Bridge rectifiers

[jamesmason]

Turing AC into DC

I have bean reading the board for a few weeks. After having some problems visualizing how Bridge rectifiers work I found something similar to this on the net and then promptly lost it. So I made this to hopefully help others with the same problem. This is the first gif i have, made Just a little plug I made this with the gimp  which is Easley a rival to Photoshop but much better because its free.

I have also made a anemometer using info on other power the idea of using a bicycle meter very good.  I use a small bearing from a model shop normally use for r/c cars and for the case I used a paroceatamal container. Just waiting for a still day to calibrate it now. No digital camera at the moment so no pitchers.

[electrondady1]

thats a perfect demo. i saved it and will use it to show my son. thanks! now if i could figure out how to chose the right size of capacitor ....

[K3CZ]

I think just tad more information would further Van clarify the presentation - insert the caption "AC Input" on the left side (perhaps in parentheses) full time, and "DC output" on he right side.                Van    K3CZ

[jamesmason]

hope this helps K3CZ

james

[BeenzMeenzWind]

I've often wondered that myself. The text books say choose a value which will effectively smooth 50 (or 60) Hz and no more, while experienced engineers always seem to use the biggest one they have handy.

How much is the right amount?

I've always used a physically big, higher voltage than necessary, 1000 uF - as it seems to work and bigger is more reliable my dad used to say.

[kitno455]

james, upper left diode is missing the neg bar at the tip of the triangle.

allan

[kell]

You can estimate the voltage ripple on the filter cap of a full wave rectified supply with simple multiplication and division.

Starting with the formula  C x V = I x t

where

C = capacitance

V = voltage change on the capacitor

I = current

t = time of one half cycle (about .008 seconds for 60 Hz)

So to calculate ripple, you take current load multiplied by time, and divide by capacitance.

Example:  you have a 10,000 uF (.01 F) filter cap on a 60 Hz power supply and a 2 amp load.

V = (2 amps x .008 sec) / .01 F = 1.6 volts ripple on the cap.

This formula gives a fairly accurate approximation of ripple if the ripple is relatively small, say 10 or 20% of the output voltage.  An accurate calculation of ripple on a supply with a heavy load or inadequate filter cap (which amount to the same thing) would be more complicated, but at least with this approximation you can see whether the ripple is excessive.

[jimjjnn]

Just as long as voltage for capacitor is high enough, use whatever works for your application

[jimjjnn]

If doing a wind turbine, you have varying frequencies other than 50-60 Hz,

[Ungrounded Lightning Rod]

In doing a wind turbine (assuming it's charging a battery and you aren't trying to do something like series the three phases) you don't care about trying to smooth the output.  Just run the current into the battery and let that do the smoothing for you.  It's MUCH more effective than a capacitor for filling in the holes in the charging waveform.

A battery does the same sort of job as a capacitor - but an ENORMOUS capacitor.  For instance:

A one FARAD capacitor (i.e. so big that it was considered unlikely to ever be assembled, back in the 1960s), charged with single-phase full-wave 20 HZ and discharged at 40 amps, will droop about a volt in the 1/40th second between half-cycle peaks.

An 85 amphour 12V battery (i.e. a single deep cycle of the smaller of the two sizes commonly used in travel trailers) will droop about a volt in the time it takes it to go from 100% to 25% charge - about 96 minutes, over 1 1/2 hour.  To do that well with a capacitor would take 3,840 FARAD!

(I understand the new "supercapacitors" actually work by moving ions from one plate to the other through a solution and storing them, charge and all, within the structure of a plate (though not actually reacting them into a compound with the plate material).  This makes them, IMHO, more like a battery with a linear voltage/charge curve than a true capacitor (which works by moving only electrons through the plates and attached wiring, sometimes distorting molecules or ion distributions in the dilectric between the plates, and never moving anything all the way through the inter-plate gap).)

[electrondady1]

cxv=ixt  so   c =ixt/v

ok, thanks , but i can't pick a capacitor from that. my voltage will vary from 0 to perhaps 25 volts. the frequency will also vary, on a 16 pole geni at say 60 rpm(16x60/60=16per sec.) to 500 rpm.(16x500/60=133 per)also, i think the amps would change depending on the type of load ive got. a 12volt battery will modify the voltage and amps , but an electrolitic cell could act like a dead short, i think.

 i think i understand what a cap. does, stores electrical pulses and releases them slowly in order to smooth the output of the diodes.  

[electrondady1]

ok, so it sounds like no cap. is required .

now this is perhaps another question but to enshure optimom dc purity would it be most prudent to run two diodes in series or parallel

[jimjjnn]

I forgot about the battery.ULR , you are absolutely correct. The ripple into the battery won't cause any problem. Thanks for reminding me. I think old age got to me 8>

[jimjjnn]

One brdge rectifier per phase will do the job.

[Ungrounded Lightning Rod]

You and me both, Jim.  (I get "brainos" too from time to time.  I've even posted some to this board.  B-)  )

You will get some ripple from the battery series resistance.  But DC devices are designed to handle it.  Alternator coils feeding rectifiers feeding battery feeding heavy switched loads, with no further smoothing except inside devices that need something cleaner, is how automobiles do it, too.  Wired DC supplies, like the power line and most other "raw" power sources, are expected to be somewhat unclean.

[jimjjnn]

That's what I get for working with robots as my last job was.

Guess I let them do my thinking for me. DUH !!!!

Hadn't used my electronics background . Never realized I might need it again after retirement. ULR, Your postings are very sensible and down where the fella that doesn't have the knowledge, can understand them. Keep up the good work and keep us thinking