Designing a Bench Power Supply

[tlogic]

Hello i am on the way to design a new power supply for my bench. The outputs i want to be some fixed voltages like: +3.3V, +5V, -5V, +12v, -12v about 2A (for supplying digital chips /operational amplifiers etc) and one or two adjustable outputs 0-30Volt 0-3A (for testing circuits). By this design i want to have the ability to supply any voltage that i may need to have in any project using one device and if it is need to use all the outputs in the same time.

My question is what design do you advice me to use. A main transformer and then use linear or pulsed regulators to make the multiple voltages or learn and make an smps design? What is better?

 

[shamikrudra]

SMPS is better but complicated
I will recommend you to buy an computer SMPS and use it as bench power supply
cheapest and accurate.

 

[tlogic]

And what about the adjustable output?

 

[shamikrudra]

add one lm317 circuit to it and you have very good power supply.

 

[Sajjadkhan]

I don't think that you can achieve 30V form that power supply as its max voltage is 12V.

Well tlogic i want to do the same. Do you have knowledge about SMPS? i don't but i would love to, but since it will take time to learn and i want that bench power supply within month or two so i would go with the simple power supply. I don't think that 3A is going to increase size that much?

If you have good knowledge with simple power supplies then let me know, we may start together and might make something with lots of features.

 

[shamikrudra]

usnig PC smps you can take max 24 V DC if that is enough you can use it. it is an ready made and cheap solution. if you want to make one for your self you can find lots of help in google.

 

[Sajjadkhan]

Yes but what i mean to say is that comparators and op-amps normally have voltage limit up to 32V so yes 24V is not enough. Beside i am guessing that tlogic wants to build a power supply which also reflect voltage and current on the display and something which is home made and looks good on his bench.

 

[tlogic]

Yes that's what i want to do. I know about the PC PSU i already have one on my bench but it isn't what i'm looking for.

 

[Sajjadkhan]

Well for that i first have to find an appropriate box or an old analog destroyed power supply and to design my circuit according to the box dimension.

 

[tlogic]

I think that the box is the last think you have to be worried about. First you have to design the circuit on paper, test in on a prototype and then design it to feet on a box of your choice.

 

[Sajjadkhan]

Yes of course but it happen with me many time that i have to Alter my design due to not enuff room

So the thing is that we have to use a voltage regulator, i guess LM338 is good one to start. It gets the input from a transformer followed by rectifier and a capacitor. The output of the regulator would be passing through a current sense circuit, the voltage at the output can be step down by voltage division resistors and can be fed to ADC for voltage measurement and output of current sense circuit also to ADC for current display.

A short protection circuit can be added. If higher current is required then an external transistor can be added to LM338.

What you think of it as a start?

 

[barry]

You might want to think carefully about using SMPS-they generate a lot of noise, as opposed to linears. If you are expecting to power sensitive analog circuits, SMPS might be a problem.

 

[Sajjadkhan]

Hey barry, once i opened my laptop charger. there was a copper cover inside it and inside that cover the circuit was contained. So was that copper cover to protect external devices or internal devices from noise?

 

[barry]

That shielding is mainly to minimize the power supply noise radiation. And a PC doesn't have any analog circuitry (well, maybe the audio output) to worry about. I'm not saying you CAN'T use a switching supply for analog circuits; just be aware. My company makes radar systems which are dealing with very low-level analog signals, so we avoid switchers like the plague-it's just easier to drop in a linear than to try to reduce the noise from a SMPS.

 

[SGInut]

I'm also looking to build a very similar setup. Still rather new and would help out however I can.