Proper Name/DIY Circuit For A DMM Switch Bank - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: May 11, 2019, 03:47:55 pm
content: My DIY Hobbyist work has become sophisticated enough to require multiple high quality DMMs to read various parts of a circuit all at the same time. I can not afford so many high-dollar DMMs, so I am wondering if there exists a switching bank that could handle multiple/6-8pairs probe inputs, to one/two outputs, wherein the outputs post-switch equal the inputs pre-switch exactly for volts, amps and resistance.What is its proper name? Commercial unit has gotta be expensive, right?Is it a DIYable circuit? If yes, any suggestions for a good DIY circuit. I can handle fairly extensive DIY manufacturing... KiCad boards, order them, I have a DIY Reflow Oven...Thank you for your help.

author: rstofer
author_rank: Super Contributor
timestamp: May 11, 2019, 04:22:13 pm
content: Any 2 pole rotary selector switch can select any 2 inputs and output them to the meter. HOWEVER, what are the odds that the meter is in the wrong condition (amps versus voltage versus resistance) when you change the switch? I would expect this arrangement to be a recipe for disaster. I guess you could disconnect the DMM from the switch output before changing channels but it still isn't a good situation (in my view).I can't imagine needing to make as many as 8 measurements for any of my projects. I might possibly get to 3 but it's not an issue because I have several multimeters (digital and analog).Have you looked at the Aneng 8008? It is a terrific meter for about $26 at Amazon. I wouldn't trust the safety ratings (on ANY Chinese meter, for that matter) but, of all the meters I have, it is the one I reach for. It's just a nice little meter. Dave did a review and the accuracy was outstanding!https://www.amazon.com/Alloet-True-RMS-Digital-Multimeter-Voltage/dp/B072JMBLJS

author: t1d
author_rank: Super Contributor
timestamp: May 11, 2019, 06:22:59 pm
content: Thanks, rstofer, for your help.- A simple rotary switch is going to add influences to the reading... The significance of those influences will depend on the accuracy required. That's why I thought some negating circuitry might be required. I guess it could be handled with just math... Determine the resistance the switching system adds and then account for it. But, that's not very efficient.- You make a v-e-r-y good point about having to be careful about what mode you are in, when you change the switch. I had not thought of that one and it is extremely important, as you said.- Well the only reason I ask the question, of course, is because I have a present need. For my current project, I could readily use six... It is a LT3081 PSU... So, Supply volts and amps, Output volts and amps, Iset voltage and Mon voltage. The two input DMMs could be read once and moved off, but my Bench PSUs don't have very nice/accurate/digits accuracy and I am curious as to what is going on on that side...- Good suggestion on the cheap meters. I will look at the links.I appreciate your interest, input and opinions. Thank you. I will continue to think on it.

author: rstofer
author_rank: Super Contributor
timestamp: May 11, 2019, 07:05:23 pm
content: Quote from: t1d on May 11, 2019, 06:22:59 pmThe rotary switch on the DMM itself is nothing more than spring contacts on PCB traces. Older V-O-Ms used nothing more than a rotary switch.http://www.electro-nc.com/faq.shtmlhttps://www.simpson260.com/260-3/simpson_260-3.htmTrue, the 260 never had the resolution of a modern DMM but I don't think it was the rotary switch causing the limitation.Any 'electronic' multiplexing will add complications due to scaling, injection current, limited range and so on. You can look into analog multiplexers but I don't think it's going to work out unless you want to deal with the current shunt before the MUX. But it could be done...

author: t1d
author_rank: Super Contributor
timestamp: May 11, 2019, 07:12:45 pm
content: Good stuff... Thank you.

author: artag
author_rank: Super Contributor
timestamp: May 11, 2019, 10:17:07 pm
content: More expensive bench multimeters like the Keithley 2000 often have available a 'scanner' or 'mutiplexer' card to switch several inputs. They're most commonly used with thermocouples and special cards exist for this purpose but generic cards to switch voltages and even 4-wire ohms circuits are available.Although these were once extremely expensive, you'll find that lower-resolution meters - especially if a bit bulky - can be quite cheap on the used market.

author: Brumby
author_rank: Unknown
timestamp: May 12, 2019, 02:35:44 am
content: The amps vs voltage issue can be addressed by having at least 2 meters. One set on current and the other set on voltage - switching current readings through one and voltage readings through the other. This would make shorting the non measured current sources easier, too.However, if you spend a little time with electronics, you could soon end up with 6 multimeters - and then you wouldn't have to worry about switching at all.

author: oPossum
author_rank: Super Contributor
timestamp: May 12, 2019, 03:32:02 am
content: Quote from: t1d on May 11, 2019, 03:47:55 pmThey are expensive when new. You may be able to get a HP 3488A for a price that is within your budget.

author: t1d
author_rank: Super Contributor
timestamp: May 12, 2019, 05:18:32 am
content: You all provided great information! I'm glad I asked. Thank you.

author: artag
author_rank: Super Contributor
timestamp: May 12, 2019, 02:52:33 pm
content: Re Brumby's suggestion : another way to measure lots of current sources is to have one voltmeter and multiple current shunts. They don't have to be high precision, you can calibrate them yourself. Then you can leave the shunts in circuit (no need to switch the actual current, which might make a brief break) but just measure the voltages across the shunts along with the other voltages.I do have to disagree that you'll soon have 6 meters though. You'll have far more than that.

author: tooki
author_rank: Super Contributor
timestamp: May 12, 2019, 05:02:34 pm
content: Depending on your needs, consider using INA226 current/voltage ICs with an Arduino. They are astoundingly precise and accurate, especially given that they only cost a few bucks on eBay. The only obvious limitation is that they can’t measure negative voltages, which rules out some applications. (They can do negative currents just fine.) That and that isolation could be an issue in some circuits. I talk about them more here:https://www.eevblog.com/forum/projects/ode-to-the-ina226-voltagecurrent-sensor/

author: rstofer
author_rank: Super Contributor
timestamp: May 12, 2019, 05:35:56 pm
content: Quote from: tooki on May 12, 2019, 05:02:34 pmThat's a really neat device! The solution could be elegant...I don't understand A2 and A3 but apparently it is possible to put 16 of these gadgets on a single I2C bus. I think I would need to spend a lot more time with Table 2 and section 7.5.5.xhttp://www.ti.com/lit/ds/symlink/ina226.pdfClever!ETA: Worst case, $3.37/ea qty 1 at DigiKey.

author: oPossum
author_rank: Super Contributor
timestamp: May 12, 2019, 06:08:26 pm
content: For high side only there is the popular INA219 and the triple INA3221. Both readily available on BoBs.