Full Bridge Rectifier, AC output?

Thread Starter

Hate Everyone

Joined Aug 11, 2017
3
Hello, I'm having a little problem. I made a Full Bridge Rectifier with an RS 605 and capacitor(and the other version with 4 diodes). i am getting a DC and AC ouput at the same time.But if i change the polarity in my multimeter probes im not getting AC ouput. What is going on? I tried connecting a capacitor and the DC voltage increased as well the AC output.(I'm using a transformer that converts 220v AC to 12v AC)(without capacitor DC voltage is 12v and 25v AC, and with the capacitor, DC voltage is 18v and AC voltage 40V). Please reply. Good Night.
 

#12

Joined Nov 30, 2010
18,210
The rectifier only rectifies, so it produces pulses. Most meters don't know what to do with pulses when you set them for DC. They might try to find the average or they might keep checking at different times of the wave form. Anyway, the answers are useless until you install a filter capacitor. Why you're reading 40 VAC with a capacitor installed would require a drawing of your circuit to figure out.
 

Tonyr1084

Joined Sep 24, 2015
4,373
Well, I was thinking that if you take a thing and connect it to another thing then you should see results.

OK, what the heck am I talking about? Well, I'd ask that of you as well. Can you post a drawing of your circuit? List where you're checking voltages and what you're seeing. Let us know how you built your bridge rectifier circuit as well. We can't troubleshoot without a common reference point. So help us help you. Post a schematic and list what you're seeing. If you CAN, post a video to YouTube then post a link to it in your answer so we can see what you're getting and what's going on.
 

MaxHeadRoom

Joined Jul 18, 2013
20,422
I looked at them all and it is not really clear what you are doing, a decent drawing with the readings/numbers would have been better.
I assume that is what it appears to be, a 12vac adapter?
Max.
 

Jony130

Joined Feb 17, 2009
5,157
Well your circuit is without any external load only the multimeter input resistance (1MΩ or larger) .
My unloaded bridge rectifier shows this at his output.

ind.png



And after you connect the capacitor as a load, the capacitor will charge to peak voltage 13.6V * 1.41 = 19.2V

Also, cheap multimeters on AC does not properly measure rectified voltage.
My cheap meters UT33 and MT830 showy around 32V at AC for 18.2V RMS at the input of the bridge without capacitor.
 
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Tonyr1084

Joined Sep 24, 2015
4,373
Many meters will have trouble reading a DC voltage on an AC scale. Part of the reason is because the DC is pulsating between zero and 12 volts (RMS). If you put your test meter on the AC scale and attempt to read a DC voltage, because the voltage is fluctuating the meter will have a hard time deciphering the exact value because it's looking for a sine wave that is going positive then negative. No telling WHAT your meter will read. As for why you're seeing 41 volts with the capacitor, without being able to see the full circuitry we just can't tell for sure WHAT may be going on. To be honest, and not trying to find fault or doubt your ability, but what if you mis-wired something. Honestly, I doubt you did. But a capacitor on a DC source should act like a battery and capture the peak voltage which is 1.414 above RMS value for a sine wave. So if you have a 12 volt sine wave (RMS) the peak voltage will be 16.97 volts (call it 17 volts). In your photograph with the cap I can't read what's under the numbers (the 41.7). For all we know it could be saying 41.7 mV (millivolts)
 

Thread Starter

Hate Everyone

Joined Aug 11, 2017
3
I looked at them all and it is not really clear what you are doing, a decent drawing with the readings/numbers would have been better.
I assume that is what it appears to be, a 12vac adapter?
Max.
Yes. Also at photos3.zip is the drawing of the circuit.

Well your circuit is without any external load only the multimeter input resistance (1MΩ or larger) .
My unloaded bridge rectifier shows this at his output.

View attachment 132768



And after you connect the capacitor as a load, the capacitor will charge to peak voltage 13.6V * 1.41 = 19.2V

Also, cheap multimeters on AC does not properly measure rectified voltage.
My cheap meters UT33 and MT830 showy around 32V at AC for 18.2V RMS at the input of the bridge without capacitor.
Thanks that helped.

Many meters will have trouble reading a DC voltage on an AC scale. Part of the reason is because the DC is pulsating between zero and 12 volts (RMS). If you put your test meter on the AC scale and attempt to read a DC voltage, because the voltage is fluctuating the meter will have a hard time deciphering the exact value because it's looking for a sine wave that is going positive then negative. No telling WHAT your meter will read. As for why you're seeing 41 volts with the capacitor, without being able to see the full circuitry we just can't tell for sure WHAT may be going on. To be honest, and not trying to find fault or doubt your ability, but what if you mis-wired something. Honestly, I doubt you did. But a capacitor on a DC source should act like a battery and capture the peak voltage which is 1.414 above RMS value for a sine wave. So if you have a 12 volt sine wave (RMS) the peak voltage will be 16.97 volts (call it 17 volts). In your photograph with the cap I can't read what's under the numbers (the 41.7). For all we know it could be saying 41.7 mV (millivolts)
Cause I can't measure the signal wave (my oscilloscope died after a lightning) your answer helped me
 
Last edited by a moderator:

ModemHead

Joined Nov 1, 2010
13
First, the A-to-D converter in most handhelds like this one integrates the input waveform over a short period of time, which means it responds to, and displays the *average* of its input, even if it is pulsing.

Second, inexpensive DMMs like yours use a VERY simple AC to DC converter that consists of a single diode with no DC blocking capacitor. Thus it responds to the average of the positive half-wave rectified input, which is then scaled by multiplying by 2.2. This works reasonably well for displaying the RMS voltage of symmetrical sine waves. The voltage drop and non-linearity of the single diode leads to increasing errors at lower voltages, which is why the lowest ACV range is always 200V on this type of DMM.

As a result of this simplistic design, the AC voltage range will display 2.2 times the average of *all* positive-going input, and nothing for negative. Try measuring a 9V battery both ways on the ACV range to see the effect.

So the ACV reading of this DMM of the bridge's output is not meaningful, but the DCV reading is.
 
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