In Power Protection Circuits, I outlined some methods of circuit protection. In this article, I'll take a closer look at the practical aspects of using a crowbar circuit for reverse polarity protection.
The second part of this series looked at using a series diode and found this simple method lacking. The crowbar circuit also relies one the one-way conduction property of a diode. In this circuit, with correct power supply polarity, the diode is revere-biased and does not conduct.
If the power supply is connected with reversed polarity, current flows through the diode and (hopefully) the fuse blows. This eliminates the diode voltage drop of the series diode method; in normal operation, the only component in the current path is a fuse. This isn't much more complicated than the series diode approach, adding only a fuse.
The downside of thie protection is that the fuse blows if the power supply connections are reversed. The circuit is protected but there's an added hassle of having to replace a fuse if an accident occurs.
Correct sizing of the fuse in this circuit is critical. Obviously, it must be sized to handle the load of the circuit in normal operating conditions. Let's consider the ADC/bargraph circuit we talked about in Power Protection Circuits - A Closer Look at the Series Diode. Let's say we have a 40-segment bargraph with each segment drawing 15 mA. With all the segments illuminated, the current draw of the bargraph will be 40 segments x 15 mA = 600 mA. Add to this a couple LEDs in the rest of the circuit, the power for the controller and so on. Let's say the circuit could draw a maximum of 700 mA if all the LEDs are illuminated. A 1000 mA fuse might seem like a reasonable choice to allow a little headroom for in-rush current.
To power the circuit, we'll use one of my ever-popular cell phone chargers. For a buck a piece at the thrift store or PC recycler, it's hard to beat these handy supplies. I have a nice Samsung unit rated for 5 volts @ 700 mA. This is perfect, and handles the maximum load of the circuit.
When the power supply is connected with the correct polarity, the circuit will receive the regulated 5 volts without any voltage drop of the series diode. The ADC reference voltage is stable no matter how many bargraph segments are illuminated and the circuit works well.
What happens if I make a mistake and connect the power supply backwards? The fuse blows instantly and the circuit is protected, right? Sadly, no. That's not going to happen in this case! In AC-DC Power Supplies - Using Wall Warts - A Brief Look at 5 Volt Supplies, I plotted the output voltage of a power supply like this.
If the current draw is over the rated limit, the voltage falls off. This supply can't supply enough voltage to blow the fuse! The voltage will fall, probably to the VF of the diode but our circuit is still supplied with some level of reversed polarity voltage!
The power supply must have enough current output to decisively blow the fuse in the event it's connected backwards.
Once again, the simple appraoch has been shown to be lacking. In normal operation, this method has no impact on circuit operation but to ensure reliable reverse-polarity protection, the power supply must be oversized. The cost of this simple approach is a more expensive power supply and less system efficiency.
Note: There are alternatives to the fuse in this method which will automatically reset when the polarity is corrected. The limitations will be the same as for the fuse.