Question regarding Low Voltage Disconnect for a Solar Charge Controller

[MikeLU]

Hi everyone,

I have a question regarding the implementation of a Low Voltage Disconnect for a solar charger I am working on.

I am having a hard time figuring out how I would implement this disconnect directly from my microcontroller. Since the disconnect is intended to remove the load from the battery when the battery voltage is sensed to be below it's low voltage threshhold, the contact must be able to withstand typical currents for the load.

I am trying to build my charger as generic as possible so it has a wide variety of uses. Since I plan on powering the low voltage disconnect directly from the 5v output of my microcontroller (40mA max current) I am limited on my options, The only affordable relays that I have found that can be powered from a 5v signal are only rated at 3A on the contact side which I fear is far too low.

I was thinking about using a mosfet or BJT for this, but cannot figure out how I would maintain the VGS voltage above threshhold if I am simply applying 5v to the Gate, and have a load being powered from 12v.

I searched this forum to no avail, I have also tried finding schematics for the charge controllers on the market and that too has left me in the dark.

Can anyone shine some light on how I would manage to power a LVD contact (relay or FET) from a 5v source with 40mA max current, that is able to withstand high load currents?

Thanks in advance,

Mike

 

[tpetar]

Why you insist to power this control circuit from 5V?


See this:

 

[MikeLU]

Thanks for the reply tpetar,

I am insisting on using 5v because it is a microcontroller controlled charger and I need go be able to power the LVD from the microcontroller output, which is only 5v.

The other issue I have is that I am using isolation to keep my microcontroller and charge circuit separate, and for that reason I can't mix my microcontroller ground with the loss ground.

In the provided picture, is q2 the load disconnect? If so, isn't it bad practice to switch the negative of a supply? The load will still have 12v supply but a floating ground?

Thanks for the reply!

-Mike