# Best op-amp for 50a 75mv shunt.

I want to measure current between my 60v solar panel to 48v battery. I have 50a 7mv shunt.
I want to connect is on low side for current sensing.

which OP amp is best to amplify shunt drop voltage.
option are
OP07
LM741
LM386
LM324

or u can check yourself on

thanks

Do you mean amps when you say "50a" and miilivolts when you say "75mv" - the symbols
for ampere and volt are "A" and "V" respectively, the case is significant.

Normally a shunt would be described by its resistance, should we assume this is a
1.5 milliohm shunt (75mV for 50A...)?

75mV is a pretty small voltage so would require a precision opamp (you'd probably
want an input-offset voltage spec of 1mV or so - however you haven't said what resolution
and accuracy of current measurement you want - will you ever have 50A flowing?

For convenience a single-rail opamp would be convenient - precision single-rail opamps
aren't that commonplace though, doubt any of those parts are both (OP07 is precision
if I remember correctly).

The OPA320 appears to me to be a suitable op amp. It can run from a single 5V supply, has rail-rail I/O, and 150uV max input offset voltage - so the current measurement error due to input offset voltage will be 100mA max, if the shunt drops 75mV @ 50A.

However, I would consider using a Hall current sensor instead, which has the added advantage of providing isolation. If 50A is the maximum output of your solar cells, then the ACS758LCB-050U-PFF-T would be a suitable choice.

I’m a bit sceptical about needing to read 50A though, a 60V solar panel is usually limited
to the 8A or so of a typical 6-inch wafer in full sun, and 120 wafers is a huge panel, more
likely 5 inch or smaller wafers or half wafers, so lower current.

Is the solar array 3KWs?
50 * 60 = 3000
That seems like a reasonable moderate size but I have no solar experience.

Isolation is a good idea when bringing power level signals into sensitive electronics even if you are working on the low side. There are many ways to do that.

The op-amps you have selected are not very good for the reasons dc42 gave: poor input offset, lack of rail to rail performance, split supplies. I agree that the OPA320 is good choice. With calibration, you should be able to get 1% or better resolution and accuracy.

http://www.ecircuitcenter.com/Circuits/instamp1/instamp1.htm

Hi look this up guys, an instrument amplifier is the best way to amplify a low level DC, the circuit is designed to counteract drift and offset.
You can also adjust offset and gain.

Tom.......

Normally a shunt would be described by its resistance, should we assume this is a
1.5 milliohm shunt (75mV for 50A...)?

Actually, ( and unreasonably as it seems, to me ), 75mV seems to be the standard industry terminology.

michinyon:

Normally a shunt would be described by its resistance, should we assume this is a
1.5 milliohm shunt (75mV for 50A...)?

Actually, ( and unreasonably as it seems, to me ), 75mV seems to be the standard industry terminology.

Ah, yes, a bit specialist, but I see the reason, people buy different shunts for the same
meter and the meter assumes a particular FSD voltage level. Still if you own a meter and
are buying a shunt you really ought to be fluent in ohm's law!

The problem is not "knowing Ohm's law". The problem is, is that 75 mV specification, the voltage drop across the shunt at full scale ( 30 Amps ), or is it 75 mV per Amp ? It could reasonably be either.

There are special devices designed for amplifying current shunts. Here is one that says is will work with high or low side shunts.

Supplier search terms are current sense amplifier, IC OPAMP CURR SENSE, or current shunt amplifer. Try the popular vendors.

Here is page at TI

here is a page at LT:

There is a wide variety. Note that many are for high side measurement. This is because many systems require a low impedance ground and cannot tolerate a current shunt in the ground path.

I've used shunts, Hall Effect devices of several different types and currrent transformers (UggH)... I currently own 12 LEM HAL 300-S that are great because the offset and gain are adjustable, Although for a higher current load, it is a bi-lateral sensing device and so measures current in both directions and is accurate to +/- 5% DC to 50 KHz.
For smaller work Ebay sells the Allegro ACS712ELCTR-30A-T +/- 30 A at 50 KHZ for not so very much money US \$3.80
and this one for \$4.00 http://www.ebay.com/itm/111085883206 (5 A version) there are 3 ranges 5, 20 and 30A
There is also the ACS756 for 50A + and can be purchased on a PCB with screw terminals.

Doc

Doc and Docedison have suggested Hall effect sensors. They do not require a shunt. The Allegro ACS756KCA-050B-PFF-T available in distibution for \$6.50 each which is about the price of the 50A shunt. The datasheets look like the parts are super simple to use. They are isolated. This is clearly a very good choice.