Reverse Polarity Catch 22

[ljbeng]

I have a device (Unit A) that will require DC power 10 to 24v DC in. The power must be supplied by the installer. Unit A will be connected through a serial port to another device (Unit B). Unit B has it's own power source. It is powered by AC lines and creates it's own internal DC power.<p>I want to reverse polarity protect Unit A. If I put a diode across the DC source with the banded end towards the +DC. Then I can put a fuse in line with the +DC. Now if the installer connects the DC power backwards to Unit A, the diode will conduct and the fuse will blow. <p>The problem lies in the fact that there is still a path to ground through the serial connection to Unit B. And since the power may be connected backwards, the +12v side will now be connected directly to ground through the serial connection.<p>Should I fuse both the +DC and Ground wires going to Unit A? Should I fuse the ground link of the serial connection to Unit B? <p>Any other ideas?

[toejam]

hook a bridge rectifier up plus to plus minus to minus and allow for doide drop voltage and it will help.

[bodgy]

Or if the two diode voltage drop is too much for your project, then a N channel MOSFET wired with Gate to +V, Source to circuit to circuit 0v and Drain to 0v supply, will do the job.<p>Choose a MOSFET with a very low Rds(on) and the voltage drop will be in m or uV.<p>Connect the circuit up the wrong way the MOSFET will not turn on and the intrinsic diode will block supply.<p>Colin<p>[ October 09, 2003: Message edited by: bodgy ]</p>

[dyarker]

If Unit A's power supply is floating (circuit ground not connected to earth), then a single diode in SERIES in A's power in is enough. Almost no current flows, so everything is protected. (Think bird sitting on power line. His feet might be at 10000 Volts; but so is his head as long as he doesn't touch another wire or the tower, no current, no problem.)
Take toejam's suggestion of full wave bridge, then it doesn't matter how the user connects A's power.<p>If Unit A and B have circuit ground connected to earth ground, then put opto-isolators in the serial lines. This also eliminates ground loop currents. (Is this a HF, VHF, UHF or microwave radio project?)<p>Cheers,