Help troubleshooting homemade (Ludens) ESR meter - Page 1
Forum section: electronics / beginners

author: ELItheICEman
author_rank: Newbie
timestamp: March 05, 2012, 04:46:19 am
content: Ok, I built an ESR meter using the schematic and instructionshere(I think it's fairly popular, maybe one of you guys have built this before?). I've went through the process of tracing all the components through the circuit and comparing to the schematic, All seems well. The problem most likely lies in my knowledge of transformers, I ordered the EA-77-188 Tiny double E core and the same gauge magnetic wire used in the guide (AWG #36 and AWG #26). On my first attempt I initially started winding the #36 wire perfectly around the bobbin, each loop adjacent to the last winding. That is until I realized it would probably take at least an hour to wind 400 turns like this. So I kind of sped up the process of winding while negatively affecting the uniformity and neatness. I finished up with some masking tape and started the next run (20 turns of #26 wire). Again I used masking tape to finish up. I then slid the E cores into place (although I didn't glue them in any way). I then put the transformer onto my circuit board and applied voltage to the circuit. The needle on the uA meter jumps ever so slightly upon turning on the circuit but nothing else. I've tried a few caps and resisters on the test leads and adjusted the trim pot in every position. I'm getting a steady 5V on the board, but I haven't really found many other helpful results from other test points. I have an oscilloscope but I'm not exactly fluent in using it...

author: amspire
author_rank: Super Contributor
timestamp: March 05, 2012, 05:31:14 am
content: The link above has some extra "http" stuff at the front. The proper link is:http://ludens.cl/Electron/esr/esr.htmlThe neatness of the transformer winding will not effect operation.Start off by powering the device with a battery, put a 10 ohm resistor across the test leads, and see if you have a signal across the resistor. If you don't, connect the oscilloscope ground to the battery negative, and see if you have a signal coming out of pin 1 U1A. It should be a square wave of about 50KHz. The reason for powering off a battery is that it means you can connect the ground lead of the scope to anywhere in the circuit. If you use a grounded power supply, then you can only connect the scope ground to the power supply ground. If anything gets hot, disconnect the power.If you are getting a 50KHz voltage across that 10 ohm resistor I mentioned, then problem is in the circuit around U1B. See if you have a waveform on Pin 7 of U1BIf you are still stuck, capture the circuit as an image, add voltages and waveform sketches (rough is fine) at different points on the circuit, and post the resulting annotated schematic here. Then we can help. For voltages, attach the black multimeter lead to the battery negative, and measure the DC voltages on the op-amp pins and power supplies.If you are not confident on using the oscilloscope, then this is the perfect time to learn.Richard.Edit: Is that regulator a 7805? If it is, have you got the regulator in the right way round. It is hard to tell but it looks like it is reversed. Do you have 5V on the regulator out?

author: ELItheICEman
author_rank: Newbie
timestamp: March 05, 2012, 05:58:04 am
content: IIRC the schematic had the pinout wrong for the 7805, I remember cross referencing it, I am getting 5v out of it. I am powering the circuit from the 12v rail of a atx power supply mounted in a custom enclosure. I probed around with the scope in several places but never got any actual signals, although I may have had a setting wrong on the scope of something. I will do some further testing when I get the time, Thanks for all the helpful information and the quick reply!

author: amspire
author_rank: Super Contributor
timestamp: March 05, 2012, 06:58:04 am
content: I would get a 9V battery for the testing. Using an ATX power supply is a bit scary, particularly if you are not sure where it is safe to connect the ground lead. If you keep using the power supply, at least add a series resistor to the 12V - 100 ohms should work fine. Otherwise if you have a fault or misconnect the scope ground, you can get 20 amps+ into either the circuit or the scope.

author: baljemmett
author_rank: Unknown
timestamp: March 05, 2012, 04:24:39 pm
content: Quote from: ELItheICEman on March 05, 2012, 04:46:19 amI built that one last summer -- I didn't use the exact transformer specified, but substituted a core and bobbin I could get easily through my usual suppliers. Unfortunately I undersized it a bit so had to scale the windings down to something like 100:5!QuoteThis is an ideal opportunity to play with the 'scope and get used to using it! When I first powered my build up it did exactly the same thing; although I suspected I'd cocked up when winding the transformer I figured I might as well troubleshoot it methodically and get some oscilloscope praxis in at the same time, so traced the signal out of the oscillator side, to the transformer primary, and then could find absolutely nothing when looking across the secondary. A quick continuity test showed the secondary was open-circuit (which it wasn't when I wound it, hmm) so I redid that winding and got the expected output.So in other words, exactly what amspire's said already, with a dose ofEEVblog #17for good measure -- and definitely consider running it off a 9v battery for testing, you don't want rich chunky amps from an ATX supply ruining your day if you misprobe something.