Author Topic: Bridge rectifier giving negative volts instead of positive?  (Read 3294 times)

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Offline raspberrypi

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Bridge rectifier giving negative volts instead of positive?
« on: February 19, 2017, 10:47:21 am »
As far as I can tell this is wired up correctly. But I thought where the two stripe ends of the diode meet up that was the negative side and the two non striped ends were the positive. AC goes into where a stripe and non stripe join up. Meter is plugged in right and and on DCV
I'm legally blind so sometimes I ask obvious questions, but its because I can't see well.
 

Offline Zero999

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Re: Bridge rectifier giving negative volts instead of positive?
« Reply #1 on: February 19, 2017, 10:58:59 am »
I thought where the two stripe ends of the diode meet up that was the negative side and the two non striped ends were the positive.
No, the positive output is taken from where the two cathodes join and the negative output is taken from where the two anodes connect.

Look at a half wave rectifier. Notice how the positive output is taken from the cathode?
 

Offline raspberrypi

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Re: Bridge rectifier giving negative volts instead of positive?
« Reply #2 on: February 19, 2017, 11:02:03 am »
Thats opposite of this isn't it?
I'm legally blind so sometimes I ask obvious questions, but its because I can't see well.
 

Online Ian.M

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Re: Bridge rectifier giving negative volts instead of positive?
« Reply #3 on: February 19, 2017, 11:11:25 am »
The stripe end of a diode is the cathode, (bar in diode symbol) and indicates the way that (conventional) current will flow.  Taking the bridge as a whole, current from the transformer secondary connected to the AC terminals will flow OUT of the positive terminal (pair of cathodes) and back IN to the negative terminal (pair of anodes).
Its the opposite way round to what you may expect because the bridge is supplying current to the load, not consuming current from a DC supply rail.

If that doesn't satisfy you,  draw the bridge schematic using individual diode symbols, labelling the bridge terminals AC1, AC2, DC1, DC2, with a voltage source across the AC terminals and a load resistor across its DC terminals,  Let AC1 be positive with respect to AC2, and follow through on the schematic which diodes conduct current, which block, and the direction of the current through the load.  Repeat with AC2 positive with respect to AC1.
 

Offline NottheDan

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Re: Bridge rectifier giving negative volts instead of positive?
« Reply #4 on: February 19, 2017, 11:22:55 am »
Meter is plugged in right and and on DCV
Meter may be plugged in right but you have the common connected to the positive output of the bridge.
 

Offline Zero999

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Re: Bridge rectifier giving negative volts instead of positive?
« Reply #5 on: February 19, 2017, 11:45:47 am »
Thats opposite of this isn't it?

That graphic is correct.

Here's how a bridge rectifier should be connected up.

https://en.wikipedia.org/wiki/Diode_bridge

Notice how the positive output is taken from where both cathodes meet?
 

Offline Brumby

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Re: Bridge rectifier giving negative volts instead of positive?
« Reply #6 on: February 19, 2017, 12:01:31 pm »
But I thought where the two stripe ends of the diode meet up that was the negative side and the two non striped ends were the positive.

You have that exactly backwards.


I like this animation:
 

Offline Zero999

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Re: Bridge rectifier giving negative volts instead of positive?
« Reply #7 on: February 19, 2017, 12:24:44 pm »
Using conventional current flow. . .

Current flows into the anode and out of the cathode. If the diode is connected in series with a positive power supply (say for reverse polarity protection) then the anode should go to the positive supply and the cathode to the positive terminal of the circuit being powered.
 

Offline Vtile

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Re: Bridge rectifier giving negative volts instead of positive?
« Reply #8 on: February 19, 2017, 04:30:10 pm »
Here is more "traditional" way of draw the diode bridge. Much easier to remember.

 


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