New Prototype Testing

September 20, 2013 in 8 Channel PDU by Joshua Oster-Morris

I’ve been testing the latest prototype I built last weekend. Specifically, I’ve been testing its ability to clamp extremely high capacity short circuits. This is important because the PDU is a smart power controller as well as a circuit breaker so it needs to be able to handle short circuits.

The PDU is designed to supply up to 100A and that amount of current will generate a lot of excess heat unless the output impedance is EXTREMELY low. Not wanting the Motobrain to run hot, I’ve designed the PDU’s output impedance to be ≈ 5mΩ per channel. At 5mΩ impedance, each channel can source 2800A @ 14V when dead shorted! That is an absurdly large amount of current and more than enough to destroy the PDU unless the circuit is shut down quickly (just a few of microseconds). On-board, the PDU accomplishes this through analog feedback circuitry that links the switched state of the output transistor to the current flowing through the transistor. We compare the amount of current flowing through the transistor to a known value and if exceeded we shut off the transistor to protect it.

I wish it were that easy, but of course there are complications. For example, a lamp will draw much more than the maximum allowed steady state current of 15A for a short period of time while it warms up. After warm up, the draw settles down to its normal current level. If we just shut down any circuit that went over the 15A limit we would have a tool that was not very useful to us. Therefore, I take a ‘wait-and-see’ approach with the current feedback system and wait the amount of time you would wait for a fuse to blow in a standard fused system. This requires some sophisticated electronics to do, but the outcome is an intuitive system that responds like a standard automotive circuit but provides the durability and simplicity of a solid-state circuit breaker.

Below is an oscilloscope screenshot showing a load at the moment it is short circuited. The yellow trace is a representation of the current flowing through the transistor. The green trace is a signal turning the transistor on and off. You can see in the second image that the transistor was “fully enhanced” (switched on) when I shorted it. The PDU immediately begins to shut the transistor off and then begins to pulse it on and off for half a second to confirm the circuit is truly shorted before giving up and shutting it off. I shorted the circuit with a piece of large copper wire.

If a short occurs while you’re using your PDU, the circuit will shut down and you’ll be notified of the tripped circuit. You can reset the circuit using the app or by cycling the vehicle’s power. Simple to do, but there is lots of stuff under the PDU’s hood to make it so.

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