Multimeter Current Autoranging with MOSFET - How? Looking for a Forum Thread - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: June 11, 2018, 09:09:32 pm
content: Hello,just thinking about some improvements of a multimeter design I am considering. Well - not exactly a multimeter, but there is a part of a circuit that does excatly that: Measuring current accross very wide range: 400uA to 10A.I do not want to use mechanical switches - reliability issues, rather high power consumption, slow, all sorts of problems.I vaguely remember the circuit was configured like this, using parallel connected shunts, switched with antiparallel pair of mosfets. There was a trick, that keeping the shunt voltage low (<200mV???) the leakage through the substrate diode is negligible.There was a thread on this forum, that discussed exactly this. Someone even posted some measurements of that leakage, including some temperature dependent measurements. I can not find the thread any more. Can you please point me to it?One different question though: As the instrument can be easily disconnected from power while still connected with the current measuring circuitry, I need a protection not to blow up the mosfets on the highest current range. (10A through a substrate diode is a LOT of heat!) I figured out the best would be to use a NC type relay across the current input jacks.What type of relay would you use there, considering 10A continuous NC contacts are needed, while requiring the least amount of current consumption and small space? Voltage rating is almost irrelevant, as under normal circumstances, there never will be more like a 1V across an open contact.Thx as alwaysY.

author: Yansi
author_rank: Super Contributor
timestamp: June 11, 2018, 09:43:47 pm
content: Never mind, I have finally found it! Yaaay!https://www.eevblog.com/forum/blog/eevblog-929-designing-a-better-multimeter/50/But the question for the relay is still open.

author: JS
author_rank: Frequent Contributor
timestamp: June 12, 2018, 04:09:24 am
content: Υου could use latching relays instead of mosfets for the range swapping, the problem with those is the unknown start-up condition, a firmware line or two should reset them at start-up.For the NC relay you would be drawing quite some power, while on and measuring. Keeping a mosfet ON takes virtually no power, so if your product is battery powered you could supply the highest amp range selected as default with small leakege while the device is off. 100nA for an IRF540. If you could expend that energy during power off condition you should be fine.JS

author: Yansi
author_rank: Super Contributor
timestamp: June 12, 2018, 05:36:19 am
content: Well, keeping mosfet ON takes no power, however exactly no power is available to the device when disconnected from mains or its power supply.I don't know, but even the battery powered multimetrs, I do not trust the solution "keep mosfet on" is valid in any way. Or do these multimeteres say in the manual, not to pass any current while battery is drained or not present in the device? Or while multimeter switched off? It is kind of dangerous in my opinion, as 10A through any substrate diode is a lot of heat to at least desolder the mosfet from the boardIt would be nice to have some depletion mosfet type (normally ON) that would hold that amount of current.Y.

author: JS
author_rank: Frequent Contributor
timestamp: June 12, 2018, 10:52:59 am
content: Note that most DMMs have a separate path for the higher range. It isn't common to see ranges over 3A state the path with lower ranges.For your case 3A could be managed by protection diodes, which in low burden voltage shunts might be shoktty diodes dissipating a reasonable ammount of heat. Granted the shunt resistors must be over rated but shunt resistors always are. You might want an 0.03Ω for 3A, to have 100mV of burden voltage, that's 0.3W. At 0.3V you already have 10A, 3W so fuse should blow before diodes kick in and you need about 3W resistor so self heating isn't much of a problem. For lower ranges the diodes will shunt the current and fuse will blow before a 1Ω 1W resistor or a small 100Ω resistor have an trouble.If you want to deal with higher ranges becomes harder as you need bigger and bigger diodes, in any case the higher range shunt should hold more current than the fuse and lower ranges go in series with each other so diodes kick in first. If the mosfet diode can't handle the higher current fussing you could use shoktty diodes. You shouldn't be getting 0.3V without counting the fuse as low burden V, so leakage should be manageable.JS