LEM IT 60-S Ultrastab Interface - Page 1
Forum section: electronics / testgear

author: slugrustle
author_rank: Frequent Contributor
timestamp: August 30, 2025, 08:10:30 pm
content: I recently acquired a used LEM IT 60-S current sensor (Datasheet:https://www.lem.com/sites/default/files/products_datasheets/it_60-s_ultrastab.pdf). The output of the current sensor is a ±100mA current, proportional to the ±60A input current. The datasheet indicates the use of a <60Ω burden resistor on the current sensor output, with a 0Ω minimum.I want to interface this sensor with a bench DMM (Rigol DM3068) and am considering connecting the current output of the sensor directly to the current input of my DMM and set it to ammeter mode. The DMM internal shunt would be the burden resistor. Does anyone see a problem with this?A Vishay VPR221SZ foil resistor probably has better TCR, but the DMM ammeter specs are good enough for my purposes, and I think it's easier to wire directly to the DMM.

author: Martin72
author_rank: Super Contributor
timestamp: August 30, 2025, 09:28:14 pm
content: Hi,QuoteZero ohms would not be good because then no voltage would drop across it.QuoteNot at first.It depends on what current strength you usually want to measure.Based on the transmission ratio of the LEM, the following currents are generated:60A: 100mA1A: 1.67mA100mA: 167µAIf your Rigol can reliably measure currents in the mA/µA range, I see no reason not to go with the “direct” solution.We also have these LEM current transformers at work for our ZES LMG450 analyzers.On Monday, I can take a look at how the LMG processes the signal from the LEM.

author: Hydron
author_rank: Super Contributor
timestamp: August 30, 2025, 10:12:35 pm
content: I've done this successfully with a different ultrastab, used a DMM6500 and put the ratio in as an equation to get direct reading output. Even with a 600A (1500:1 ratio) full scale sensor it was usable down to mA levels with the DMM autoscaling.

author: slugrustle
author_rank: Frequent Contributor
timestamp: October 18, 2025 02:53:31 am
content: Quote from: Hydron on August 30, 2025, 10:12:35 pmThank you. This ended up working well. My readings via DMM matched the E-Load I was using to ±1 count in the last digit on the E-Load. Had to use the measurement because the E-Load wasn't in calibration and it's policy. E-Load claims ±0.05% reading + ±0.05% full scale on the 30A range. DMM claims ±0.05% reading and ±0.015%/±0.003% full scale on the 20mA/200mA ranges so not too much better.I took an offset on the 200µA range first and waited until that stabilized on the trend chart before doing the other measurements. Hope that helped. Offset was only 5.08µA output so 3.048mA input referred. Pretty great for a unit I got off eBay.You mentioned DMM autoranging. I am pretty sure the Ultrastab needs an output shunt to always be connected for proper operation. I think it directly outputs the compensation winding drive current as the measurement output... I used manual ranging and shorted the DMM input whenever I changed ranges so relay switching in the DMM front end wouldn't interrupt the Ultrastab output current. Maybe that's overkill, but it made me feel better.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 18, 2025 08:49:51 am
content: Autoranging can cause some disturbance, but if there is no design flaw there should be no excessive burden voltage even for short times. Often they have make before break logic / relays and at least the protection diodes in parallel. So the voltage drop should no go higher than some 2-3 V even for transients. 60 ohms and 100 mA would be 6 V maximum drop.

author: slugrustle
author_rank: Frequent Contributor
timestamp: October 18, 2025 03:15:45 pm
content: Very good to know. Thank you. I'm going to go measure this effect with a current source when I get some time, just out of curiosity.