# Voltage activated switch?

#### JohnnyAwesome

Joined Mar 26, 2021
3
Hey, I am going to upgrade the ventilation in my cabin that is off-grid solar powered (12V)

I am looking to upgrade from one of those fans with a built in solar panel, to a computer case fan connected to my solar cell and batteries.
The fan that i have is ineffective, noisy and unreliable. (If the built in solar panel gets a little dust on it, it wont work..)

What I would like is to have is the fan running when the solar panel is charging (in the winter that is a couple of hours per day at most where I live), so that it can't empty the battery. Basically it would work the same way as the one i currently have, just with a better fan and with more reliability.

So what i need is a controller that switches the fan on when the solar panel is charging. I would like to either build one out of electronic components, or use something "off the shelf" and make it as simple/reliable as possible. Maybe something that switches on at say 12.7V? Or is there a better way? Any suggestions on a circuit/component that would do that?

#### Yaakov

Joined Jan 27, 2019
2,381
What are you using for a solar charge controller?

#### JohnnyAwesome

Joined Mar 26, 2021
3
What are you using for a solar charge controller?
I am using a "Epever Landstar LS1024B" I have several buildings on the property and would prefer a solution that would fit any and all of them, in case i want to install the same ventilation to them. They all have different controllers, some from 20 years ago. One is an "Epeves Landstar LS0512R", and cant remember the rest off the top of my head. That being said, if you have a solution in mind that would fit most controllers i could upgrade the rest.

#### Yaakov

Joined Jan 27, 2019
2,381
I am using a "Epever Landstar LS1024B" I have several buildings on the property and would prefer a solution that would fit any and all of them, in case i want to install the same ventilation to them. They all have different controllers, some from 20 years ago. One is an "Epeves Landstar LS0512R", and cant remember the rest off the top of my head. That being said, if you have a solution in mind that would fit most controllers i could upgrade the rest.
The problem is you have to detect that the panel is charging the battery. The question is, where would you measure the voltage to know that?

a circuit to detect a change in voltage is easy, knowing there is a meaningful change may knot be.

Have you measured the voltage in the two states? Do you know where to measure? Do you have numbers about the change?

#### JohnnyAwesome

Joined Mar 26, 2021
3
The problem is you have to detect that the panel is charging the battery. The question is, where would you measure the voltage to know that?

a circuit to detect a change in voltage is easy, knowing there is a meaningful change may knot be.

Have you measured the voltage in the two states? Do you know where to measure? Do you have numbers about the change?
I don't think i have ever measured the voltage while charging, but if i remember correctly i measured around 12.5V when the batteries are full. I am an electrical engineer, so i know the theory behind this, but i have very little experience in electronics on this level.
My thought process was, my full batteries generally measure around 12.5V give or take. And when i charge my batteries for example at home or on my boat (where i have volt meters always connected) the system voltage quickly rises over 13V, if the batteries aren't very empty. Knowing this I assumed that the simplest way of building the controller, would be that it switches on when the system voltage is for example 12.8V, since my batteries should never have 12.8V unless they are being charged.

And unfortunately I can't go do any measurements in a couple of weeks until the ice melts.

#### Yaakov

Joined Jan 27, 2019
2,381
I don't think i have ever measured the voltage while charging, but if i remember correctly i measured around 12.5V when the batteries are full. I am an electrical engineer, so i know the theory behind this, but i have very little experience in electronics on this level.
My thought process was, my full batteries generally measure around 12.5V give or take. And when i charge my batteries for example at home or on my boat (where i have volt meters always connected) the system voltage quickly rises over 13V, if the batteries aren't very empty. Knowing this I assumed that the simplest way of building the controller, would be that it switches on when the system voltage is for example 12.8V, since my batteries should never have 12.8V unless they are being charged.

And unfortunately I can't go do any measurements in a couple of weeks until the ice melts.
I think the only way for you do do this will be actually measuring. You have to characterize the problem more fully to develop a sustainable solution. You might find that, say, putting a current sensor on the cell output makes more sense (if can be a hall effect sensor with no electrical connection), or you might find that there is a charging signal generally available on the controllers. But even if you use your first idea, you have to know what signals you have to work with.

The obvious thing is a comparator with an adjustable voltage reference, but that's a blind suggestion.

