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As you know, with solar panels, current lags voltage by a lot.

I want to run a DC pump straight from a solar panel. Simple, easy to maintain, low cost setup (relay, SS relay, diodes, etc.)

In the past I have used a high torque, low volume, low current (12V, 1.5A) pump and it started almost as soon as the panels (12V, 9 amperes) got sunlight and had no problems in broken, cloudy weather.

I want to use a different pump (higher volume, 12V, 7 ampere, same panels) and need a simple circuit that will not energize the pump until the panels are delivering enough current.

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    \$\begingroup\$ Use a car headlight bulb and measure the voltage. When it gets to twelve volts then switch the feed from the lightbulb to your pump. When the voltage drops, the relay will change back to the lightbulb. Instead of a lightbulb you could have a resistor or other load that consumes 7A or thereabouts. \$\endgroup\$
    – Kartman
    Commented Dec 26, 2020 at 10:56
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    \$\begingroup\$ Maybe keep the low volume pump to get some flow when there's not enough insolation/current to run the high volume one. \$\endgroup\$
    – greybeard
    Commented Sep 1, 2023 at 14:39

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A 7 amp motor will have about 70A stall current (I just multiplied by 10 it's a good approximation), you're not going to easily start that while connected to a 9A solar panel only.

you're going to need some sort of battery or supercapacitor to provde the starting current.

probably 1 to 10F will be enough capacitance.

schematic

simulate this circuit – Schematic created using CircuitLab

In this crrcuit R3 acts as a load until sufficient voltage is developed across it to flow through the zener diode and activate the SCR, the relay then switches out R3 and switches in the motor, R3 will develop about 70W of heat so it'll basically be some sort of heating coil.

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  • \$\begingroup\$ Thank you Jasen. What would the zener value be? 12v? I'm a software engineer with basic hardware training. I tried something similar, but did not have the SCR, D1, and R1. The relay would chatter in many cases. I knew why, but could not figure out how to latch the relay effectively (without resorting to a battery, or manual reset, or ...). \$\endgroup\$
    – tsb
    Commented Dec 26, 2020 at 15:02
  • \$\begingroup\$ 12 V/2 Ω is a whopping 6 A. Thinking you included R3 to not trigger SCR1 when the panel's current is way too low to run the motor at 7 A, but a motor rated 7 A will draw rated current with (steady state internal +) rated torque. I expect more than 1.5 A needed, and would start with 4.7 Ω, expecting the pump to continue turning when current drops to, say, 3.5 A. (Damn - I'm thinking rotary pump without anybody saying so.) \$\endgroup\$
    – greybeard
    Commented Sep 1, 2023 at 14:37

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