9 volt overly complex(I think) power supply

[coolbeer]

I'm just starting out in this field of "home improvement"(although I do have a few years of school on the subject behind me), and I thought my first project could be a simple 9v power supply.

I set about designing the beast, and 4 revisions later I was stuck on some 2n3055's and MJ2955's. After banging my head at that wall for a few hours, I kinda thought to myself, why don't I start a bit easier?
5 Revisions later I was banging my head against some zener diodes, not to mention beeing on my 27th revision of a pcb...
Soooo, simple simple simple is now my mantra, i hereby present simple:http://img269.imageshack.us/img269/6673/snapshot1o.jpg.
Now, what I really don't know is if this design will work in practice, in theory(in my head) it should, although it's a bit overly complex with the dual rectifier bridges, but where is the fun in doing everything by the book.
I was thinking of strapping the bridge diodes with some caps, but then we are pushing on the overly complex bit again :P
So my questions are: will this work, why should I not do it this way, have I missed something and do you have any suggestions?

I should probably mention that I haven't ordered any parts yet, so anything can be changed if needed. And the use for this psu would be powering my next project wich will probably be a distortion pedal for my guitar.(yes yes, I know the ampage is waaay overkill, but what if I want to run a few pedals at once? ^^ ).

[star882]

You can greatly simplify the circuit by center tap rectifying the transformer and using only one regulator.

[martin clark]

Yes, use one regulator and add an external pass transistor to deliver the current you need.

[sawreyrw]

Paralleling the LM317's is not a great idea. They will not try to regulate at exactly the same voltage. So the one that wants to produce the higher voltage will supply all of the current and the other one will not supply any current. This is the case where the load current is less than the current limit of the 317. If the load current is higher, then the second 317 will supply current, too.

If you really want to parallel the 317s, then you need to add low value resistors between their outputs and the output caps. You may still need caps right at the outputs of the 317s to ensure they don't oscillate.

I don't know what your load is, but the VA rating of the transformer may be low.

[coolbeer]

sawreyrw: Awsome reply, just the constructive criticism I need to evolve my knowledge.

Yes I know the design is a bit... over the top, especially when the loads we are talking about are below 1 amp(I dunno, 0.2 to 0.5 amps per device perhaps).

Although I'm thinking that to get 9v regulated it should be enough with a 9vAC single secondary transformer, 9x1.414 = 12.73, then we loose about 1.4v in the bridge, so 12.73 - 1.4 = 11.33. About 1.3v goes in the regulator, so down at 10.03v, witch gives about 1volt go go on, not sure how much goes away in the caps, but it's close, too low perhaps, for let's say 9v 1A?

So in theory, 9v / 10VA should be just on the hair enough for 9v 1A regulated power, unless the current over the bridge will be way over what the transformer can give(Belive I read something about that somewhere).

I'll start work on some new schematics you guys can rip apart

[coolbeer]

New and improved design, now stripped down to just one rectifier/regulator. Please let me know if my calculations for the transformer is way off, I know I'm on the limit, but how close can I cut it?

Hopefully I can start ordering stuff next week(salary and stuff), and then it's two-three weeks before I can expect stuff delivered(ebay, alot cheaper than local, even national).

[coolbeer]

Doh, forgot the image...
Here we go: http://img684.imageshack.us/img684/9793/snapshot2t.jpg

[torrence]

Looks really familiar. Here is a circuit I built about six months ago. The first 317 is being used as an adjustable current limit, and the switch is a range select. If you often use 5V, you can do your regulators a favor and provide a lower input voltage so they don't have to work so hard. Most of this is straight off the 317 data sheet.

[woody]

Cool, a 9v 10va transformer will not be large enough for a 9v 1A dc supply. The short answer is when you rectify ac you get more voltage but you also get less curent and charging the caps puts more of a load on the transformer than a resistor
would . Your transformer just might be large enough for a 9v 1/2 A supply.

[coolbeer]

I'm back!
Due to some personal issues I've been gone a while, but now I'm ready to continue forward with my many projects.

I have rethought the design of the power supply and has currently landed on doing a discrete regulator, I've been simulating it to death in LTSpice and while it looks good there it's still a simulation not real life.

Schematic:


An earlier version was using a 9v transformer, but I have more or less decided on using a 12v/1A transformer instead. It will give me more headroom for filtering at the expense of having to drop a few more volts.

The maximum current draw without the voltage dropping too much/something blowing up is probably around 500mA. Shorting the output gives about 2A of current, so fusing it for 500mA will protect the transistors from blowing up.

The transistors in the regulator is FJN965, rated at 5A, but only .75W, they have exceeded my expectations, one thing is letting the magic smoke out, but having the transistor still working afterwards is hot(and it was VERY hot). This resistance to blowing up is perfect as it gives the 500mA fuse time to blow, I could probably toss in a slow fuse and they would still live. I have shorted the output over 5 seconds continuously without anything dying, but I was getting concerned about killing the transformer and my breadboard

I have gotten the case for the supply and did a test-fitting of the frontpanel components earlier:


Happy DIYing!
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Kolbjørn