DMM input protection - Page 1
Forum section: electronics / beginners

author: SG-1
author_rank: Regular Contributor
timestamp: August 30, 2018, 05:56:47 pm
content: We use 6VDC door bells for continuity checks & I would like a better way to protect them other than MOVs in parallel with the bell & a fuse in series. It is not practical to de-energize the control circuit during this process because everything will change states.How do DMM manufactures like Fluke protect resistance & continuity settings from live circuit voltage ?

author: IanB
author_rank: Super Contributor
timestamp: August 30, 2018, 06:12:08 pm
content: You should look at some of the posts and videos by joeqsmith on this subject. You will find very in-depth information on the subject.Very briefly, and grossly oversimplified, the input is protected by series resistors and PTCs to limit the fault current, and then whatever voltage does appear on the other side of the resistors is clamped to limit the maximum voltage that can be presented to sensitive components downstream.

author: SG-1
author_rank: Regular Contributor
timestamp: August 30, 2018, 06:52:54 pm
content: I just watched Dave's Video, I will check out joeqsmith.

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2018, 12:44:02 am
content: Ohms or continuity mode places current source in parallel with the inputs while measuring voltage so there are two separate circuits to protect.The voltage inputs use a high impedance buffer or amplifier so a high value resistor in series with the buffer input (after the high impedance voltage divider or shunt) can limit the current without affecting the reading. 10 picoamps of input bias current through a 1 megohm series resistor is only produces a 10 microvolt error but that same 1 megohm resistor limits the overload current to 100 microamps per 100 volts. The series resistor needs to be a high voltage type and normally a low value high voltage capacitor is placed across it to limit noise and swamp the effects of the input capacitance of the input buffer and any shunt diodes if AC measurements are made.The current source is more difficult to protect. Since it only sources current in one direction, a diode can protect it against high positive input voltages but what if it is pulled negative? Then the current source's voltage compliance is potentially unlimited. High voltage compliance can be built in with a high voltage output transistor but this still allows high power dissipation when high voltage is present. (1)A common solution in older DMMs is simply to add a low current fuse. An old solution I like is to add a high voltage low current incandescent bulb which essentially acts as a better PTC thermister. A better design would probably do this and disable the current source when a high negative voltage is detected limiting power dissipation which is easy enough to do.(1) It is not quite that dire. Most DMMs only specify their ohms mode to be protected up to 400 volts so connection to a rectified 240 volt AC source will not damage ohms mode. 400 volts at 1 milliamp from a typical ohms converter is only 400 milliwatts which is not too demanding of the ohms converter's output transistor.