amp switching circuit

[wormslapper]

Could someone please help me with a circuit that will switch on a relay when it detects 4amps and then switch of the relay when the current drops below 4amps. This is for a 12volt system.I need to be able to remove the load from the charging circuit when the panells produce less amps than the load is drawing so as not to drain the battery supply to low.

 

[VEBill]

If you're not going to use the battaries, you might as well remove them from the system. ;-) A better approach would be to disconnect the load when the battery voltage drops below the safe lower voltage limit. As a bonus, it's a slightly easier circuit than sensing current.

(A Voltage Sensing Relay for Solar-power system - I expect that it is probably available commercially.)

 

[cbarn24050]

You need a current operated relay, you can convert a normal relay quite easily.

 

[OperaHouse]

Actually, sensing current might be easier. Choose a rersistor that will produce about .7V at 4A. Feed that voltage through a 100 ohm resistor to the base of a transistor. The transistor will drive a 12V relay. Selecting a resistor that will give you about 1V is probably easier and using an adjustable 100 ohm pot across it will give you a little adjustability.

 

[logbook]

I am actually doing a very similar design at the moment.

You have a continuous source of charging current and a load. VE1BLL has pointed out that if the load is always less than the panels supply, the battery has very little purpose.

The postings suggesting that sensing current is easy are perhaps misleading. 4A x 1V = 4W. That is quite a waste of power. If you use a voltage comparator and a 100mV drop you will get 10x less power wasted.

The problem with sensing the battery voltage is simply that the battery voltage when on charge is at best a poor indication of charge state. If the battery trip shuts off the load at say 10.9V then at what point should it switch the load back on? In my scheme I just switch the load off and let a user switch it back on at their own convenience, many hours later. Otherwise an automatic re-start would cycle horribly even with a volt of hysteresis on the switching level.

 

[itsmoked]

logbook makes good points. You would want something more sophisticated to do the switching or the switching should not be automatic or you will get nasty galloping.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[VEBill]

If the batteries happened to be already charged (likely, since you're not really using them...), and you've disconnected the load with the relay, then how are you ever again going to get the required 4 amp current? Your solar panels are connected to an open circuit relay...

Also, you need to watch out for the power consumption of your entire circuit. No point if it takes half the total average output (including nights) from the solar cells...

This (solar cells plus batteries) is one of those systems that is several times more complicated than you might initially think.

 

[OperaHouse]

4W isn't so bad to waste since it is probably wasted any way! The solar panels are usually 19-21V (for a 12V system) to compensate for the usual losses and occasional leaf. Unless you use a max power point controller, that extra power is just wasted.

The real problem is where do you get the 4A before you have the load connected? You would have to have a dummy resistance load to simulate the current and then switch to the actual load. That leaves little ability to charge batteries. This is a simple problem that may require a more complex solution. Having the load come on when the battery reaches 14V and shut off when it drops to 13V with appropriate time delays may be a solution.