DMM not reading AC voltage on circuit - Page 1
Forum section: electronics / testgear

author: gotwood
author_rank: Contributor
timestamp: November 06, 2013, 07:36:50 pm
content: I have a load in parallel with a Zener shunt regulator w/capacitor .when I read the AC voltage across the capacitor and load the voltage goes down until there is no reading. This is with my Klein MM2300 DMM.When I use my College DMM(fluke) can't remember model number ...I get .817VAC across Capacitor and .004 VAC across load.Any idea why my meter is behaving this way?|----------R------------------|-----------200 Ohm-----------|---------------|| E | | |13V C 100 MicroF 1N4735 Zener 270 Ohm| T | | ||----------------I-------------|--------------------------------|--------------|

author: drtaylor
author_rank: Regular Contributor
timestamp: November 06, 2013, 08:01:39 pm
content: Assuming I'm interpreting your schematic correct, there should be no AC other than ripple. If the input is 13VAC and those letters represent a rectifier, then across the 100uF cap will be DC with some minor amount of ripple. Then the voltage is further reduced through the 200 Ohm and then regulated to 6.2V by the Zener. The nominal 13VAC will be ~18VDC - two diode drops assuming a bridge rectifier, so lets say ~17VDC. Then the 17VDC is divided by the 200 ohm and 270 ohm resistor to ~9.7V if the zener wasn't there. With the zener there around the 200ohm resistor had to dissipate 10.8V which is ~54mA (this better be at least a 1W resistor). At 6.2V the 270 resistor takes 23mA of that leaving 31mA flowing through the Zener, about 200mW.So putting your DMM in the lowest AC range on the Cap will show some ripple depending on loading. Now if there is no rectifier, then the cap represents a short and you still won't read any AC until the cap fails which it will.So I guess I'm asking you, why do you expect AC? This is a simple AC to DC regulated output with limited power output. It should be measured only in DC after you get past the rectifier. Take out the Cap and I guarantee you'll read plenty of AC.

author: gotwood
author_rank: Contributor
timestamp: November 07, 2013, 08:30:47 pm
content: correct, I am trying to read the AC ripple in both cases....across load is should be near .004V other should be .8 isnI'm looking for a reason why my meter is not showing this voltage but the Fluke meter at my school does.

author: deth502
author_rank: Frequent Contributor
timestamp: November 07, 2013, 09:18:06 pm
content: does your klein meter read down to .004vac

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 07, 2013, 09:52:35 pm
content: Some meters do not perform well when measuring AC in the presence of significant DC offsets. Try a 0.1uF film cap in series with the meter.

author: gotwood
author_rank: Contributor
timestamp: November 13, 2013, 04:32:03 pm
content: tried using the capacitor you suggested.....no help still reads 0

author: turbo!
author_rank: Regular Contributor
timestamp: November 13, 2013, 05:07:05 pm
content: Interesting fact about DMMs. When it says "true RMS" it often means "AC only". A TRUE "true RMS" provides the DC equivalent voltage as seen by a resistor . What this means is that say you have a 10 ohm resistor. A reading of 10v RMS is something that produces the same amount of heating effect at the resistor as 10v battery connected to it. It would obviously depend on the integration time which you can not change on portable DMMs.When you rectify an AC power with diodes and capacitors, you will always have ripples in interval of 2x AC Hz. While the direction of voltage doesn't cross about the 0v line, the ripple component is called the AC component and the "floor" that trough is standing is the DC component.http://www.home.agilent.com/agilent/editorial.jspx?cc=US&lc=eng&ckey=458371&nid=-33272.0.00&id=458371You'll need to share us a DC coupled scope screen shot for us to have any idea. In AC coupled mode, it will block out the DC part.

author: macboy
author_rank: Super Contributor
timestamp: November 14, 2013, 01:45:41 pm
content: The numbers from the Fluke sound about right. In simulation, I get 880 mVRMSacross the capacitor, and 18 mVRMSacross the 270 ? resistor. The simulated Zener is not a 1N4735, but another 6.2 V one, so the amount of ripple is only a guesstimate.When trying to measure, is your Klein meter auto-ranging to the lowest (600 mV) range? If so, and you still read zero, then it seems to be defective.

author: turbo!
author_rank: Regular Contributor
timestamp: November 14, 2013, 01:54:21 pm
content: Quote from: macboy on November 14, 2013, 01:45:41 pmHe didn't say anything about Hz. It could be 50Hz, 60Hz or hundreds of Hz from a car alternator.