On the right is a diagram of a possible dump load: two 12V 50W downlights (MR16) are connected in series to provide a 100W 24V load.
FETs are very sensitive to static electricity, so it is a good idea to put a large resistor (say 5 MOhm) between the two outside pins of the FET (Gate and Source), as well as a small resistor (say 50 Ohm) between the incoming signal and the Gate pin. This prevents damage to the FET when there is no signal connected, and also while connecting or disconnecting power.
Another option is to use high resistance wire instead of light bulbs. The advantage would be simpler and more compact construction. On the other hand, the bulbs are nice because they make it clear when current is being dumped. Note that I hve found that using bulbs results in the FETs sometimes blowing, probably because a cold filament has a very low resistance. High resistance wire is thee way!
I use three of these in parallel to dump up to about 420W at 28.5V. Here is what it looks like. The bulbs can give off a lot of heat, so there is a steel plate to protect the box.
Each FET has a heat sink - they can get very hot! Note the separate connections for 'voltage sense', and power. To detect the battery voltage accurately, wires are needed directly to the battery terminals. This is because the voltage on the power connectors will fall below the actual battery voltage when several amps of current are drawn.
Note that a motor is not suitable for a dump load, as the rapid switching on and off causes inductive spikes which can damage the electronics (and the motor).
It is essential to run separate wires to the battery for the 'voltage sense' connection!
|On the right is a 'combo' dump load I installed at a site. It has two 12V CFL bulbs which switch on via a relay at medium voltage with a wide hysteresis. There are also six 50W bulbs which operate as a convential dump load.|