# Convert ac to dc

• posted

Hello, I have a power transformer that converts 110 v ac to 12 .6 v ac. What is easiest way to change 12.6 v ac to 12.6 v dc to run a 12 v fish pond pump continuously. Thx..........Vic

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View in Courier:

+------+ 120AC>---+ +---|~ +|-----+ P||S | | |+ R||E | | [PUMP] I||C | | | 120AC>---+ +---|~ -|-----+ +------+ FULL-WAVE BRIDGE
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JF```
• posted

```--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.```
• posted

A rectifier will convert 12.6 Vac to 17.8 Vdc. A 12.6 Vrms has an amplitude of 17.8 V (multiply the rms voltage by the square root of

2).

You'll need to drop the 17.8 V down to 12 V. Either a 12V voltage regulator which can handle the pump current, or even a power resistor in your application, can be used.

Mark

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As long as doesn\'t use a reservoir capacitor after the rectifier he
shouldn\'t have to use a regulator  since 12.6VRMS does the same work```
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** No it will not ~!~!!~!~

The average DC output voltage of a perfect ( ie no diode drop) bridge rectifier is 0.90 times the rms input voltage.

Look it up.

....... Phil

• posted

"John Fields"

** Not with a motor as the load.

A rectified sine wave has a lower average value ( by 0.90) than its rms value.

....... Phil

• posted

Yeah, I was assuming a cap in there. Phil's right though, for a full wave bridge the average voltage is 90% of the rms value (ignoring diode drops). Assuming a 0.6V drop, Vic will get

0.9*12.6-0.6 = 10.7 V for the average voltage.

Chances are that's enough to turn the motor. I don't know what the effect of the 17V peak voltage would be, but I'm not a motor expert.

Mark

• posted

a bridge rectifier will get you 11.4 (-ish) ... close enough ?

Bye. Jasen

• posted

I will try the rectifier, and let you know......

Thanks again.........Vic

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The bridge rectifier will drop, say, about 1.2 volts (assuming a crappy silicon rectifier) off the peak ac voltage. He'll have about 16.6 volts at the peak of the ac cycle, which is where any filter cap he throws on there will charge to. Now, depending on the pump and the size of the filter cap he could have anywhere from your 11.4-ish up to about 14 volts, give or take.

I don't think that unfiltered 120Hz dc would be any good for the pump, Victor, so a filter cap is a really good idea. Radioshacks tend to carry

4700uF electrolytics for a few bucks a pair; they'd be a good fit.

-ben

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Hi guys.......This transformer I have has 5 leads, 2 black on the primary side (110 v in ) and 2 yellow and one black on the sec. side. The black I assume is the common, and 6.3 v at 3 A on each of the yellow wires.How do I get 12.6 volts??? I connect the 2 yellow and I get a short??? I guess this sounds silly, but I'm afraid I'm not too swift on electronics HELP!!! ( There was no paperwork with the transformer) Thanks......Vic

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You'll not be using that black wire on the secondary side, just tape it off and tuck it away. Each yellow wire will go to one of the AC leads of your bridge rectifier and then your filter capacitor will hook across the + and - leads of your bridge rectifier. Take care to observe polarity! The capacitor - lead must go to the bridge - and the cap + lead to the bridge +. If you get it backwards the cap will let you know with a big bang. Then you hook your pump up across the cap and you should be in business. Good luck!

-wlb

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Hi again guys........ Thanks for the info on the transformer Ben . It seems to work fine. Only problem is,..... I could only get a 3300uf 25 volt capacitor. The tech told me it should work, only problem now is I have an output of 18.89 volts dc. which I think is probably not good for the pump. Pump is designed to run from 12 v to 16 v dc. Would a 4700uf 12v cap. solve the problem???

Thanks again .......Vic

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Sorry, wlb in my last reply I gave credit to Ben for the info. Thanks again to all of you

....Vic.....

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[snip

Only if your problem is how to make something go 'bang'.

A capacitor can only work up to a maximum safe voltage, after which it goes, well, bang.

Your 3300uf should be adequate for filtering. Stepping the voltage down is another issue, and to suggest a circuit, we'd need know what current the pump needs.

• posted

Try it with no capacitor.

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No. Your 3300 uF cap is fine. You can drop the voltage down with some diodes in series between the pump and the supply. Use diodes with a current rating equal to or higher than the current rating of your rectifier. Each diode you add will drop roughly .7 volts (depends on how much current the pump draws).

Start with 2 diodes in series, and measure the voltage across the pump while it is running. Add diodes as necessary to get to roughly 14 volts.

Ed

• posted

Was this measurement taken with the pump operating? because if it wasn't it's not telling you anything useful.

If you want less voltage, using a smaller cap may do it. A little, or even a large amount of, ripple is unlikely to damage a motor. This sort of equipment is designed to withstand vibrations: a thyristor speed control gives worse ripple than a full-wave rectifier.

Bye. Jasen

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Hello John......Disconnected the cap. and now I have a steady 13.3 volts. Hopfully the problem is solved.... I'll let you know.

Vic.....

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