Questions on DIY stuff

[Lunat1c]

With summer fast approaching I'm going to be having some free time on my hands and I would like to build my own PSU for my homelab which I only started recently. I have searched the forum regarding similar threads however I didn't quite find what I was looking for. The thing is that even though I have a good idea of the features that I'd like in my PSU, and the basic circuit topologies of a regulated PSU, I don't think I'm knowledgeable enough to design the whole circuitry from scratch by myself.

The features I would like are:
- Dual rail supply, with variable voltage ranging from -15V to +15V (or just from 0 to +15V and then connect accordingly when -ve voltage is required)
- Current limiting capabilities
- And perhaps an LCD that displays voltage/current just to make it interesting?
- Any other features that I might have missed which you guys think are necessary

From my knowledge I am assuming that first of all, I will have a full-wave rectifier that would convert the mains AC voltage to an unregulated DC voltage. This stage would then be followed by a buck converter to step-down this unregulated DC voltage. However I'm not sure how to proceed from there to make the regulated dc voltage variable, to implement current limit capabilities and to display the voltage on an LCD (probably done by some PIC?) I won't be experimenting with the mains whatsoever until I simulate the whole circuitry and know for sure what I'm doing.

With this said, I would like to know how I should approach this in the first place. I don't know what I require exactly. I am in no hurry at all and I'm doing this to learn however I require some directions.

Any suggested readings regarding the above mentioned stuff? Or perhaps are there any schematics which one can find online and could build on? I'm open to suggestions  

[Simon]

You basically want to build a mains driven switchmode PSU. If you have not dabbled in this before i suggest you make low voltage SMPS's first. You vary your output by playing with the feedback network and the DC voltage produced will be regulated.

Your best bet is a power transformer, drop it to something like 25-40V and then use a SMPS to regulate and control the voltage output

[Armin_Balija]

I'm not sure if I'm going to help but here is what I would do, I'm probably wrong though..so correct me if I'm wrong.. I don't want to kill anyone

1. I'd start with stepping down the mains to 30V @ 60Hz (15v dual rail) using a center-tap transformer...I'm assuming you can find one online.
2. I'd get a transformer with a larger VA margin than what you'll use for safety.
3. I would then put a bridge rectifier on the +/- output of the transformer and an LM317 with capacitors on the input, gnd, and output of the regulator to smooth out the voltage.
4. Then I would use a resistor and a potentiometer on the output of the regulators to give me the 15 volts I need ( ceramic cap after the voltage divider ).
5. I would use a pot so I can make the PSU adjustable.

I haven't worked with LCD's before so I assume you'd just using voltage from the output of the regulators to feed the LCD the info it needs along with it's own separate circuit.

Maybe I'm wrong though.

[alm]

1. I'd start with stepping down the mains to 30V @ 60Hz (15v dual rail) using a center-tap transformer...I'm assuming you can find one online.

I would prefer one with two independent secondaries, that way you can just use the same circuit (with positive regulators) twice, as opposed to a positive and negative version.

2. I'd get a transformer with a larger VA margin than what you'll use for safety.

Agreed. Keep in mind that for full bridge rectification with smoothing, DCV = ACV * sqrt(2) - 2V (drop over bridge rectifier, two diodes), but IDC = IAC * 0.6 (not 1/sqrt(2)) (fudge factor because the capacitors are only charged at the peak of the AC voltage). Also account for some drop in the regulator. So don't expect 21V/2A of output with a 15V, 30VA transformer.

3. I would then put a bridge rectifier on the +/- output of the transformer and an LM317 with capacitors on the input, gnd, and output of the regulator to smooth out the voltage.
4. Then I would use a resistor and a potentiometer on the output of the regulators to give me the 15 volts I need ( ceramic cap after the voltage divider ).
5. I would use a pot so I can make the PSU adjustable.

That will work, except that the LM317 won't go below 1.2V and you will need some extra circuitry for an adjustable current limit (eg. a second LM317). There has been a fair amount of discussion about DIY lab supplies, including discussion about several designs that eliminate these issues (two LM317 with the adjust pin pulled below 0V, LM723, the two op-amp design most good commercial designs use, secondary side buck converter).

I haven't worked with LCD's before so I assume you'd just using voltage from the output of the regulators to feed the LCD the info it needs along with it's own separate circuit.

You'll need either a micro controller to control the LCD, something like an ICL7106 (note that they expect seven segment displays, as opposed to HD44780-compatible text LCDs), or standard digital panel meters. Note that the latter usually need a power supply galvanically isolated from the signal they're measuring.

[Zero999]

An LM317 is a good start.

You could build a basic 1.25V to 15V supply. Once you've got it working expand it to include additional features such as operation down to 0V and current limiting, although there are better ways to perform the latter than adding another LM317.