Bridge rectifier giving negative volts instead of positive? - Page 1
Forum section: electronics / beginners

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 19, 2017, 10:47:21 am
content: 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

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2017, 10:58:59 am
content: Quote from: raspberrypi on February 19, 2017, 10:47:21 amNo, 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?

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 19, 2017, 11:02:03 am
content: Thats opposite of this isn't it?

author: Ian.M
author_rank: Super Contributor
timestamp: February 19, 2017, 11:11:25 am
content: 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.

author: NottheDan
author_rank: Frequent Contributor
timestamp: February 19, 2017, 11:22:55 am
content: Quote from: raspberrypi on February 19, 2017, 10:47:21 amMeter may be plugged in right but you have the common connected to the positive output of the bridge.

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2017, 11:45:47 am
content: Quote from: raspberrypi on February 19, 2017, 11:02:03 amThat graphic is correct.Here's how a bridge rectifier should be connected up.https://en.wikipedia.org/wiki/Diode_bridgeNotice how the positive output is taken from where both cathodes meet?

author: Brumby
author_rank: Unknown
timestamp: February 19, 2017, 12:01:31 pm
content: Quote from: raspberrypi on February 19, 2017, 10:47:21 amYou have that exactly backwards.I like this animation:

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2017, 12:24:44 pm
content: 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.

author: Vtile
author_rank: Super Contributor
timestamp: February 19, 2017, 04:30:10 pm
content: Here is more "traditional" way of draw the diode bridge. Much easier to remember.