sábado, 26 de marzo de 2016

Rectifier 220/10 V

1 Description
This is a small salvaged rectifier - I do not have a clue about its source, any small home apparel could be!

A couple of pictures:

On the secondary it features a rectifier bridge with an electrolytic capacitor, no more fancy features! The diodes, 1N401, have a nominal capacity of 1A, so that's the nominal intensity of the machine. For an output voltage about 10 V, that's approx. nominal 10 w. The capacitor is rated at 16 V, so little margin.

Actual measurements with no load:
V_rms Primary: 225 V
V_rms Secondary: 8.2 V
V_dc output: 11.2 V

2 Simulation
The circuit, modeled in QUCS, is here
A snapshot of the circuit, with some results:
 (I have adjusted the transformer ratio to fit the measured Vcc).

Crest Factor = (Vpeak/Vrms) and for a sine wave, the crest factor is sqrt(2). Then, if Vrms is 225, Vpeak is 315 V, as in the model. 
My DMM has read 8.2 Vrms on the secondary, while the simulation shows a sine wave with a peak-to-peak amplitude of (-0.79,+11.8) V.
The Vrms of this voltage is:
Vrms = (11.8+.79)/2 + (11.8+.79)/2/sqrt(2) = 6.3 + 4.5 = 10.8 V
So, what's wrong?
It is possible that my DMM (Promax FP-2b) is deriving the rms value from average or peak values, which might be not correct for a dc-shifted wave.
Not clear... will have to check with the scope if the secondary is behaving as shown in the model.

I have checked with my USB oscilloscope and the results at the secondary are:
8.2 Vrms
23.5 Vpp
18 mVdc
50 Hz
And at the load probe:
11.0 Vcc

Weird secondary...

4 Inverting the Rectifier 
What would happen if the voltage source is placed at the output?
For a DC source nothing: the diodes and the capacitor would block it.
And an AC source should blow the electrolytic capacitor.

5 Removing the Capacitor
If the capacitor is removed, the output shows a superimposed AC wave of 2.9 V of amplitude. 

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