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kbpc

I bought this voltage rectifier, this should be a simple Graetz rectifier, however, there are 2 strange issues I run into:

  1. The output DC voltage is higher than the input AC voltage. Why? For example I input 15.9V AC and it gives out 21.4V.

  2. What I have is the 50A version, yet if I put on a 1A dummy load this diode bridge keeps turning itself off, why? It is not the transformer I use, because I measure the AC and DC voltages with 2 meters at the same time and it gets stable AC voltage and keeps turning off.

I have tried to add some filter capacitors to the output, but it doesn't change anything.

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  1. Expected voltage at the output of a bridge rectifier with a light load and large output filter capacitor is \$\text V_{IN}\cdot \sqrt{2}-V_F\cdot2\$ where Vf is the forward voltage of the diodes. That's because the capacitor charges to the peak voltage of the input AC waveform. We generally specify AC power in RMS volts, and the ratio of peak-to-RMS is the square root of 2.

    So 21.4V is quite reasonable and expected.

  2. It's just a rectifier bridge- four 25A diodes- it can't turn itself on or off. Either it's defective and a connection is going open (which seems unlikely to me) or your measurements are somehow faulty.

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1, The output DC voltage is higher than the input AC voltage why? For example I inputting 15.9V AC and it gives out 21.4V.

This is where the RMS and the peak values are defined. You probably measured it with your multimeter. Multimeters measure the RMS (root of the mean of the square, or root-mean-square in short) of the voltage. So, when you say 15.9VAC, it's the RMS voltage. The term RMS is related to the heating effect of the AC voltage. You can research more if you want.

When you rectify it with the bridge rectifier, the output is the "peak" of this input sine.

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Image Source

The peak is simply 1.414 (or square root of two) times the RMS voltage: 15.9 x 1.414 = 22.5V. Taking the forward drop of two series connected silicone diodes inside the bridge into account, the peak of the rectified will be between 21V to 21.5V. So it's normal.

2, What I have is the 50A version, yet if I put on a 1A dummy load this diode bridge keeps turning itself off, why? It is not the transformer I use because I measuring the AC and DC voltage with 2 meters in the same time and it gets stable AC voltage and keeps turning off

You probably didn't put a large enough filtering capacitor. If there's not enough capacitance at the output, the voltage will fluctuate more. If you load the output more (i.e. more output current) then the fluctuation will be even worse and the output may hit zero. That's probably what you meant with "keeps turning itself off".

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  • \$\begingroup\$ What is "large enough"? I have connected 3 electrolytic caps in parallel (2200uF + 6800uF + 6800 uF) and as I said it haven't change much. I doubt that it's faulty because I have ordered 2 and both is doing the same thing. \$\endgroup\$ yesterday
  • \$\begingroup\$ @ThermalTake I just guessed because you did say you added filter caps but you did not mention what types and values you used. Anyway, having 2 brand new faulty bridge rectifiers is unlikely. In addition to what Spehro said about measurement or connection errors, I'd like to say that the source might be the source of your problem. I don't know what you are using but it might be faulty, or gets overloaded and activates its protection (if any). \$\endgroup\$ yesterday

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