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)

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 enough,
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.

The fans do run noticeably stronger, so it's not just an imaginary
effect either.
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

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. The
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

On Tue, 14 Aug 2007 23:19:01 -0700, ChrisPikula
<ChrisPikula@gmail.com> 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)
>
>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 enough,
>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.
>
>The fans do run noticeably stronger, so it's not just an imaginary
>effect either.
>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
>
>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. The
>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

I'm thinking...ripple voltage.


D from BC

ChrisPikula 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)
>
> 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 enough,
> 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.
>
> The fans do run noticeably stronger, so it's not just an imaginary
> effect either.
> 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
>
> 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. The
> 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
>
Your rectifier does not charge your capacitor to the measured AC
voltage, but to the peak ac voltage(minus 2 diode voltages, ~1.4V).
So the change from 6 to 8.5 is about what you would expect.

ChrisPikula 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)

If this arrangement works, perhaps you should stop there.

> 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 enough,
> 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?

An AV voltage of 7.9 volts RMS, (as read with a volt meter)
means that this AC voltage will heat a resistor the same as
a 7.9 volt DC voltage would. But twice a cycle, AC sine
wave voltage is zero volts. At the two peaks of the sine
wave, it is 7.9*1.414=11.2 volts.

When you use a bridge rectifier on this waveform, you get
11.2 volt peak ripples minus the 2 volts or so of rectifier
drop. A little of the voltage also drops across the
resistance of the transformer windings as you bring the load
current up. So without the capacitor, you had a rippling
voltage that had an average value of about 6 volts. That
means the average of the zero volt periods and the peaks and
everything in between was about 6 volts. Obviously, some
parts of that rippling voltage had to be higher than 6 volts.

When you added the capacitor, it tended to charge up to the
peak voltage of the rippling waveform, and supply the motor
current during the low parts, with the rectifier supplying
current only just before and during the peaks of the
waveform, instead of all the time. So the new voltage
waveform ahas peaks almost the same as before, but the
valleys are almost filled in by the capacitor, so the
average voltage must go up. At no load, it should go up to
almost the 11.2 volts peak of the AC minus about 1.2 volts
of unloaded rectifiers or about 9 volts. Under load, the
diode drops increase to about 2 volts, the capacitor voltage
sags a bit between the peaks, and the transformer drops
more voltage from the large capacitor charging current
peaks, so the voltage is less than 9 volts.

> 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.
>
> The fans do run noticeably stronger, so it's not just an imaginary
> effect either.

Agreed.

> 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
>
> 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. The
> analogue agreed as close as I could tell to the digital, so I don't
> think they are suspect.

I see nothing strange. you have just demonstrated the
properties of a capacitor filter on rectified sine wave AC.

> Any and all help appreciated, teach me more physics, etc, etc.
>
> Cheers!
> Chris

See:
http://www.kettering.edu/~bguru/PE424UCR/FWRC.pdf

"ChrisPikula"
> 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.


** DMMs set to read DC volts will read the *average* value of a wave
connected to them.

Fine, if it is steady DC - but can be misleading if it is not. In your
example, the wave is a sine wave that has been rectified so each half wave
has the same polarity.

The average value is less than the peak value seen at the top of each sine
wave crest - by a factor of 0.637 ( or 2/pi). So, when the DMM read 6
volts DC, the peaks were 6 / 0.637 = 9.42 volts.

Adding that filter capacitor raised the average DC value UP to almost meet
the peak value - however adding 20 fans pulled the voltage down due to
resistance losses in the transformer's copper wires and the diodes in the
bridge.

Nothing strange is happening.




......... Phil

ChrisPikula 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)
>
> 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 enough,
> 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.

No, it's completely right.

Without the capacitor there, what you had was the average voltage of a rectified
sine wave. When you add the capacitor, it charges up to the *peak* voltage of
the rectified sine wave.

All quite normal.

Graham