Simple bridge rectifier not functioning as expected - Page 1
Forum section: electronics / beginners

author: RLSprouse
author_rank: Contributor
timestamp: November 30, 2014, 02:59:17 pm
content: Beginner-level electronics hobbyist here.I am simply trying to implement a simple four-diode full-wave bridge rectifier like you can find in any electronics textbook. For example, the first attachment is a diagram from "Student Manual for The Art of Electronics," page 66.I have built this simple circuit several times, but I just cannot get it to function as expected. Attachment 2 is my latest version, built on a tiny breadboard for clarity.I built the circuit arranged more or less to mimic the circuit diagram in attachment 1. I used four 1N4007 diodes for the bridge, and I believe I have them oriented correctly. The "load" is a 10K resistor. The yellow wires simply connect the load to the appropriate terminals of the bridge. The black and red wires provide a clean sine wave signal from my workstation's Function Generator.If I hook my oscilloscope probe to the input terminals, I get a nice clean sine wave plotted on the scope. The problem is, if I hook the probe across the load, I get a half-wave rectified plot as shown on Attachment 3 rather than the expected full-wave rectified signal.So, can anyone tell me what I am doing wrong?Thanks in advance for any assistance.~ Russ

author: mariush
author_rank: Super Contributor
timestamp: November 30, 2014, 03:09:46 pm
content: It has to do with the oscilloscope and function generator earthing.Please see the following videos (jump to about 6:10 in the first video if you're not patient):

author: c4757p
author_rank: Super Contributor
timestamp: November 30, 2014, 03:22:15 pm
content: Yup, I've been working with electronics students lately and pretty much all of them did this during the bridge rectifiers lab. Common mistake. Lots of lab equipment has its 'ground' tied to the power system's earth ground, which means they're all tied together - and when you ground multiple pieces of equipment to different points, they all short together and bypass whatever components were between them.A good few function generators donothave earth-referenced grounds, which is awfully nice - for the rest, spend some time looking up how to probe a signal differentially. That is, use two probes, one on either side of the load (and with ground to the same point as the function generator's ground), and have the scope subtract the signals for you.

author: RLSprouse
author_rank: Contributor
timestamp: November 30, 2014, 04:48:35 pm
content: Thanks, you guys, you nailed it. After watching the videos, I "un-grounded" my workstation and now the scope plots the expected full-wave rectified signal.I'm not sure what risk this may cause. It seems safer than un-grounding the scope. I guess the best solution is to have a scope with isolated channels, but it sounds like those are expensive.Now I will mess around with using the math function as shown in the videos. But this raises the question, how do you know when this is needed? Is this problem common in working with real circuits?Anyway, I've learned a lot. Thanks again!~ Russ

author: Seekonk
author_rank: Super Contributor
timestamp: November 30, 2014, 05:18:35 pm
content: Consider yourself lucky. The last time I heard this same story, the bridge rectifier was connected to a line operated variac. The scope probe became toast.

author: SeanB
author_rank: Super Contributor
timestamp: November 30, 2014, 05:46:40 pm
content: Use an isolation transformer ( for this a 600R isolation transformer on the FG output will be better) to give you a floating voltage source to apply to the bridge rectifier. That is a lot safer, and is also a demonstration of a transformer with 2 isolated windings providing isolation.

author: c4757p
author_rank: Super Contributor
timestamp: November 30, 2014, 05:53:14 pm
content: Quote from: RLSprouse on November 30, 2014, 04:48:35 pmWell, this is a real circuit, is it not? You'll know when you need this because you want to probe a voltage but can't do it directly without shorting something out!

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 30, 2014, 05:55:44 pm
content: Quote from: RLSprouse on November 30, 2014, 04:48:35 pmIt looks like you have a scope with 2 inputs. Just use both probes without the grounds connected and ADD both channels and INVERT one.