#### MisterBill2

Joined Jan 23, 2018
8,214
There is another variable that can allow a simpler scheme to switch on a fan or other device only when the solar cells are delivering a charging current. This scheme depends on the charge controllers not allowing current to flow back into the solar cellswhen they are not producing enough current to charge the batteries. The scheme would use a small sensitive reed switch with just a few turns of "heavy" wire around it to act as a current sensing relay. The "heavy" wire should be the same size as the conductors from the solar cells. The beauty of this scheme is that it is totally isolated from the charging circuit and it is simple, reliable, and cheap.
The number of turns of wire will depend both on the charging current and on the sensitivity of the reed switch. So a bit of experimenting will be needed.
This is not an original idea, so I can't take credit for it. But I did not see anything like it mentioned here.

Joined Jan 15, 2015
5,876
Pretty sure if a battery is charging the current flows one way and if discharging the other way. The ACS 712 (Allegro) is a current sensor which senses current flow in either direction. They come in modules for 5, 20 and 30 Amps or actually +/- 5, +/- 20 and +/- 30 amps. The Vout is VCC / 2 or with a normal 5 VDC applied with zero current the Vout is 2.5 volts.
66 to 185 mV/A output sensitivity
Output voltage proportional to AC or DC currents

Depends on range but figure if the battery is charging the Vout is above 2.5 volts and if discharging below 2.5 volts. Not knowing your limits (charge or discharge) I can/t choose a range. From there a simple comparator.

Ron

#### MisterBill2

Joined Jan 23, 2018
8,214
Pretty sure if a battery is charging the current flows one way and if discharging the other way. The ACS 712 (Allegro) is a current sensor which senses current flow in either direction. They come in modules for 5, 20 and 30 Amps or actually +/- 5, +/- 20 and +/- 30 amps. The Vout is VCC / 2 or with a normal 5 VDC applied with zero current the Vout is 2.5 volts.
66 to 185 mV/A output sensitivity
Output voltage proportional to AC or DC currents

Depends on range but figure if the battery is charging the Vout is above 2.5 volts and if discharging below 2.5 volts. Not knowing your limits (charge or discharge) I can/t choose a range. From there a simple comparator.

Ron
The reed relay system I described does not sense the direction of current flow, thus it does need to be in a lead that only carries charging current. AND it costs a whole lot less than any module, or even the module's shipping charge. In addition, ALL modules with active components use some power.

Joined Jan 15, 2015
5,876
So it cannot be in a lead with bidirectional current as in charge / discharge. The module I mentioned is about $1.50 or less on Flea Bay. Another module to use a set point is another buck and a half if that. Amazon Prime is free shipping for those who have it. All in all maybe$5 or slightly over and right into the battery main line. The thread starter can do whatever he/she wants. Merely a suggestion.

Ron

#### AnalogKid

Joined Aug 1, 2013
9,043
Late to the party; Reload might have covered this ...

A hall-effect current sensor gives a DC voltage that is directly proportional to both the current amplitude and its direction. There are clamp-on versions for systems where you cannot remove a wire and feed it through the core. LEM is a good company.

What is the peak charging current?
What is the minimum charging current you want to cause the fan to run?
Hysteresis? - Fan comes on at charging current A, turns off at charging current B

What power source is available to run the sensor system?

ak

#### Rich2

Joined Mar 3, 2014
179
My solar panel gets hot during the day. Could you control the fan with a thermostat?

#### ResFiber

Joined Mar 9, 2016
6
You might also consider some type of photodetector module that would be powered from your battery. When its light outside, the photodetector can activate a relay to turn on your fan.

#### MisterBill2

Joined Jan 23, 2018
8,214
None of the alternative methods described so far is nearly as simple a the current operated relay made from a few turns of wire and a small reed switch. Nor as inexpensive.

Joined Jan 15, 2015
5,876
My solar panel gets hot during the day. Could you control the fan with a thermostat?
I don't see why not.

Ron

#### sghioto

Joined Dec 31, 2017
2,266
Any suggestions on a circuit/component that would do that?

Adjust Vr1 to activate relay at desired voltage.

#### AnalogKid

Joined Aug 1, 2013
9,043
Re: #16

When the cell output drops below the pin 6 threshold voltage, what turns off the fan?

ak

#### MrChips

Joined Oct 2, 2009
23,073
Presumably you need to improve air circulation when things get hot.
A fan controlled by a thermostat is all that is needed as already suggested.

#### sghioto

Joined Dec 31, 2017
2,266
Re: #16
When the cell output drops below the pin 6 threshold voltage, what turns off the fan?
ak
Then pin 3 goes high and the relay drops out

#### AnalogKid

Joined Aug 1, 2013
9,043
Don't think so.

ak

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