Help reverse engineer this circuit - Page 1
Forum section: electronics / projects

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 25, 2017, 08:04:03 pm
content: Im working on reverse engineering a certain part of a motor protection relay, this one:It is the circuit that measures the input voltage from one of the input phases, as this is for 3 phase motors in high voltage installations. 3 neutral connected transformers are connected directly to each of the inputs just like this:So, here's a general view of the 4 voltage inputs (although normally only 3 are used), they are identical to each other:And here's a closeup of one of the sensing modules. The input is AC, up to 200V, and somehow this isolated module outputs digital data at the digital isolator (ADUM3100) with only 2 comparators in the circuit. It's quite clear that it is powered from the same voltage it is measuring but I don't think there is even circuitry for a PWM, any ideas?(The 12 zener drives the gate of the mosfet, and the 5V zener powers the digital isolator)

author: bktemp
author_rank: Super Contributor
timestamp: February 25, 2017, 08:16:38 pm
content: There is a low value ceramic capacitor next to the LMV393. My guess would be, the comparators work as a VCO or some other voltage to pulse density modulator converting the voltage into a digital data stream.I would apply a variable input voltage and look at pin 1 and 7. There should be either a changing frequency or some other voltage dependent digital signal.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 25, 2017, 08:47:59 pm
content: Quote from: bktemp on February 25, 2017, 08:16:38 pmThat is something I've should have done already but haven't as I don't have it with me at all times. Still, if this works as I suppose, I'll be amazed at the low cost low parts count solution.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 25, 2017, 10:17:46 pm
content: I'm going to guess that they implemented either a Delta Sigma ADC or a PWM modulator with the comparator. Given that it has a bridge rectifier, they're probably looking at the zero crossings to check the phase order. It would help to show if there's anything on the other side.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2017, 01:27:04 am
content: No surprises here -- people frequently underestimate the power of a simple dual comparator.For example, I can realize most of a UC3842 with an LM393, it just takes a ton more resistors and some support transistors.Tim

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 26, 2017, 06:41:02 am
content: Quote from: NiHaoMike on February 25, 2017, 10:17:46 pmNothing on the other side

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 26, 2017, 06:43:25 am
content: Quote from: T3sl4co1l on February 26, 2017, 01:27:04 amYour sentiment matches that of Jim Williams,"Comparators may be the most underrated and underutilized monolithic linear component. This is unfortunate because comparators are one of the most flexible and universally applicable components available."http://cds.linear.com/docs/en/application-note/an13f.pdfLater this week I will take the time to reverse engineer the traces among the different components and take scope screenshots

author: eb4fbz
author_rank: Regular Contributor
timestamp: February 26, 2017, 08:39:48 pm
content: It could be a voltage to frequency converter.

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 10, 2017, 04:00:37 pm
content: Congrats to all who voted for PWM, it seems they are generating a "cheap" 260khz triangular signal at the output of one of the comparators with an RC, diode discharged. This wave is fed to the second comparator with the rectified input. The output is then fed to the digital isolator. I guess they correct the non-linearity digitally.