Cheap method to measure I (30mA to 500ma), with 1.5Vcc? - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: January 11, 2018, 06:28:45 pm
content: I need a circuit that will measure the motor current of a small dc motor that operates from a single AA cell. The circuit is powered from a single AA cell and I need to detect when the current is less than 30mA and greater than 150mA.The signal will be used to drive comparators that have a 0.2V internal reference.I could use a series resistor (say 0.1and a RR opamp with a gain of 57, however perhaps there is a dedicated IC for this application (needs to work down to 1V or so)Any suggestions?

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: January 11, 2018, 06:41:50 pm
content: IDK, but 0.1R and 30 mA => 3 mV and motors are noisy, you're going to need a good filter. A comparator is faster than an opamp (the opamp will go into saturation => not good, and slow).

author: Jester
author_rank: Frequent Contributor
timestamp: January 11, 2018, 07:01:18 pm
content: Quote from: GeorgeOfTheJungle on January 11, 2018, 06:41:50 pmIdeas for a better approach?

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2018, 07:24:27 pm
content: Does the current sensor also need to be powered from the same 1.5V source?What voltage drop is acceptable?Does it matter whether the current sense resistor is on the positive or negative side of the motor's supply?The trouble is, not just the voltage drop, but getting components which can run at such low voltages. To get decent battery life from an alkaline cell, you need the circuit to work down to 1V or less. I've just Googled and found the TS1001, which can work down to 0.65V. Another possible candidate is the LMV951, which can work down to 0.9V.https://www.silabs.com/documents/public/data-sheets/TS1001.pdfhttp://www.ti.com/product/LMV951Setting the op-amp gain high, for as lower voltage drop as possible, may seem like a good idea, but it can cause the input offset voltage to become significant, so there will be a compromise between cost, voltage drop and accuracy.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: January 11, 2018, 07:27:19 pm
content: Quote from: Jester on January 11, 2018, 07:01:18 pmNot that I know of. I was just thinking out loudand honestly, I doubt I would know how to make that work with just 1..1.5 volts... (sorry!)The motor is brushed or brushless?

author: Jester
author_rank: Frequent Contributor
timestamp: January 11, 2018, 07:38:10 pm
content: Quote from: Hero999 on January 11, 2018, 07:24:27 pm

author: martys
author_rank: Regular Contributor
timestamp: January 11, 2018, 11:16:18 pm
content: Might be worthwhile to use something like a tiny MCP640 boost convertor that will work down to Bat V > .67V and create 90% efficient boost to a 3.3 or 5V stable voltage supply so you can then use a cheap LM393 or LM339 comparator and even have stable operation over the full life of the single cell 1.5V battery.

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2018, 10:12:25 am
content: Quote from: Jester on January 11, 2018, 07:38:10 pmWhat are you using for a comparator?What tolerance are you after?I'd be more tempted to use a higher sense resistor and less gain. The TS1001 and NCS2001 have an input offset voltage of 500µV and I doubt you can get better than that for such a low voltage op-amp. A 0.1R sense resistor would only result in 3mV at 30mA, which is only six times the op-amp's offset voltage. A 0.5R sense resistor would give 15mV, which is 30 times the offset voltage. The voltage drop would be 75mV at 150mA, which is hopefully acceptable.You could use two separate amplifiers, for the different current levels. One with a gain of 13.3 for the 15mV threshold and 2.67 for the 75mV.I no longer bother working out gains. I use this calculator to help work out the resistor values:http://sim.okawa-denshi.jp/en/rercal.phpVOUTon the op-amp circuit is V1 on the calculator.VINon the op-amp circuit is V2 on the calculator.V1 = VOUT= 200mVV2 = VIN= 15mVE24 valuesR1 = 1k6R2 = 130RE96 valuesR1 = 1k69R2 = 137RV1 = VOUT= 200mVV2 = VIN= 75mVR1 = 2kR2 = 1k2