current measuring - Page 1
Forum section: electronics / beginners

author: DarkZero
author_rank: Regular Contributor
timestamp: May 22, 2016, 02:26:04 pm
content: Im trying to build ampere meter and i built one that works with a op-amp configured as a differential amplifier but as the voltage goes up on the sense resistor i found out that i would need to find op-amp with higher vcc rating.now my question is how does multi meter does this or even better how is meter that works on 5v for example doing this i cant find anything on the internet on this or i just lack terminology

author: joeqsmith
author_rank: Super Contributor
timestamp: May 22, 2016, 02:55:29 pm
content: very low shunt resistance.

author: Kalvin
author_rank: Super Contributor
timestamp: May 22, 2016, 03:00:04 pm
content: The multimeter measures the voltage across the shunt resistor relative its own ground reference which is floating as the multimeter is connected into the circuit using only two measurement probes.

author: coppice
author_rank: Super Contributor
timestamp: May 22, 2016, 03:02:42 pm
content: If you have problems with the voltage across the shunt being too big you are making some odd choices about your shunt value. You want to keep the shunt as small as possible, to avoid wasting power in it, to avoid changing its value by toasting it, and to minimise the effect you are having on the rest of the circuitry by introducing a voltage drop.

author: DarkZero
author_rank: Regular Contributor
timestamp: May 22, 2016, 03:18:07 pm
content: im using 0.1 Ohms shunt resistor to measure 0-20v 2A PSU the problem is i have to supply op-amp with a 22v in order for it to work so if i had 0-50v psu i would need 52v supply for the op-amp and such op-ampso my question is how does those cheap amp meters do it of the 5v power supplies that they use and whats the practical circuit to do it

author: TheAmmoniacal
author_rank: Unknown
timestamp: May 22, 2016, 03:20:39 pm
content: It's very easy to calculate the best value shunt resistor to your needs, if you need 22 mOhm or 100 µOhm even..V = I*R

author: joeqsmith
author_rank: Super Contributor
timestamp: May 22, 2016, 03:27:10 pm
content: So 0.2V is is into the op-amp. Something is wrong but I guess you knew that already. Are you within the common mode range of the amplifier you are using?Quote from: DarkZero on May 22, 2016, 03:18:07 pm

author: DarkZero
author_rank: Regular Contributor
timestamp: May 22, 2016, 03:44:40 pm
content: i attached the pictures in effort to make my question more clear im trying build stand-alone meter for some monitoringfor example if i set PSU to 20V, 0.05 ohms will develop some small voltage drop which will give some result on the output of the op-amp but op-amp needs to be supplied with more then 22v to be able to do that's and that where my problem is i would like my current meter to work for example on 5v rather then always higher then what load is seeing.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 22, 2016, 03:53:03 pm
content: Those both don't look right. Where is the center for the +/- supplies? Tie your ref to that.

author: DarkZero
author_rank: Regular Contributor
timestamp: May 22, 2016, 03:55:36 pm
content: Isn't the GND of the power supply my ref ?

author: Jeroen3
author_rank: Super Contributor
timestamp: May 22, 2016, 03:58:25 pm
content: http://www.ti.com/lit/ml/slyb194a/slyb194a.pdfYou need to be aware of thecommon modevoltages in your circuit.A multimeter does not have this problem because the multimeter does not share the same ground.

author: Brutte
author_rank: Frequent Contributor
timestamp: May 22, 2016, 04:07:11 pm
content: You have applied 20V to a comparator that is Vcc = 5V powered.What about "absolute maximum ratings" chapter?

author: DarkZero
author_rank: Regular Contributor
timestamp: May 22, 2016, 04:10:28 pm
content: Quoteop-amp i used is mc1458 i dont know how picture ended up with that part number

author: Brutte
author_rank: Frequent Contributor
timestamp: May 24, 2016, 06:49:57 pm
content: Quote from: DarkZero on May 22, 2016, 04:10:28 pmOk, then you have applied 20V to anop-ampthat is Vcc = 5V powered.Quote from: TI, mc1458 maximum ratings chapter

author: rstofer
author_rank: Super Contributor
timestamp: May 24, 2016, 09:06:25 pm
content: What you need is a high side current sense amplifier and a low resistance shunt:https://www.maximintegrated.com/en/products/analog/amplifiers/MAX4172.htmlThese devices are all over the place. In addition, Maxim has an app note on how to use this device up in the 200V range.

author: danadak
author_rank: Super Contributor
timestamp: May 24, 2016, 09:32:11 pm
content: http://www.analog.com/library/analogDialogue/archives/48-02/diff_amp.htmlhttp://masteringelectronicsdesign.com/differential-amplifier-calculator-2/These might help.Regards, Dana.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: May 24, 2016, 10:03:49 pm
content: There's no reason your op-amp ground needs to be the same or 5v below your power supply ground. All the op-amp cares about is if V+ minus V- is within the required supply voltage range and the voltage at its two input pins (both sides of the shunt resistor) are within the common mode voltage range, that's it. Who cares where the ground reference for your power supply goes? Also, why do you need to use a high side shunt resistor?If you don't need a high side shunt, then put it on the low side and give your op-amp +/- 5V relative to the power supply ground.If you do need a high side shunt, then consider using an isolated +/- 5V supply for the op-amp, and tie the common to either side of the shunt resistor to center it in the +/- 5V range.