I'm building a load bank for testing high power lead acid batteries, and expect to pass up to 800 amps at 12 VDC. I could do it with contactors but would like to use an SCR with a pot and allow the current to be ramped up with the pot. Any thoughts?
I could do it with contactors but would like to use an SCR with a pot and allow the current to be ramped up with the pot. Any thoughts?
It's a foolish idea for at least two reasons: -
- An SCR will latch once conducting and stay latched hence to disconnect the "load bank" from the battery you will need a contactor. Of course it makes sense to just use the contactor for full functionality.
- Current won't ramp up with either a contactor or an SCR with or without a pot. If you require a variable load impedance then use something else that can be controlled on/off in a PWM manner. I'm thinking banks of MOSFETs and an inductor here.
There will be kilowatts of heat to get rid of so design your load bank rig carefully.
SCRs are not appropriate switches for DC because they are difficult to turn off. There are commutation circuits that you could build, but it would be a lot easier to use an IGBT or MOSFET.
To control current, you would need to use pulse width modulation (PWM). With PWM, the current drawn from the batteries would be pulses, not steady DC. You would need to determine if that is appropriate for testing the batteries. The current could be smoothed with filtering, but that adds to the equipment required.
The use of the plural "contactors" implies that you are thinking of a bank of small resistors rather than one large resistor. You could easily use a potentiometer to sequence the connection and disconnection of resistors. You can use contactors, solid state relays, MOSFETs or IGBTs.
If you are designing something for industrial use, you should consider a system that would return power to the AC power system. That is much more complex, but it would reduce operating cost and save money in a situation where the testing is going on for an extended period of time.