# Convert ac to dc

Discussion in 'Electronic Basics' started by Victor MacQuarrie, Apr 4, 2007.

1. ### Victor MacQuarrieGuest

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

2. ### John FieldsGuest

View in Courier:

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

4. ### redbellyGuest

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

6. ### Phil AllisonGuest

** 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

7. ### Phil AllisonGuest

"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

8. ### redbellyGuest

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

9. ### jasenGuest

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

Bye.
Jasen

10. ### Victor MacQuarrieGuest

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

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

11. ### Ben GustaveGuest

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

12. ### Victor MacQuarrieGuest

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

13. ### wlbGuest

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
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

14. ### Victor MacQuarrieGuest

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

15. ### Victor MacQuarrieGuest

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

.....Vic.....

16. ### Randy DayGuest

Victor MacQuarrie wrote:

[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.

the voltage down is another issue, and to suggest a
circuit, we'd need know what current the pump needs.

17. ### John PopelishGuest

Try it with no capacitor.

18. ### ehsjrGuest

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
roughly .7 volts (depends on how much current the
pump draws).

across the pump while it is running. Add diodes as
necessary to get to roughly 14 volts.

Ed

19. ### jasenGuest

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

20. ### Victor MacQuarrieGuest

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.....