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nate-costello asked

What is the Smart Solar MPPT 150/45 Remote On-Off Input Impedance?

I would like to use an opto-isolator to turn off and on MPPT charging (this is fed by a charge enable signal from my BMS). The opto-isolator I'm using is an 817. The maximum collector current on the 817 is 50 mA.

The MPPT Manual states:

"3.5 Remote on-off

The left terminal is connected to the internal 3,3V supply, with a resistor in series for short circuit protection. The right terminal (marked as + or marked as H) will switch the controller on if >3V is applied, and will switch the controller off if <2V is applied or if the terminal is left free floating. The recommended use of the remote on-off is:

a. A switch wired between the left and right terminal

b. A switch wired between battery plus and the right terminal.

c) A switch between the right terminal and the charge disconnect terminal of a VE.Bus BMS"

I would like to know what the approximate value of the resistor is to ensure that when my opto-isolater is on, the collector current doesn't exceed 50mA. Can anyone comment on this value?

MPPT Controllerssmart solar set-up help
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kevgermany avatar image kevgermany ♦♦ commented ·
If you don't get an answer, use the opto isolator to switch an SSR wired across the terminals.
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nate-costello answered ·

TLDR: With the 'H' terminal shorted to the 'L' with an ammeter, the current flowing into the 'H' terminal is 77.3 uA. Input impedance into the 'H' terminal is somewhere between ~18KOhm and ~270kOhm.

Here's the method in case anyone wants to check themselves:

  1. Temporarily disconnect the left and right terminals from each other.
  2. With a voltmeter measure voltage between the 'L' terminal and 'VBatt-' to determine 'L' terminal output voltage [measured +4.85V]
  3. With an ammeter measure current between the 'L' terminal and 'VBatt-'. [measured 0.1mA / 106.3 microA]
  4. Based on 2. and 3., determine in line resistance on the 'L' terminal. [R = V/I = 4.85V / 0.0001063A = 45.626KOhm]
  5. With a voltmeter measure voltage between the 'H' terminal and 'VBatt-' to determine 'H' terminal voltage [measured -3 milliV]
  6. With a voltmeter measure voltage between 'VBatt+' 'VBatt-' (at the MPPT inputs) [measured +26.1V]
  7. With an ammeter measure current into the 'H' terminal when 'VBatt+' is applied to it [measured 0.1 milliA / 96.0 microA]
  8. Based on 5. and 6., determine input resistance of the 'H' terminal. [R = V/I = 26.1V / 0.0000960A = 271.875KOhm]
  9. With an ammeter measure current between the 'H' and 'L' terminal [measured 0.1 milliA / 77.3 microA]
  10. Based on 2., 5., and 9., determine resistance in the current flow path from 'L' to 'H' [R = V/I = 4.85V / 0.0000773A = 62.743KOhm]

Note, this R doesn't equal the sum of the other two calculated resistances like I would have expected. However, it does show that current will not exceed max collector current when an opto-isolator is fully conducting.

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sharpener answered ·

I would be very surprised if a logic input takes more than 50mA, why not connect a multimeter and see?

N.b. you only have 0.3V margin between the 3.3V available on the left terminal and the 3.0V HI threshold voltage so you may need to drive it quite hard or alternatively connect your optoisolator as b) above for guaranteed operation as here.

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nate-costello avatar image nate-costello commented ·

Unfortunately, I'm on the road and only have an inaccurate clamp on ammeter.

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