Large current 12V power supply.

Boris

For one of my projects, i need a 40A, 12v DC power supply. Voltage does not need to be precise or smooth. So I can basically take a 120v->12v Transformer, and then use a bridge rectifier and a capacitor. Does anybody know where i can get a cheap 120->12 transformer, that coud handle 40A?

Thank you,

Boris.

Beau Schwabe

Cheap? ...is a relative term. How much are you willing to spend? Many low voltage 12V outdoor lighting transformers are rated at 40A.

Cheap would be to find one pulled from something at a surplus store.

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Beau Schwabe - Mask Designer III

National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

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Robert Kubichek

The quickest way would be to take car battery charger/jumper that is large enough, and use it, but I would add some regulation of V using an appropriate regulator with enough pass transistors to handle to current.


Bob

Paul Baker

Those specs fall in the range of battery chargers, so you may want to look for one of those. A quick check via search engine pulled this up: http://www.absak.com/catalog/product_info.php/products_id/392·though I think $169 is beyond what your looking to pay, but at that amperage you need a UL listed and properly designed (like fault tolerance in the event of shorting) or you'll regret not spending a $100 more on a proper unit when your house burns down.

Your taking a raw transformer would require a transformer that puts out more than 12V AC in order to recify it to 12V DC, AC->DC transformation always ends up with a DC voltage less than the AC voltage out of the secondary windings. And be sure to place a proper fuse or GFI circuit and proper heat dissipation measures. I have a SLA brick charger rated at 30A and charging my 50AH cell from a·dead state makes the unit quite toasty.

Boris

Am i missunderstanding something here or is this:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=42881&item=3872545210&rd=1
what im·looking for for $5.95+ shipping.
What exactly does Enclosed Transformer 120V - 12V 50VA (New) mean?
Is 50 the current in amps???

Paul Baker

That Item is 50VA or volt amps, which is the same as watts but for AC instead of DC, its only a transformer meaning 120V AC in 12 V AC out and 4 and 1/6 Amps max draw, after you rectify it you'll end up with arround 9V DC and about 4 Amps.

Paul Baker

If your getting a transformer and making your own supply you'll probably want a 5:1 >500VA (120V->24V, rectify, capacitors, regulate to 12V) and please don't forget a 50Amp fast blow fuse.

Boris

Paul Baker said...
If your getting a transformer and making your own supply you'll probably want a 5:1 >500VA (120V->24V, rectify, capacitors, regulate to 12V) and please don't forget a 50Amp fast blow fuse.

You realy think that the output of the transformer should be 24V AC? 1/2 of the voltage is going to be lost across the Bridge Rectifier? I wasnt planning on adding any kind of regulation to it, but how does regulation work?

Paul Baker

Ok look at the two figures I've included. The first is the sine wave coming out of a 10:1 transformer, meaning 12V out. The second is an idealized full wave rectified version of the first figure. The second line is the output after you have used a large capacitor to turn the recitfied power into DC, as you can see it is lower than the amplitude of the recitifed power, this is because the capacitor is taking the integral of the recified power, effectively dumping the power above the line into the dips below the line. If you took the integral of the rectified·power over the time period, you would get what that voltage level that resultant DC power is.

Now here's the caviat, there are different ways of expressing the voltage of an AC signal: Peak, Peak to Peak (P-P), Average, and Root Mean Square (RMS aka Effective). Heres a link that explains them: http://www.bcae1.com/voltages.htm· V(RMS) = .707xV(Peak), V(Avg) = .636xV(Peak). Performing AC->DC arrives at V(Avg) in ideal conversion, but all electrical systems have a loss (mainly through heat dissapation). It has been a decade since I took power theory, but if I remember AC is ussually expressed in RMS. So if the voltage from the wall is expressed in RMS and the transformer is 10:1 the voltage out is 12V(RMS). Accounting for losses conversion of the 12V(RMS) could get you in the ballpark of 12V DC, depending on the quality of the transformer, rectifier bridge and the capacitors.

A regulator takes some DC voltage in and produces a lower DC voltage which is clean and stable and very close to the specified Vout. But there is a required headroom needed for this conversion to take place which is typically on the order of 1-3V (with the exception of Low Dropout type, but LDO is more common on low current type regulators). This means to get a good clean 12V DC with lots of current, your going to have to feed it with 15V DC or more. In this type of application the regulator will need a good heatsink to operate properly. You can also place more than one regulator in parallel to get higher currents (you could take 2 12V/20Amp regulators and get 12V/40Amp).

Hope this answers some of your questions,
Paul

I still suggest getting an off the shelf part to do this unless you really can't afford to do so. And if you do build your own, I would test it first for an extended period of time using power resistors equivalent to the resistance of your load which are capable of handling the current and measure voltage, current and temperature of the parts, if your parts get too hot, depending on thier design they can get into a runaway thermal condition causing more and more current to flow until something burns, this is why a fuse is important, if the fuse blows you know you designed it wrong.

Boris

Thanks for the info.
I found this Transformer:
"Transformer
Not exactly a center-tap, but close -- this chunky chassis-mount transformer accepts 120 VAC and puts out 12 VAC and 6 VAC simultaneously, or 18 VAC if output is tied together. (Ignore the label that lists the AC voltages as DC.) Maximum current at 12V is 10A, and at 6V is 4A. Measures 2-5/8" x 2-3/6" x 1-9/16" with (8) 5/16" leads. (4) mounting holes.
·34080 120 V TRANSFORMER $2.00 / EACH· "
at



http://www.sciplus.com/category.cfm?category=138

can 5-6 of these be connected in parallel to provide the required amperage?

Paul Baker

In theory yes, though it may require additional circuitry, this is treading in a direction I cannot help you in. Like I said my power systems class was in '92 and I'm very rusty, additionally it's one of the few classes I sold my textbook back at the end of the semester. Perhaps someone else can answer this more fully.

Paul Baker

How about getting a couple of computer power supplies off of ebay (like http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=44949&item=6741916590&rd=1) but I would buy two of the same brand.