Adding Dump Load to Solar Control Charger

Ok, I have established that the gate drive signal to the mosfets in the controller is 11.5v. The solar chage controller is on a 24v system.

Why do you need a dump load with a PV system? Solar cells don't overspeed with no load...

It sounds like he wants to divert power to another application when the CC has charged the batteries for some other use, non specific though. Hmmm. Only 2 things come to mind, 1 is a still.

A picture of your controller's circuit diagram would help.

As already stated by respondent, dump load is not necessary with PV solar. A mechanical DC generator or an induction generator / alternator starts to race faster and generate higher destructive voltages, if load is removed and input source(hydro or steam or wind etc) continue to give input side power.

In PV solar, output voltage is limited by Voc and no damage takes place if sun keeps shining and no load is connected or battery is over charged and you stop drawing power. Dump load concept is meaningless in this context.

The reason I want to do this is just to make use of the energy for heating water rather than it just going to waste. I need to find a way to switch on a dump load when the charge mosfet is off.

Use an SPDT SSR or Mosfet and use another interphase relay recommended.

The link may help. http://www.jameco.com/Jameco/Products/ProdDS/2179471.pdf

I like the idea of a spdt ssr but can't find any on ebay. As for using a mechanical relay, it would make a loud ticking noise about 50 times per second as that is about how fast the pwm on my controller is. Plus it would not handle 30+ amps my panels can give.

Circuit current depends on the resistance of your load, in shortcut 30+ Amperes is the capacity of your panel can give, but certainly, you can have loads <30 Amps.

What's the PWM doing in your controller? I thought this is from a PV cell, out put is DC (straight signal, not pulse width modulation)

Your idea again was, after the battery was charge, you are going to use the PV cell energy with some other purposes just like heating.

I guess, what you mean to say is inverter.

Charging the battery, wont need inverter, but using it on home appliances will require inverter.

This is with $10 charge ok with you? nah just joking.

Not knowing the circuit makes it a little hard to offer any meaningful info as we don't know where the charge Mosfet fits into the circuit, but basically when a Mosfet is off, it will have a raised voltage across it. It should not be too hard to pick this voltage off to switch another circuit on, via an open collector buffer comes to mind for the purpose. Again, not knowing the circuit severely limits meaningful suggestions

Incidentally, you don't need to pull a PNP transistor base down to 0v to switch it on, you only need to make it sufficiently -ve with respect to the emitter to cause saturation.

Its difficult to be precise as your infos are misleading at best......

But if you always want to heat water when charging is finished, you only need a changeover relay, of sufficient current carrying capability (I would pick one with at least 60 amps at 24 volts) to switch between charger and resistive heating (dummy load is misleading!).

A simple switching circuit to switch the relay/contactor should be very easy to build.....I would suggest that a small micro computer/PIC or similar could be so programmed to assess the charge level on the battery and to switch over to water heating when solar energy is still available..

Right here is a poorly laid out diagram. the top mosfet is internal to the controller. The bottom one connected to the tank is what I want to add. All mosfets are n channel and I need the dump load mosfet to switch on when the battery mosfet is off. They will flip back and forward like a spdt relay. I just don't know how to have my dump mosfet on when the battery mosfet is off.

From this diagram, you don't need a mosfet on your damp load anymore.

Connect right away at the negative terminal, anyway your battery and damp load is parallel.

When battery is charged, the mosfet will saturate and automatically no current will flow through the battery but directly to your load.

Question, why not also remove the mosfet on the battery? The battery will automatically charge when discharge and supply your load circuit when sun is out.

But then at night, the battery discharges fully and I wake up to a tank of hot water and a destroyed battery.

A photodiode interphased with a dry contact will serve as your battery disconnect switch during night time, or a simple tumbler switch will do.

or best thing you would do is buy a temperature controller or a thermostat, connect the probe to your tank, once actual value is below preset, it automatically trigger on the heater once set value is achieved it will trigger off the heater, then, your battery wont be discharge by night and you have a stable hot tank all through one whole day.

Good idea but my water heater is at least 600 watts and when my panels can't give 600 watts, such as when it is cloudy, the battery will become more discharged than I would like. I want something totally automatic as I am mostly at work.

buy some more battery or buy a solar water heater by this, you need not to worry about batteries and other stuff.

A solar water heater will not fit on the shed as there is so much panels on it already that they hang over the edge. I just want to figure out a dump load setup which incorporates my existing controller.

You can do anything with enough budget. I believe all is given already, all you need to do is innovate and make it happen. --- an easy job for a cowboy

Unfortunately, that circuit does not show the control section of the charger, which is the critical part to knowing how to make this work.

A consideration - whether this is a series controller or an MPPT, both use PWM for their charge control (the series type is always in PWM mode, the MPPT normally only when in float mode), so when in this mode, your charge Mosfet is going to be rapidly switching the battery into and out of the solar charge circuit. Many brands of both types will also switch the charge off momentarily when in battery voltage sense mode. Therefore you will need to include some form of time delay to slow the connection of the alternate load to prevent it from doing the same in reverse order. A relay switcher would chatter incessantly under the above conditions.

Having said that, for the Mosfet to turn off, there has to be some circuitry upstream of the gate that is reducing the gate voltage to zero. You may be able to use that voltage change to initiate the alternate load switching, possibly via an inverting buffer (either open collector or drain) or maybe a P channel Mosfet will do the job.

A comparator/flip-flop/FET circuit will do what you want.

Something like an LM339?

Here is a revised diagram. I imagine that the 2n3906 will act like a not gate, being off when the battery mosfet is on and being on when the battery mosfet is off.

Are you sure about this? good luck with your project, fun, fun learning anyway