Author Topic: variable gain current amp  (Read 4260 times)

0 Members and 1 Guest are viewing this topic.

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: variable gain current amp
« Reply #25 on: May 15, 2020, 07:12:35 pm »
Hi,

I think that double measuring and shorting shunts is the way to get really high dynamic range.

Dave did a review of device designed to this in EEVBlog 1304. Look at 31:00 mins plus. There are multiple shunts and a pair of shorting MOSFETs.

Quitech Otti USB power measurement and battery analyser: https://www.qoitech.com/

The question is how good do you need to be?

Regards,
Jay_Diddy_B




they seem to have used some opamps that look quite neet https://www.analog.com/media/en/technical-documentation/data-sheets/14901afd.pdf but actually have 100µV offset to the 3µV of the LTC6102
 

Online Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 2766
  • Country: ca
Re: variable gain current amp
« Reply #26 on: May 15, 2020, 09:41:26 pm »
Hi,

You live with a large offset if you have a large signal. I think that there are three shunts in the Qoitech device:



One is 1 \$\Omega\$
Another is 22 \$\Omega\$
I suspect that the third one is a low value.

Regards,
Jay_Diddy_B
 

Online Vovk_Z

  • Super Contributor
  • ***
  • Posts: 1478
  • Country: ua
Re: variable gain current amp
« Reply #27 on: May 16, 2020, 12:43:34 am »
it will go to a panel meter. I can get meters that measure 200mV, 2V, 20V. I don't need it to be highly accurate.
- but there are ready-made manufactured 5 Amps 5 digit ammeters?
Look here.
« Last Edit: May 16, 2020, 12:08:50 pm by Vovk_Z »
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: variable gain current amp
« Reply #28 on: May 16, 2020, 08:35:46 am »
You want me to trust something off ebay? I know I said I did not need it highly accurate but I did say it needs to be repeatable. What if those things drift?
 

Online Vovk_Z

  • Super Contributor
  • ***
  • Posts: 1478
  • Country: ua
Re: variable gain current amp
« Reply #29 on: May 16, 2020, 09:25:07 am »
It is more repeatable than you can make at home.
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15157
  • Country: de
Re: variable gain current amp
« Reply #30 on: May 16, 2020, 09:26:04 am »
These cheap high resolution panel meters usually use an MCP3421 ADC and µC. So there are not that many parts to drift: mainly the ADC internal reference and the shunt. Both would be relatively small changes in the scale factor, but not offsets. So not a big deal for moderate accuracy over a large dynamic range. There should be low offset drift, which would be the critical factor.

However the ready made panel meters may need an isolated independent supply.
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: variable gain current amp
« Reply #31 on: May 16, 2020, 09:34:35 am »
It is more repeatable than you can make at home.

i am using murata panel meters that cost £25 each. That is why I am trying to come up with the simplest circuitry possible to interface them to a switched shunt. At 200.0mV readout they have done most of the work for me. Speaking to another forum member on the phone last night we figured that i may have been over complicating it for myself as i was not switching the inputs i was trying to just switch the shunt.

If i switch the shunts and switch the inputs to the correct shunt then I have solved everything. I could use mosfets for the shunts and a relay for the meter inputs or an analog switch of a DPDT relay would do it all.
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: variable gain current amp
« Reply #32 on: May 18, 2020, 07:24:51 am »
Anything stopping me using mosfets to switch my panel meter input locations?
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15157
  • Country: de
Re: variable gain current amp
« Reply #33 on: May 18, 2020, 08:41:58 am »
Anything stopping me using mosfets to switch my panel meter input locations?
The supply / common mode range for panel meter could be a factor.

One also needs a voltage (e.g. more positive for an N channel MOSFET) for the gate. A ready made CMOS switch chip may be easier in some cases.
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: variable gain current amp
« Reply #34 on: May 18, 2020, 08:50:16 am »
No problem there. The sense resistors are on the low side. They are connected to ground via mosfets than switch them on and off. The sense connections go through mosfets practically to ground
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15157
  • Country: de
Re: variable gain current amp
« Reply #35 on: May 18, 2020, 09:12:10 am »
The circuit as shown should work. The FET for switching in the higher value shunt could be a smaller type. For the FET used to switch the small shunt one could consider to swap source and drain. This way the drain to source diode would provide protection for the 1 Ohms shunt in case the current gets too large for positive currents. As shown the protection would be for negative currents.

Alternatively one can get away with 2 FETs for the current (+ 1 inverter): both shunts in series and always measure the voltage over both combined.  1.01 Ohms instead of 1 Ohms may not be a problem and could be corrected with some 100 Ohms parallel to the 1 Ohms if needed.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf