Multiple Inputs For My DMM - Page 1
Forum section: electronics / general

author: Keith
author_rank: Contributor
timestamp: September 12, 2011, 12:50:05 am
content: Anyone know if there is a simple box that takes 4-6 separate probes and multiplex down to the one going into a DMM? Simple box with a mechanical switch that allows me to hook up multiple test points throughout the circuit and switch between them. Sounds easy enough to build one. Thanks

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 12, 2011, 01:26:55 am
content: Quote from: Keith on September 12, 2011, 12:50:05 amThat's trueJust get a high quality switch and lots male-female banana plugs.

author: Unknown
author_rank: Unknown
timestamp: September 12, 2011, 01:50:03 am
content: Quote from: Keith on September 12, 2011, 12:50:05 amIt is an easy enough thing to build. I don't know of an exact commercial match and easy is a relative term.This could be a useful beginner project to hone soldering and mechanical construction skills. It can be surprising what you can learn from the simplest of circuits.If you do decide to go ahead, two things to consider first up are1. Ensure the cables, connectors, insulation, test leads/clips, switch etc are all rated for the the voltages you plan to test.2. Ensure your selected switch is Break-Before-Make NOT Make-Before-Break. (A trap for young players.)

author: Keith
author_rank: Contributor
timestamp: September 12, 2011, 02:20:57 am
content: thanks guys!

author: Unknown
author_rank: Unknown
timestamp: September 12, 2011, 03:28:04 pm
content: Keithley has some appnotes about automated switch system, maybe some of them are applicable to smaller systems. In commercial settings, this is often called a scanner. Some bench meters have optional scanner cards, basically a bunch of relays and connectors. Issues are isolation (eg. CAT III 600V), resistance (any additional resistance will act as series resistance with a normal 2-wire resistance measurement), current rating and offset voltages (for low voltage measurement).

author: Bored@Work
author_rank: Super Contributor
timestamp: September 12, 2011, 04:15:03 pm
content: Agilent sells automatic multiplexers for lots of $$$, e.g. 34970A or 34972A.But it looks as if a box with a switch might be all you need. Do run the numbers first. It might be that the sum of the cost of a good enclosure, quality switch, safety 4mm sockets, probes etc. might be as much as another multimeter.

author: Unknown
author_rank: Unknown
timestamp: September 13, 2011, 12:15:42 am
content: Quote from: BoredAtWork on September 12, 2011, 04:15:03 pmAll depends on the detail of the problem at hand. A second (or second and third meter) may solve the OPs problem and offers a number of advantages. The proviso I'd add is to make sure any additional meter is of a reasonable quality or accuracy. I'd rather a basic Fluke and a switch over a box load of junk meters.An extra meter will offer a comparison reference for any existing meter and twice the real time display capacity, while constructing a switch box might be a way of gaining some construction experience without too much risk of escaping smoke.Hell if budget allows why not do both.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 13, 2011, 01:08:24 am
content: At work, someone once constructed a USB controlled switch board to allow many automated measurements to be made with a single DMM. (There were 10 or so precision relays connected to a MAXQ microcontroller.) Then they got a PXI box with a multi channel A/D card and the board went into storage.It's pretty easy to build such a switchbox, although you'll need really good relays if you want low crosstalk.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 13, 2011, 03:04:17 am
content: Quote from: Kiriakos-GR on September 12, 2011, 01:26:55 amHigh quality stands for silver plated mechanical switch.I bet that at the six poles switch you will had good clearance between the contacts so to get even up to 600V.But it best to double check this factor, by reading the specifications.

author: Keith
author_rank: Contributor
timestamp: September 14, 2011, 12:15:04 am
content: Thanks to all of you for the help. I'm looking to set up multiple test points on my circuits and perform some low power current and voltage measurements. Low being less than 30 volts and not more than an amp. I have a few meters so this is for fun.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 14, 2011, 03:32:19 am
content: I think that you have to forget the idea about Amp measurements with the box.If you are smart you will understand the why !!

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 14, 2011, 11:59:41 am
content: a good idea! but not so good. if i want to make it automated, i want to do a single switch and it will do both range and probe switching. pushing a switch and then switching the range knob (2 task) still looks like the old way, just a slight simpler, but... YMMV. as i said, a good idea.

author: Joshua
author_rank: Regular Contributor
timestamp: September 14, 2011, 03:36:14 pm
content: Also keep in mind that on all the meters I've seen, you actually have to move your positive lead on the multimeter between voltage and current measurements.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 14, 2011, 03:42:39 pm
content: i think thats what he's trying to do.

author: Lawsen
author_rank: Frequent Contributor
timestamp: September 15, 2011, 07:01:54 pm
content: I read you want to make a manually controlled 4 inputs to one input to a simple multimeter. You will need a rotary switch and four banana input jacks as Kiriakos mentioned in the reply. I remembered bounce less digital switch in my electronics text book, but a resistor in the circuit will alter your measurements. It is best to keep the wiring short and simple to not alter the measurements and use a quality rotary switch and banana jack.Multiple input digital multimeters are usually specialized data acquisition devices with multiplexer logic switching. These are costly instruments. Most are used at an university academia place for research in measuring voltages from multiple places or product development in a business. I have no experience in this. I did remember from college that I have used a three channels MPLI (discontinued) by Vernier to convert the displacement of a tension tester by moving a Bourns linear precision potentiometer changing resistance in a Wheatstone bridge circuit, thus changing the voltage applied to keep track of the displacement position of the tension tester. It was a method to convert a physical motion to an electrical signal for the computer to record the tension tester's position and give a digital read out.http://www.fluke.com/fluke/caen/Data-Acquisition-Equipment/Portable-Units/Hydra.htm?PID=56279http://www.vernier.com/products/interfaces/sdaq/http://www.bourns.com/ProductFamily.aspx?name=potentiometershttp://en.wikipedia.org/wiki/Wheatstone_bridge

author: Keith
author_rank: Contributor
timestamp: September 16, 2011, 07:41:18 pm
content: Thanks again to all for the advice. I was going to purchase SPDT(break then make) switches that will allow me to insert my probes into various test points on my circuit and watch the changes at those points after I change a variable. I would simply manually switch between the ports allowing me to see what was happening on that particluar point. I was trying to avoid multiple DMMs and also to have some fun building a small project.

author: Unknown
author_rank: Unknown
timestamp: September 16, 2011, 11:41:43 pm
content: Quote from: Keith on September 16, 2011, 07:41:18 pmThat's what it's all about. Your project, your way, with plenty of learning along the way.