FAQ – Linear Current Boosters


Frequently Asked Questions: Linear Current Boosters


Linear Current Booster
What does a linear current booster do?

At times when the sun is non optimum, a Linear Current Booster boost the current in the pump motor to produce the torque to turn the pump motor. It does this by sacrificing voltage on motor, hence the motor runs slower. This results in a motor that may be pumping slower (there is after all less solar power), but this is better than having a stalled pump pumping nothing which would happen solar direct.

The net gain varies with pump type and we at Solar Converter have heard conflicting numbers and ideas everywhere form 30 % to 100 % more water and is likely more of a function of torque requirement versus size of panel. If you put enough solar panels on to supply enough current all day (expensive), you will not see much gain. If you size the panels to match the motor torque requirement, (fewer panels = less cost), a gain of over 50 % is not unrealistic. If the panel is under sized compared to motor, the gain is 100 % as the pump will not go at all solar direct but will run with an Linear current booster.

Why does this happen?

In a motor, basically the current represents the torque, and the voltage represents the the speed of the motor. Note, a DC motor produces torques even when not rotating.

To illustrate, lets us assume it takes 3 amps of current to produce enough torque to turn the pumping mechanism and we have 3.5 amp panels – say approx 60 w panels.

In the morning when solar direct, there must be enough sunshine to produce the 3 amps of current. With a 3.5 amp panel this will happen at say 11.00a.m. in the morning. When the panel can produce 3 amps of current the motor will start to turn, and pumping will start at whatever voltage (speed) the panel can support the 3 amps of current (or above).

In the morning with an Linear current booster, the solar panel voltage is not tied to the motor. Nothing happens until the panel voltage hits approx 15 V at no current. Then current is available. The motor current starts to rise. BUT the motor is stalled meaning zero motor volt. However, there is winding and wire loss, say 1 volts worth at 3 amps of currents.

As the current in the motor rises to 3 amps and pumping starts. The question is HOW much current is required of the solar panel to support this current in the motor.

If the output voltage is 1 V and the input is at 15 V, the output is 1/15th the input in voltage. For Power in = Power out (neglecting the approx 4 % loss), the output current must be 15X the input, or conversely THE PANEL CURRENT MUST BE 1/15th the output. If it takes 3 amps to run the motor at 1 V it takes only 3/15 = 0.2 amps of panel current at the panel 15 V. At what time of day is 0.2 amps available from a 3.5 amp panel?? — likely about 8:00 a.m. in the morning.

The motor illustrated starts pumping at 8:00 am instead of 11:00 am.

Conversely, the solar direct will stop pumping when the panel cannot produce 3 amps, likely about 4:00 pm and theboosted motor will stop when the panel cannot produce the .2 amps, likely say 8:00 pm.

During Optimum sun there is little or no difference in pump performance.

Clearly the motor is pumping alot longer during the day equates to alot more water pumped! In this example the pump solar direct starts at 11:00 am and stops at

What size of linear current boosters do we make?

At the 12 and 24 V level 3, 7, 10, and 15, 30 and 40 amps
At 36 and 48 V 10, 20 and 40 amps
At 90 volts 6, 15 and 30 amps

Is a motor fuse required?

Many of our current boosters have an internal motor fuse to protect the unit from motor jam or output shorts. It may be beneficial if the motor is a lot smaller than the unit in rating to add an external fuse to protect the motor itself.

Can I add more solar panels to get more water when the day is cloudy?


Historically, linear current boosters work well in low sunlight, but many times more water is needed so extra solar panels do a good job of increasing water flow when the sun is weak. BUT next day when the sun is good the motor is drastically overpowered and burns out.

All of our linear current boosters feature voltage limited output. This means that no matter how many solar panels you have on your system, the motor voltage will not go above the voltage limit on it. The Linear current booster does an excellent job of maintaining flow in poor sunlight but to get good water flow all the time, extra panels can be added to get good flow even in poor sunlight without burning out the motor in good sunlight.