Project Design & Layout Critique - Page 1
Forum section: electronics / projects

author: DrMag
author_rank: Regular Contributor
timestamp: May 03, 2016, 06:31:00 pm
content: I'm working on a project to measure small currents (<100 uA) at high voltages (3-5 kV). Unfortunately, our setup precludes a low-side current shunt, so the measurement has to be done at the high voltage end. I've come up with a scheme that I believe will work, by measuring the light output of an LED put in series with the high voltage circuit. Below is the schematic and board I've designed to test this idea, and I'd really appreciate any feedback you might have to offer! I'm a physicist, not an engineer, so a lot of this is self-taught, and I'd love to learn better practice.A few notes about the design:This is a prototype, and will not be at high voltage. The distance between the LED and photodiode is set by a light pipe, and in the final board the high voltage isolation from the rest of the circuit should be quite good. I might even put a slot in the board between to be safe.The ADC is measuring the photodiode voltage, the actual voltage at the shunt resistor on the LED (to indicate what the current actually is), the 4.096 V reference voltage (gives 0.25 mV/bit), and the supply voltage. (I had another ADC input, so why not?)The four LEDs (labeled 0,1,2,3) are simply for debugging the software in the MSP430.The system we are testing does not change quickly, so the 10V/us slew rate of the OPA322 is more than sufficient; the noise level of the op-amp looks great for this project.I've attached images of the board layout both with and without the ground fill visible. There are a number of vias scattered in key places to stitch the two sides together and get connections to the needed places.Comments on anything from component choice to layout style are more than welcome! Thanks, everyone!Edit: I just realized I completely forgot to connect my reference voltage to the ADC.Fixed, and attached images are now correct.

author: spudboy488
author_rank: Regular Contributor
timestamp: May 04, 2016, 11:49:01 am
content: It's a nit but it is something that has bugged me (and burned me) on some legacy designs at my company: You are inconsistent with your connector orientation. I'm not sure which connector you plan on using for P1, P3, P4, and P5 but, for example, if using MTA-100 headers, P1 and P5 are correct where pin 1 will match the connector. P3 and P4 are "backwards". Maybe you did this on purpose for cable routing and plan on mounting the connectors to match the pin numbers and having the wires exiting the connector in-board. If that's the case, then I'm fine with it. But mixing which end of a connector is pin 1 on a PCB if using the same connector is not nice.

author: RobK_NL
author_rank: Frequent Contributor
timestamp: May 04, 2016, 12:10:50 pm
content: Your photodiode amplifier looks very strange, where did you get that?The most common way is something like this:

author: Ian.M
author_rank: Super Contributor
timestamp: May 04, 2016, 12:58:39 pm
content: LED light output is *NOT* linear with respect to If and also varies with temperature and age. Your solution will drift badly and require frequent recalibration to even hold 10% accuracy.To do this right, you need to convert to a digital signal at the high voltage end, before you optically couple down to your MCU. You therefore need to choose a modulation scheme for the optical link e.g pulse rate or PWM modulation, or put a MCU with an ADC on the high voltage board and simply send a serial data stream with RS232 framing.Obviously all these choices require isolated low voltage power sitting at your HV voltage. Probably the easiest option is a high isolation DC-DC converter e.g.http://www.digikey.co.uk/product-detail/en/recom-power/REC3.5-1205SRW-R10-C/1486-REC3.5-1205SRW-R10-C-CHP/5120356. It rapidly gets more expensive if you need more than 5KV working isolation, and depending on the application, batteries, or solar cells (+ optionally a bright lamp to illuminate them) or a miniature DC motor generator set coupled by a perspex shaft or an insulating belt may be a more cost-effective choice. Solar cells are particularly attractive if you need high isolation with minimum electrical noise.