltage between pins 2 and 3 so we have a 0.1R resistor there on the main high current path, the voltage difference needs to be 0.6V to initiate the current limit so using ohms law we can easily calculate that the current limiting will happen at 0.6V/0.1R = 6A.

There's a really nice toroidal transformer which will deliver plenty of current.

It's all a bit of a bodge:

The case looks OK:

It's not bad for a few hours work. The bench looks like a bomb has hit it:

good, egh?

I've been fiddling today and have made myself another bench PSU.

I was looking for a 28V supply the other day, so I've knocked up something to give me a 28V, 12V and variable output.

The 28V line is a simple LM317 - there are plenty of calculators for that voltage regulator on line.

The 12V line is simply a 7812 regulator, I think I need to add a big heatsync through because the power dissipation at 500 mA will be 22 - 12 = 10V * 0.5 = 5W so its going to run quite hot.

The variable supply gives an adjustable output between about 2.5V and 15V; it's based on my favorite the old lm723:

The first pass transistor is a BD139 and then the main one is a TIP3055.

The current limiting is dictated by the voltage between pins 2 and 3 so we have a 0.1R resistor there on the main high current path, the voltage difference needs to be 0.6V to initiate the current limit so using ohms law we can easily calculate that the current limiting will happen at 0.6V/0.1R = 6A.

There's a really nice toroidal transformer which will deliver plenty of current.

It's all a bit of a bodge:

It's not bad for a few hours work. The bench looks like a bomb has hit it:

good, egh?