# Bridge Rectifier Problem

• posted

Hi, I am using the following Bridge Rectifier

I applied the 50HZ , 16V sin Wave at the input and seeing 4.35 volts at the output using digital voltmeter and 194 volts using oscilloscope. My questions are

1. Is 4.35 volts correct output? If yes than why full wave bridge rectifier is stepping down the voltage?
2. Why scope is showing me 194 volts?

I have attached 22uF capacitor at the output too , no difference.

Thanks John

• posted

"john" schreef in bericht news: snipped-for-privacy@t21g2000vbo.googlegroups.com...

You must have made more then one mistake or maybe your equipment is crap.

How do you know you applied 16V/50Hz? Where did that voltage came from? Did it still measure 16V/50Hz after you connected the bridge? And after you connected the cap?

Your digital meter should show over 16Vac when measuring the open voltage source as you have no load. Usually digital meters are not designed to measure pulsating dc voltages like rectified sine waves. With the cap connected you should measure about 22Vdc or higher. Your scope sure is set to an 1:10 probe while you're using a 1:1 probe or no probe at all. Think you did not connect the bridge properly. Did you notice both ac connections at the same side of the device?

petrus bitbyter

• posted

Hi,

The input is coming from the function generator. SO, I measured it using the scope than connect the output to the bridge and see the input again on the input pins of the bridge. It was 16 volts peak to peak but the output is 140 volts peak to peak measured with the scope.

Yes I did notice the ac sign on the bridge.

Thanks John

• posted

"john"

** That IS the expected voltage.

The AC input is 16V p-p so after rectification becomes 8V peak MINUS the voltage drop of TWO diodes or about 1.1 volts at low curent.

The AVERAGE value of a rectified of a sine wave is 2/pi times the peak and DC meters will read that value.

6.9 x 2 /pi = 0.44

and 194 volts using oscilloscope.

** You need to set the scope's input to suit the probe in use.

** See above - no such thing is happening.
** Operator error.

** Try again, it should make a big one.

... Phil

• posted

If one side of the function generator output is grounded and one side of the scope input is grounded to the same point (possibly through the power cord green wire) you can get some quite unexpected readings on the scope (but that likely wouldn't account for the 140V you report).

Neither side of a bridge rectifier output can be connected to one side of its input - if you do connect them, you are shorting out a diode or two in the bridge.

```--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca  ```
• posted

"Phil Allison"

Typo:

** 6.9 x 2 /pi = 4.4
• posted

let me see if I understand you.

You have 16 PP going into the bridge and have 140 V PP coming out?

That does not make a lot of sense to me unless you have something else attached to the bridge output that is inductive..

• posted

Double-check your connections AND range settings in both cases. For the scope, that sounds like a bad connection - the transformer supplying the signal is acting like a capacitor from the mains to scope input and return is via scope's ground - so the circuit you allege does not exist. In the case of the DVM a similar fault exists but the "ground" is via capacitive coupling from DVM to your hand and then more capacitive coupling to ground (all in series now "row, row your boat"...).

• posted

Probe set to x1, scope thinks it is x10. Set probe to x10.

Best regards, Spehro Pefhany

```--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com```
• posted

Obvious cockpit errors. If you don't know how to use your test=20 equipment you will get wild measurement errors.

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