Power Supply Design - Is there a cheaper way? Need help

Ok here is my current design for a power supply for my project, here are the outputs I need:

5V able to drive up to 5A of current.
12V able to drive up to 1A of current.
-5V able to drive up to 1A of current.

As you see from my design, this is quite expensive for a power supply. The transformers alone are $30. Add the Regulators and its $40. Add the Misc components like the rectifiers and capacitors and we are talking $50 for this supply.

Are there any inefficiencies in this design? Any way to make this power supply cheaper and have the same capabilities? Any input would be appretiated.

Whenever you build one of something you're kinda stuck paying quantity one prices. $50.00 does not seem like a big price tag for this power supply.

Cost savings:
Is there any advantage in using the same 2.5 A transformer for both parts. I see no advantage in using a different smaller transformer for part of the supply.

2200 uF is a pretty beefy capacitor. Check the voltage ratings and see if you can get by with smaller capacitors on the inputs of the regulators.

Use a full wave rectifier made of diodes instead of a bridge rectifier.

It's not much but you gotta start somewhere.

I would be tempted to run the whole thing from one transformer, as I can see your not afraid of using a switching regulator. But, then, the thing that drives transformer cost is often the current rating, so I can see why you thought it would be better to go with a higher voltage and just switch down to +5V. The thing that bothers me about this is that a 5A switcher of such a simple configuration is going to be noisy, both electrically and magnetically and may be a hassle, forcing you to add some additional output filtering which may reduce your efficiency. I wonder if it might be better to go the other way, that is to just get only one transformer that can put out enough power for all outputs, then use a linear regulator for the 5V output (perhaps the transformer is a 6.3V type), and switching regulators to generate the +12 and -5 at 1 A. This will need a bit more thought.

By the way, your circuit can't work they way you've drawn it, since you can't get a negative voltage from a positive voltage using a 7905. And your rectifier configuration on the output of T2 doesn't make any sense. Why have you taken the output from the terminal 2 of D4 rather than terminal 1?

Can you use an ATX power supply, most have plenty of current on the positive rails, not sure about the -5 current (you might be able to regulate the -12) They power up by shorting the green wire to any black wire.

Here's a nice wiki on such a project

http://wiki.ehow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

RadioRon said:

I would be tempted to run the whole thing from one transformer, as I can see your not afraid of using a switching regulator. But, then, the thing that drives transformer cost is often the current rating, so I can see why you thought it would be better to go with a higher voltage and just switch down to +5V. The thing that bothers me about this is that a 5A switcher of such a simple configuration is going to be noisy, both electrically and magnetically and may be a hassle, forcing you to add some additional output filtering which may reduce your efficiency. I wonder if it might be better to go the other way, that is to just get only one transformer that can put out enough power for all outputs, then use a linear regulator for the 5V output (perhaps the transformer is a 6.3V type), and switching regulators to generate the +12 and -5 at 1 A. This will need a bit more thought.

By the way, your circuit can't work they way you've drawn it, since you can't get a negative voltage from a positive voltage using a 7905. And your rectifier configuration on the output of T2 doesn't make any sense. Why have you taken the output from the terminal 2 of D4 rather than terminal 1?

Click to expand...

Sorry about the error in T2, that wasnt intentional I just made an error being lazy and copying and pasting and not paying attention, I wouldnt actually hook it up like that

What do you guys think the minimum filter caps I can use after the bridge retifier? Also, I wanted to use 2 transformers because of size considerations. And I can't use an ATX power supply because this power supply is just part of a bigger project i'm working on thats going to be enclosed in its own case.

Also, yeah i looked at the spec sheet and it does require a negative voltage for the negative voltage regulator, what was i thinking. Would I have to use some kinda of rectifier that only gives me the negative part of the AC or something?

I've been spending a little time trying to learn about switched mode power supplies (SMPS). My first comment simply supports what's been said already - building one of anything can be expensive. The learning and sense of accomplishment might be priceless.

One of several advantages the SMPS approach is said to have is in the transformer. For safety you need to separate the main incoming supply from the output of your supply with a transformer. If you put the transformer(s) on the higher frequency side the transformer gets much smaller and from a production standpoint it's less expensive. Now, can you get these less expensive transformers, one at a time, at less cost - that remains to be seen. For my own purposes I would approach the design as you have -by first isolating the incoming supply. I would not do switching of mains supply without having a lot more skill than I have at present.

Why not use a center tapped transformer.

another suggestion for you, since I think the big cost of your project is spent on big bulky power transformers?

try googling a bit for "offline switchmode power supplies" and/or "isolated offline smps"

"offline" refers to the switcher handling direct mains line voltage, bypassing the need for a bulky power transformer.

