Full-Wave Bridge Rectifier Only Half Rectifying - Page 1
Forum section: electronics / beginners

author: aaruff
author_rank: Newbie
timestamp: March 18, 2022, 07:59:04 pm
content: To further understand diodes I put together a full-wave rectifier circuit found in "Practical Electronics for Inventors". I am driving the AC signal with my Siglent SDG 1032x at 50hz, 6Vpp, 0 Offset, and 0.0degree phase. When I connect the probes (channel 2) to the output only the positive polarity signals are passed to the output, as shown below:More oddly when I disconnect the ground lead from the probe, keeping the positive lead connected, I get the following:At first I thought one of the diodes are faulty, but after checking them I have confirmed that they all work as expected. The diodes used are IN4001 with a Iav of 1A and Vrrm of 50V. The resistor is 10k Ohm.Below is the breadboard wiring with a pink arrow pointing to where I connected the oscilloscope probes. I removed them when taking the picture to reduce visual clutter.I suspect there could be some issue with grounding since the output voltage plot closer approximates what I expect when removing the probes ground connector, but as a complete newbie I don't know exactly what the grounding problem could be.Thanks in advance!

author: Ian.M
author_rank: Super Contributor
timestamp: March 18, 2022, 08:20:37 pm
content: A perennial 'noob' problem. You aren't the first and you certainly wont be the last to be caught out by a bridge rectifier with a ground referenced supply feeding it.See my comments in reply #5 here:https://www.eevblog.com/forum/beginners/full-wave-rectifier-measured-on-one-oscope-channel/for details and explanation.Be grateful your signal source was current limited and overload protected as getting grounding wrong in a power rectifier circuit is rather dangerous and can do many hundred bucks worth of damage.

author: aaruff
author_rank: Newbie
timestamp: March 18, 2022, 10:03:17 pm
content: Thanks Ian. From what I understand from Dave's video is that I can safely probe a circuit driven by a mains referenced function generator only if a differential probe or an isolation transformer is used. Is there a lower cost solution?

author: Benta
author_rank: Super Contributor
timestamp: March 18, 2022, 10:37:00 pm
content: Cheapest is probably two back-to-back connected mains transformers. eg,120 -> 24 -- 24 -> 120.Any secondary voltages are OK, provided they are the same.

author: Gyro
author_rank: Super Contributor
timestamp: March 18, 2022, 10:44:37 pm
content: Or, as you're doing breadboard stuff, just use the step-down transformer. It's pretty rare to be experimenting with a bridge rectifier unless you're building a PSU, this is one case where a small transformer is simpler than using a Function generator (or differential probe).

author: Ian.M
author_rank: Super Contributor
timestamp: March 18, 2022, 10:49:39 pm
content: For low(ish) frequencies and low(ish) voltages you can use two channels with the probe tips connected to the two points you want to probe across, then use waveform math to display the difference e.g. Y1-Y2. Either connect the ground clips to true ground, or remove them. The channels must be set to exactly the same gain and input coupling, and if using a x10 probe, the probe compensation must be carefully matched. I wouldn't expect decent results over 100KHz and wouldn't risk probing over +/-50V with respect to ground, or the scope's max input voltage, whichever is lower.Note that mains transformers are lousy for signals much above line frequency, getting more and more lossy over a few hundred Hz. An Eagle LT44 audio interstage transformer or a 600 ohm line transformer from an old MODEM should be reasonable from around 100Hz to 10KHz,

author: aaruff
author_rank: Newbie
timestamp: March 18, 2022, 11:15:32 pm
content: Would assembling a cheap battery powered function generator kit also work to isolate the bridge rectifier circuit -- for learning purposes? For example:https://www.amazon.com/Weewooday-Generator-Function-Frequency-Adjustable/dp/B08SMB892N

author: Gyro
author_rank: Super Contributor
timestamp: March 18, 2022, 11:28:51 pm
content: My son has one. You can't really go wrong at the price for a little portable signal source. Good soldering practice too.

author: Zero999
author_rank: Super Contributor
timestamp: March 19, 2022, 08:23:59 am
content: Quote from: aaruff on March 18, 2022, 11:15:32 pmBear in mind that one works off a mains adaptor. Check the negative rail is isolated from earth, which is guaranteed is the plug doesn't have an earth. Also note that many mains adaptors have a significant capacitance between the mains and secondary sides. This is often of the order of nF, which can couple enough current to case a tiny, harmless shock, but more importantly interfere with your measurements. Running it off a small battery or unearthed, medical grade power supply (this is a bit mor expensive, than a standard mains adaptor, but not hugely expensive) will eliminate this issue.

author: Gyro
author_rank: Super Contributor
timestamp: March 19, 2022, 10:28:36 am
content: Yes, battery powered would be the way to go. A typical XR2206 circuit only pulls about 20mA.P.S. You want a 9V battery, or AA / AAA holder, The input socket is 2.1 x 5.5mm. You can them with pre-terminated leads on ebay if needed. A useful reference...https://z100lifeline.swvagts.com/articles/XR2206FunctionGenManual.pdf