# Thread: want to build a power supply

1. ## want to build a power supply

Hello,
I'm going to need 2 power supplies
1) 48vdc @ 15amp
2) 48vdc @ 24amp

I searched ebay for the PSU for few months now , no luck there. So i decided to build them.

i need to know what size of toroid transformer, and cap is need. I saw couple transformer on ebay they cost bit high.

i'm not a electronic person so is you can draw a diagram that will be really helpful.

2. i have just read couple thread about the transformer. There is one thread talking about wiring the power supply is parallels to get double AMP. could someone explain this to me.

thank you very much
EZ

3. basically if you have 2x 20v outputs, you can wire them as either 40v with the coils in series, or 20v in parallel. Note that either way you only get the same power out, the VA rating of the transformer.

if it was a 200VA transformer, then at 20 volts you could draw 10A, and at 40 volts you could only draw 5A.

4. If you search the forums you will find lots of posts describing how to build an unregulated supply, so im not going to repeat it all here for you.

Basically i think you want a single 48v 40A power supply. This would require a torroid with a 2000VA rating, which arent easy to find. The simplest way would be to buy two 1000VA rated 34Vac out transformers, and two bridge rectifers. Connect the secondary windings of each transformer to a seperate rectifier, connect the outputs of the bridge rectifiers to your capacitors.

you should be able to find the formulas for calculating minimum capacitance required on the forum (ive posted them before, just cant remember off the top of my head).

• For those who can't or won't use the search feature on this message board, I submit the following for your perusal:

Daisychaining ATX power supplies:

http://www.cnczone.com/forums/showth...015#post142015

Linear P/S design/construction
http://www.campbelldesigns.com/files...ply-part-1.pdf

Even though you can now point and click, I'd still encourage you to do a search for "power supply" on this website. The topic has been discussed ad nauseum numerous times.

More basics:

When you hook two transformers or p/s or batteries in SERIES, you SUM the voltages and should plan on being limited to the current output capacity of the SMALLER of the two. Again, basically speaking.

When you hook up the output of two transformers or power supplies or batteries in PARALLEL, you can pull current from both simultaneously therefore you SUM the output current potential of the two - the voltage stays the SAME.

Hint: when you want 48 volts at nearly 50 amps DC, you are talking LOTS of power and power costs money.

If you're at all squeamish about electricity due to a lack of education, spend the money and buy the p/s. Otherwise, plan to do some studying and reading and spending because, whether it is parts or knowledge, it never comes totally "free".

NOTE: For the sake of simplicity, the above examples assume that you're using matched whatevers to hook in parallel or series....

• check Campbell design website for full outline on building an unregulated supply. It has pictures too.

• Wouldn't it be more economic and power saving to use a switching power supply design instead of a linear for such high output currents ?

• fiski: depends what you mean by economic, ive got an unregulated 60v 20a linear supply sat here that cost me £12 in scrap parts Personally i like to use linears as i can get parts cheaply, and if something goes wrong its an easy fix. Things would be very different if i wanted a regulated output.

• Take a couple of these 34VAC 14amp transformers, couple of 50amp 600PIV bridge rectifiers, couple of 28000 uf capacitors and build a powersupply. You'll also need a couple of cases.

http://www.surplussales.com/Transformers/PowerTra.html

Follow directions on many threads here on how to build a powersupply. You can build several and tie the DC outputs together to form one positive and one negative. Kind of like linking batteries in parallel. Should be able to weld with this.

RipperSoftware

• Here seems to be the recurring problem with power supply questons:

A person sees a spec and then goes looking for an exact match for the particular group of parts they've collected. This can be done but not easily with surplus parts or DigiKey/Mouser sourced items.

A person who understands the "lingo" of transformers knows what VCT means as well as the difference between series and parallel connections. They also know the elementary math needed to engineer the "custom" power supply that this message board gets peppered with. I hesitate to explain exacly how as an improperly wired system could hurt or kill someone if not perform improperly which leads to more issues of unhappiness.

In this case, the TP 07( ) "surplus" one would satisfy the current, but, oops, too much voltage (51 vs 48). Although this may or may not make a difference (I'd guess not), it is hard to say with the info availabl. The COL ( ) one would be a candidate, more on this later...

To get 48 volts at 24 amps, one could buy two of the same parts and wire them in parallel but that gets pricey - and again, you'd have 51 instead of 48 volts - and a lot of money spent.

By the way the voltages above are AC volts. When properly rectified and filtered, the 51vac turns into about 72. OOPS!!!!

For 48 volts rectified and filtered DC, you'd need roughly a 34 vac output - hence the COL one.

For the 15 amp (assumed at full load) system, go for a 510va min rating (15x34) at full load. For the 48 amp side, go for a 1632va min rating. A larger va rating means that as you draw more and more current up to the max rating, you'll see less voltage drop at high/peak current.

Since most transformers do tend to "droop" volage wise as current demand increases, it is customary to run the voltage a bit higher to still have full power (volts times amps) when you ask for it and as voltage droops.

Keep in mind that if you now start to talk 48vdc, FULLY REGULATED, you're talking big bucks as transistors that will regulate that much power are NOT cheap and neither is the support circuitry. That is why high power regulated supplies are so pricey.

This is why DIY systems are not always as cheap as they start out to be and why "kit" systems seem so expensive. A properly engineered system should work in a fashion that typifies the cost associated therewith....

You don't get something for nothing...

Yes, I may have become overly sensitive about doing the math BUT, afterall, a properly functioning DIY cnc system does have to be engineered. Sadly, not enough emphasis is placed on providing links to prior and proper explanation threads.

Conseqently , another "how do I build a power supply question???" thread gets started, a "use this" is tossed out and only folks who do or know the math know why the "use this" would work...

This is why we reinvent the wheel so often....