Current sense autorange - Page 1
Forum section: electronics / testgear

author: RedHead
author_rank: Newbie
timestamp: September 08, 2025, 10:13:26 am
content: Hello folks!I'm currently working on current measurement schematic, kind of DMM, and I'm having trouble understanding how autorange should work.In voltage measurement, it's pretty straightforward - high impedance divider and a TVS diode for overvoltage detection, and if you measuring very small voltage and than it raises to higher measurement range you can detect it and switch range up one by one until the overvoltage is no longer present.But if same situation occurs in DC current measurement, the shunt resistor will quickly blow up. If i measure, for example, microamps DUT standby current with a 10k shunt and then DUT runs with consumption like 100mA then power on shunt will be 100W. Yes, schematic should detect this overrange and switch to a higher range, but this doesn't happen immediately. According to the Vishay CRCW-HP datasheet (https://www.vishay.com/docs/20115/crcwhp.pdf), the 0805 (2010 is too big) resistors can only handle tens of microseconds before they blow, and also on 10k it causes overvoltage (1k vs 500 V max in the datasheet), which can cause damaging sense amplifier.I think about using PTCs, but they ruin the measurement scheme, so they are not suitable.Am I missing some conventional way to handle this case?Shold it be just a software solution, which detects overrange faster than shunt blows?

author: Kean
author_rank: Unknown
timestamp: September 08, 2025, 10:30:02 am
content: You will only get 100W across that 10k shunt if the current source can supply over 1000V at 100mA. And at higher currents, the compliance voltage becomes even higher.In any case, the traditional way is to clamp the voltage across the shunt with back-to-back diodes. In the 121GW schematic you can see it actually uses a DF10S 1A 1000V bridge rectifier for the lower current ranges.

author: Berni
author_rank: Super Contributor
timestamp: September 08, 2025, 10:37:14 am
content: This is the main reason why multimeters so often have a 10A and 100mA jack and why the burden voltage on the 100mA jack is so terrible. Daves uCurrent gadget uses a switch to switch in different resistor values.More fancy DMMs sometimes have relays inside their analog front end to switch in different shunt resistors.Offten still using the smallest possible shunt value and amplifying it up to reduce the range switching problems. (as it is not practical to have a separate shunt for each current range)Another option is going the electrometer route where very tiny currents are instead first turned into a voltage using a transimpedance amplifier, that voltage is then easier to measure and solves the problem of burden voltage, but resistors are still typically switched into circuit by relays.It is possible to do the switching using solid state too, there are a few products on the market that are designed for measuring current consumption of battery powered devices with a massive dynamic range (nanoamps to miliamps) that do exactly that, allowing them to switch current ranges so insanely quickly that it looks like one big continuous range.

author: Kean
author_rank: Unknown
timestamp: September 08, 2025, 12:30:18 pm
content: The Nordic PPK2 is a good example in terms of range switching with multiple stages, auto & manual switching, and a high sample rate. It also includes a basic SMU feature set allowing self calibration.It doesn't include much in terms of protection though, which is fair as it is only intended/rated for use at up to 1A and 5.5V.

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2025, 01:01:12 pm
content: Quote from: Berni on September 08, 2025, 10:37:14 amManual ranging multimeters may extend their decade divider to lower resistances so that they have a separate current shunt for each range.

author: RedHead
author_rank: Newbie
timestamp: September 10, 2025, 03:16:51 pm
content: Oh, really, I've seen Dave's video about DMM input protection, but I absolutely forgot about the diode bridge in the current path.Feel a little shame)Also, I found a Fluke 87 schematic and noticed BJTs(Q13, Q14) connected to the current path . Does this also provide some sort of protection with temperature compensation?

author: Kean
author_rank: Unknown
timestamp: September 10, 2025, 05:04:02 pm
content: The typical dual BJT protection clamp seen in a DMM input path is used for its low capacitance and low leakage properties. Note than it will typically clamp at a higher voltage than back to back diodes - more like back to back zeners at 6-10V.See this post for a little discussionhttps://www.eevblog.com/forum/beginners/multimeter-input-protection-what-are-these-bjts-doing/

author: RedHead
author_rank: Newbie
timestamp: September 11, 2025, 01:58:38 pm
content: I've tried to model this part of the schematic and saw that it provides only negative clamping. But it could be an AC measurement and protection won't do the job. It looks strange to me

author: mawyatt
author_rank: Super Contributor
timestamp: September 11, 2025, 02:21:58 pm
content: Quote from: RedHead on September 11, 2025, 01:58:38 pmDoes the npn model include reverse breakdown of the BE junction?Best

author: RedHead
author_rank: Newbie
timestamp: September 12, 2025, 07:38:29 am
content: Hmmm, interesting point. I've dived slightly deeper into the BJT SPICE model structure (https://help.simetrix.co.uk/8.0/simetrix/mergedProjects/simulator_reference/topics/simref_analogdevicereference_bipolarjunctiontransistor_spicegummelpoon.htm) and found that there is no reverse breakdown voltage parameter. It seems like you need to place diode models in parallel with the BC and BE junctions for it to work properly. That's so weird...

author: mawyatt
author_rank: Super Contributor
timestamp: September 12, 2025, 01:20:19 pm
content: There are many BJT Spice models like VBIC, MEXTRAM, HICUM which provide additional features and parameters for better modeling fidelity of the BJT behavior than the basic Gummel-Poon based generic BJT model. However, many of the popular BJTs haven't been modeled at these higher levels yet, so nice project for some undergrad studentBest

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2025, 04:59:02 pm
content: The problem I have had in the past with better transistor SPICE models is that usually the parameters of the transistor are unavailable, so would require detailed testing to determine.