PLEASE HELP!!! Rectifier issues!

Thread Starter


Joined Nov 19, 2011

I have this problem with my bridge I'm trying to convert AC voltage to DC generated out of a small 12V/DC motor.

I generated almost 8.4V/AC. I'd like to convert it using my bridge,so I made a simple rectifier using this schematic.

Added a 1000uF capacitor at 16 volts to smooth out the DC output and I followed this formula to calculate the expected voltage Vin/AC x 1.4 = Vout/DC.

And here's the result:

*Here's the problem... I made the "full wave bridge rectifier" and tested out the product I had 8.4V/AC in and I got almost 3.4 Volts DC out, I thought the voltage would be 11.76V/DC than "3.4" according to that smoothing formula "V/ACin x 1.4 = V/DCout".

What do you all think is the problem?

I used another capacitor 4700uF at 16 volts and the output was almost 2.6V/DC... What am I doing wrong? Or is it just normal?
lets say even if there was a voltage drop I thought it would be some simple number not that much of a drop :confused:.

Here are some pictures so you guys can see what I'm dealing with.

capacitor( 1000uF at 16V) + bridge:

*Bridge(Close up):

*Capacitors"1000 uF at 16V & 4700 uF at 16V":

*NOTE* The capacitor was installed incorrect in the picture. I used it right in the testing - with - and + with +"

If you are all asking about the type of motor its a brushless one.



Joined Mar 24, 2008
To me it appears you are connecting the capacitor backwards. What you have is very similar to a power supply. Try drawing a schematic as if it were one.



Joined Dec 26, 2010
We might begin by making completely sure of what type of motor this is. Many types of "brush-less" DC motors may fail to provide any useful output as generators, as their internal electronics can't generally work in that way. You might see some sniff of output with a sensitive meter, but not anything you can actually use, unless you can get back to the windings before the driver, when an alternator-like output may be seen. The waveform may not necessarily be a good sine wave.

A permanent magnet brush/commutator DC motor, used as a generator would not produce AC, but most likely "undulating" DC, i.e. with a bit of ripple and maybe some spikes or drop-outs. The voltage will change from positive to negative according to the direction the motor spindle is rotating.

Some stepper motors are not really true "DC motors", but have more in common with permanent magnet alternators. These may deliver true AC output, but again the waveform might not be very sinusoidal.

To recap, if you really have a brushless motor complete with drive circuit, this is unlikely to work as you hope unless you can access the windings directly. The raw winding voltage, or perhaps that from a stepper could give you more like proper AC, but maybe not the most pure waveform. A DC brush motor would provide rough DC, its polarity depending on the sense of rotation.

Whatever generator you have, a bridge rectifier will fix the polarity in one direction, at the cost of losing maybe 1.4V off the peak amplitude. A reservoir capacitor will tend to smooth over the ripples, raising the mean level closer to the peak depending on the size of the capacitor and how much it is loaded. Whether or not the peak to RMS ratio of the waveform would be anything near 1/√2 will depend on how the machine is made.

See Bill Marsden's note! Please ensure the capacitor is fitted the right way round if it is an electrolytic: your photo shows one with its negative pole connected to the bar ends of some diodes, which is wrong. Capacitors mistreated in this way will fail, sometimes explosively.


Joined Mar 24, 2008
It is easy to tell if it is the motor is a generator (and the comments about brushless motors is completely valid, most have circuits inside which would not let them act like generators).

Measure the AC output voltage.

Another thing, fan motors (brushless) are very sensitive about being spun too fast. The back EMF burns up the electronics.


Joined Jul 4, 2011
Does it not look like the cap is on the AC side and DC side? The wires from the motor are visible and you can see them going onto the bridge but it looks like the cap is across the inside.

From the motor you can see an orange and black wire but they look like they going to the wrong part on the bridge.

Could be the photo though?