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Linear regulated power supply design

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aonomus

Wed May 28 2008, 05:22PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Well, I'm in the process of designing a small benchtop power supply because I can't afford a commercial one (and I want to learn how to build one anyways). Attached is my draft schematic pending a sanity check. It has a positive/negative variable output, and a fixed 5v output for TTL stuff...

I already salvaged a pretty nice centertapped 16-18V transformer that is pretty beefy out of a UPS however I'm not sure as to determine its max current rating on the secondary. It has a 10A circuit breaker on the primary 120VAC side, however I doubt I would want to push it to that point as the breaker is more likely there because there was a downstream outlet on the back of the UPS hooked into the input cord through the breaker. How can I safely determine the max rating across the secondary?

Either way, the other question I had was regarding current regulation, is there a more efficient way of regulating current than simply passing it through a rheostat? I saw a few transistor configurations looking around, but I'm wondering what is an efficient method to regulate current without having the possibility of the rheostat changing resistance as it heats up?

And one last [stupid] question: what category does Eagle file a symbol for a 120VAC plug under!? I've looked everywhere that would make sense and its driving me nuts.

Edit: Forgot to include (50v 2.2uF) smoothing caps on the outputs, those will be included in the final design, the 10uF caps are to smooth out any ripple from the pot.

Further edit: forgot to mention also, the 317/337's are not the ones to actually be used, I will find either 3A or 5A adjustable regulators from Digikey and heatsink them appropriately. I only used them as placeholders since they are convenient.

dingo27

Wed May 28 2008, 11:40PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

when i was doing my ps, i found some time later that ih had the 3A 5V (but it could give 20V but low A), and ine little thing - i was always able to get more current through 317 that factor 1,5A, the max value i was able to get was near 4A ( it was overheated but it was funny working... ) i never burned any lm317...

and i think that 5V for TTL is not bad , but sometimes i use 5,5 / 6V for TTL (once i was using 7, but it was nearly to collapse ), and if i were building that PS, i would give that larger DC filtering Cap (1000-2200uF and protective diode across LM317 circuit (or that you are going to use)

aonomus

Thu May 29 2008, 02:31AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Ok so I revised with protective diodes on all the regulators, probably a good idea since I'd be using the 5V rail to power TTL stuff while I use the variable to power say, motors or other inductive loads that might have a fair bit of back-EMF

Also this is the transformer I salvaged, very clean and already terminated with quick disconnects, but can anyone make sense of the transformer markings and as to how many VA the transformer is?

Also I would not be surprised if the current regulation is non-linear and dependent on output voltage since at minimum voltage the regulator has got to send current somewhere. Either way there are some 5A or 3A TO220 packaged regulators that would work quite handily for the variable ones, only a few dollars from digikey.

Another note is, does anyone know a non-rheostat method of controlling current?

HV Enthusiast

Thu May 29 2008, 02:47AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Okay a few problems with your design.

1. You need some ceramic caps on the input / output of your 780x regulators. You need this for stability reasons, otherwise you may have some problems during load transients and get some oscillation. I typically use 0.1uF on input and output.

2. You need diodes across input to output on your 780x regulators. Because you have lots of output capacitance, you need to provide a discharge path from output to input, when you disconnect the input side from the supply (OFF). Otherwise, you'll damage the regulators.

GeordieBoy

Thu May 29 2008, 09:46AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

I'd also be concerned that you might exceed the maximum rail-to-rail supply voltage rating of that op-amp since it is connected across the un-regulated supply.

aonomus

Thu May 29 2008, 10:05PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

GeordieBoy wrote ...

I'd also be concerned that you might exceed the maximum rail-to-rail supply voltage rating of that op-amp since it is connected across the un-regulated supply.

Yea, the TL071 is listed for +/- 18v, the only problem is that a AC spike might exceed it enough. Could I tie it into the -5V supply, or does the input voltage need to be tied to the one it is regulating, thus using a pull-down resistor instead?

Edit: I went out and purchased the 7805, 7905, and the LM317/337s, I figure if I strap them onto a big enough heatsink, all should be good. I'm going to find or buy a old surplus P2/P3 slot2 heatsink since they're no longer in use.

aonomus

Sun Jun 01 2008, 07:49AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Well it would seem that theres a difference between 8V centertapped (ie: 2x 8V) and 16V centertapped. My bad. So I'm faced with either making this a 0-12V and 5V PSU, positive only, or 0-8V and 5V +/- PSU. Do many people find themselves using the negative voltage often? If there isn't a huge use for the negative voltage I'd be better off making this a 12V power supply (since it was salvaged from a UPS powered by a 12V battery).

aonomus

Mon Jun 09 2008, 03:37AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

This might be a slight bit of a necro, but I managed to find a 60V centretapped transformer for the power supply. Since the -ve unregulated rail to ground will be 30V, the TL071 won't be able to sit on it directly. I won't be frying anything by just hooking the TL071 to the -5V rail right?

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