"isolated" refers to the switcher again handing the line votlage directly, but this time, but using a transformer (instead of the inductor) on the output to provide the isolation.

take a look at the Supertex HV9910 ... it's intended as a constant current LED driver, but you can also use it as a constant voltage regulator, by replacing the current sensing resistor with a voltage divider (all the chip cares about is seeing 250mv at the feedback pin). With the proper parts, you should be able to handle even your 5a load. The 9910 is already setup to handle something like 400 volts input, so t is designed to run straight off (rectified) mains. The also have tips as to how to limit inrush current, as well as using a transformer inplace of the inductor to provide mains isolation. Supertex is just one suggestion, Linear, TI, Fairchild, ON, ST all have offerings for offline switchers.

alternately, look around at surplus shops for an "open frame" or even bare board switchmode supply ... they're designed to be built into things, and the voltages you're after are pretty common, so it should be inexpensive.

Is there anything you guys can think of to reduce costs?

Can anybody tell me what the deal is with this kind of power supply?

**broken link removed**

I currently have one of these, only costs $20, and it delivers 5V at 15amps! 12V at 2A and -5V at 1A. How in the hell does it deliver that much power for only $20? Yes I know when things are bought in mass quanities they are cheaper, but not that much cheaper.

Also, I took apart the case and looked at the parts inside, and they 3 transformers, I'm guessing one for each voltage. The thing is, the biggest one (im guessing for the 5V, isnt even all that big, its pretty small and doesnt look like a transformer that can support 5V at 15A. I wish I could get the part number on the transformer but I dont see it anywhere on it. And the transformers for the 12V and -5V are extremly dinky, like those random radio shack 100mA transformers.

Anybody mind explaining this to me? I'm very confused.

This is an off-line switcher. The transformers are custom made, so you won't be able to buy them, but if you have to make the supply yourself, it would be worth the price just for the transformers! If you were to design an off-line switcher yourself, you would have to make your own transformers.

Ok I redid my power supply a bit a brought down the cost of the parts to about $35 by settling for 4A at the 5V and bring down some of the ratings for the parts. Can anybody else think of anything else I can do to bring down the costs? Also, how do I go about getting the regulated -5V out since the current way I have it setup wouldnt work?

Make D4 into a 1/2 wave rectifier by disconnecting D6 from C29. Reverse D6 and connect it to U7. You will have to add a 1000uF cap.

Russlk said:

Make D4 into a 1/2 wave rectifier by disconnecting D6 from C29. Reverse D6 and connect it to U7. You will have to add a 1000uF cap.

Click to expand...

Would that mean that 1/2 the power from T2 would be supplying the -5V and 1/2 the power would be supplying the +12? Would this work? Because the +12 needs 12VA of power and the -5 only requires 5VA of power. So that means the +12 would only get 8VA of power instead of the 12VA it needs to drive 1A.

This is logic correct?

The transformer will supply whatever the load requires (unless it burns up). The disadvantage of the arrangement is: 1. the ripple frequency is lower, so a larger filter cap is needed. 2. Since the load is unbalanced, there is a DC unbalance in the transformer which could cause more heating. If your transformer has excess capacity, that will not be a worry.

Do you think 18VA will be enough to power the 12V and the -5V at 1A (each)?

Ok here is the final schematic I'm going with.

Do you guys see any problems before I go ahead and order the parts to test this?

I would suggest replacing D4 and D5 with a bridge?, a half wave rectifier isn't much good at 50/60Hz - a bridge as two full wave rectifiers is MUCH better.

Nigel Goodwin said:

I would suggest replacing D4 and D5 with a bridge?, a half wave rectifier isn't much good at 50/60Hz - a bridge as two full wave rectifiers is MUCH better.

Click to expand...

Do you mean replace both D4 and D5 with a single bridge or a bridge bridge replacing each diode?

I ask this because I need to be able to rectify a negative voltage for the negative voltage regulator (the -5V output). If I replace D4 and D5 with a bridge I wont be able to do this.

FusionITR said:

Nigel Goodwin said:

I would suggest replacing D4 and D5 with a bridge?, a half wave rectifier isn't much good at 50/60Hz - a bridge as two full wave rectifiers is MUCH better.

Click to expand...

Do you mean replace both D4 and D5 with a single bridge or a bridge bridge replacing each diode?

I ask this because I need to be able to rectify a negative voltage for the negative voltage regulator (the -5V output). If I replace D4 and D5 with a bridge I wont be able to do this.

Click to expand...

You do the same as D2 and D3, that's a full wave rectifier, add two more to give the negative half as well - it's normal to use a bridge to do so.

Sorry, to be mean, but if you don't understand such a simple basic circuit perhaps you shouldn't be looking at vastly more complicated switchmode regulators?.

If both are drawing 1 amp, the transformer is balanced so there is no problem. The worst case is 12 V at 1 amp and 5 V at 0 amp, but the transformer should handle that OK.