Re: Voltage concerns on a bridge rectifier-capacitor circuit
- From: Ross Herbert <rherber1@xxxxxxxxxxxxxx>
- Date: Wed, 15 Aug 2007 07:22:20 GMT
On Tue, 14 Aug 2007 23:19:01 -0700, ChrisPikula
<ChrisPikula@xxxxxxxxx> wrote:
I've made a DC power converter, and I've got extra voltage.
I'm using a physically large 15-1 transformer to reduce the voltage,
and I'm getting 7.9V AC over leads 1 and 2, and over leads 2 and 3,
with 16.2V over leads 1 and 3. I made a bridge rectifier, and on the
other side of it, with the digital multimeter I get roughly 6V DC,
using leads 1&2 for input. I then my load on it, and it works
decently. I'm happy with it. (The load was 20 computer fans, I'm
making an insert for my window). (roughly 80 ohms resistance for
each, 3 ohms when I put them all in parallel)
It's is obviously a centre-tapped secondary transformer with wire 2 as
the C.T. If you bridge wire 1 to wire 3 and connect the bridge
rectifier input to wires 1 and 2 you will get the same output voltage
but double the current capability. This way half the load current is
supplied by each of the output windings instead of all of the load
current supplied from one winding as you now have it.
enough,
To be safe, I decide to put a capacitor parallel with the load, just
to made the voltage more steady. I put a 7.5v 15000uf capacitor
across the leads, and test the voltage again. (7.5V should be
considering there's usually a .7v drop across each rectifier,
right?). I get a much steadier 8.5V across the leads?!?! (The
capacitor, load, and rectifier all connect at the same point). I
immediately shut it off, thinking WTF, where did the extra voltage
come from? I think about it, put another capacitor inline with the
first, and try again. This time I get 8.77V DC, but I'm not going to
overload the capacitor, so I feel better. There's still something
wrong, as I don't know what's going on.
If you measure the unloaded AC voltage from the transformer this will
be the RMS value, Vrms, which is = 0.707 x Vpk. When you connect a
capacitor across the dc output (no load) the capacitor charges to the
peak value of the waveform (Vpk = 1.414 x Vrms). The diodes in your
bridge rectifier will drop a total of around 1.8V on each half cycle
thus leaving you around 6Vrms to supply the load. Depending upon the
capacitor value and the load current the voltage across the capacitor
will try to maintain as near the peak voltage value as possible. In
your case Vpk = 6 x 1.414 = 8.5V which is what you read.
If the load current increases for a given capacitor value the voltage
across the load will sag a little and the ripple voltage will increase
and vice versa. Your 15000uF cap is quite large and will easily
maintain near peak voltage with your fan load.
The fans do run noticeably stronger, so it's not just an imaginary
effect either.
That is what we would expect.
The transformer still keeps the same voltages when the fans are
running or not running, and my analogue multimeter confirms the AC
voltages, and I tested both right to the wall outlet, and got a
perfectly steady 120.0V
When I put the second capacitor on, I put in inline with the first,
and put it inline with the load, not parallel to it. The reason for
this, was that I don't think it matters if it is parallel or not, as
long as it supplies the voltage when the rectifier can't, it should
work just the same. Plus, I poked myself with a wire, so I wanted to
test it as soon as possible, and didn't want to cut another wire.
Yeah, lazy, I know, but I rationalized my concern away, so it's okay,
right? :p
It seems that you may have connected both caps in series and then
onnected the combination across the load - correct? If this is so then
you effectively halved the value of the capacitor appearing across the
load and this would result in greater ripple current for a given load
current. If you connect the second cap in parallel with the first then
you double the effective capacitance thus reducing ripple voltage and
improving the ability of the circuit to maintain output voltage near
to Vpk for a given load.
The
Also, I don't think it would make a difference, but for my rectifier,
as I didn't have parts which could handle the amperage, I put diodes
in parallel where you would usually only put one, aka, a 4 diode
bridge rectifier became an 8 diode bridge rectifier.
That's *all* I have for the circuit, it's so simple, and it's
frustrating me so. I've thought and ruled out bad multimeters, dirty
power, and bad wiring. (All the wires coming off the separate fans
have been soldered into two main rails, etc, multimeters tested off
known sources, etc.) One thing that I do have a small uncertainty
about, is the reliability of my low VAC readings, as my multimeters
only went down to 200VAC measurement scales, and I had no way to
easily test their reliability at those levels, safely and quickly.
analogue agreed as close as I could tell to the digital, so I don't.
think they are suspect.
Any and all help appreciated, teach me more physics, etc, etc.
Cheers!
Chris
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