Fuse/Diode sizing for rev. polarity protection 1

[techlist]

I have a small circuit, powered by a 24VDC 1.5 amp regulated AC/DC "wall wart".

I need to protect my circuit against a user wiring it backwards. A blowable fuse is fine.

I planned to put a fuse and a diode across teh input terminal, but I'm not sure what size of each to place.

Seemed simple enougn at the start, but after reading the datasheets on the fuses, I'm not so sure. The circuit will pull a max of 1.2A, it's a uP and a step motor driver.

Can someone help me choose the correct fuse size and a diode to go with it?

Thanks, Scott

 

[techlist]

Here it is:

http://www.cincon.com/pdf/tr30r.pdf

I can't tell.

 

[jimkirk]

A typical test for this sort of thing involves shorting components that might see a failure (capacitor short, shorted rectifier, that sort of thing). Monitor supply current, smoke, fire. Is it self-extinguishing? Amount & time of smoke, things like that. One short at a time (so, as an example, two capacitors in series is a plus for safety, since a single short will not allow a DC short).

For your own edification, though not all agencies require it, is doing some partial shorts. A dead short might cause something to pop safely, while a partial short may just heat it up enough to do a long burn.

A series rectifier or proper ratings will safely protect against reversed polarity on the input.

 

[Skogsgurra]

The words "continous short circuit protection" usually means that you have a current limit and that it is equal to rated current. If it were not, continous short circuit would deliver more than rated current.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jackson73]

Two members mentioned Shottky Diode drop as 0.3 V & 0.4 V. The specs I have seen, & measured, are 0.5 V. Germanium was 0.3 V - but limited current. An old trick was using one section of a power Ge Xstr as a diode, but the Shottky is a better design parameter, if the 0.5 V is acceptable.
jackson73

 

[techlist]

Well, FWIW, I shorted the thing and it promptly blew a 1.6A fuse even though the PS isn't rated that high. The circuit isn't pulling that much so at least the PS is putting out enough to pop a fast blow fuse.

 

[ScottyUK]

jackson,

Forward volt drop is highly dependent on current density and temperature. V_f in the range 0.34V to about 0.6V is possible, with designs being optimised for different parameters. Plenty of examples at:

http://www.datasheetcatalog.com/microsemi/236/

Techlist,

Good to know things are working out for our project.


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