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reverse polarity protection diode recomedation query

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cf_corp

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Hi folks,
Ive been reading posts here for a long time but only signed up today as i think this site is very helpful.
Basically I wanted a little advice on selecting the best diode for my setup.
I have a 24v source going to a charge controller which is regulating the charge of a 6.5Ah 12vdc battery, there is also a 12v load output which will draw no more than 2-3A at 12Vdc.
My initial thought is that I need a 26-30V diode with a rating of 13-15A, any suggestions?
thanks guys
 
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Welcome, cf_corp.

First off, the diode placement you've shown, when hit with reverse polarity, will blow the diode first, then the fuse and/or the controller circuit next (if not all three simultaneously).

Might be best to put the diode in series with the fuse (either between the 24VDC source and the fuse, or the fuse and the controller, cathode on the right in the circuit [arrow towards the V+ of the controller]).
 
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Hi Cowboybob,
thanks for the quick reply, I am not sure of the total current output of the source as it is a 24v source from a truck.
So something like this?
View attachment 67740
 
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With the exception of my fuse being before the diode, does that look like it will do the job?
I just noticed my diode is before the fuse, in my diagram
 
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Don't you understand that your diode is parallel to the source? When the polarity is reversed then HUNDREDS of amps will flow through the diode which will blow it up. The wiring might catch on fire.

But if the diode is in series with your circuit then the current in the diode is low and normal when the polarity is correct. When the polarity is reversed then no current flows.
 
yes sorry, I have it in series now, my mistake. do you think that a 30v 15A diode would do the job or what rating should i be looking for?
thanks
 
yes... I have it in series now, ...

Can you stand the forward drop of your diode subtracting from the 24V source. If that is an issue and you want a series diode, you should use a Schottky diode which will have about half the forward drop of a Silicon diode, and dissipate less power

Most often, reverse polarity protection puts the diode across the power input of the appliance ( i.e. Parallel, not Series), but downstream of a fast-blow fuse.
 
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Hi,
I just saw a different charge controller than the one I was looking at, it states that it has reverse polarity protection, amongst other features. I believe the system can handle the forward voltage drop, a silicon diode has a drop of around 0.7v and a Schottky is half of that so between 0.4-0.3v?
If anyone is interested here is the unit im looking at now, it is cheaper too. **broken link removed**
 
I notice that it has "Thunder Protection", too :D
 
Yeah I see that, lol
It looks like it accepts a range of source voltages and can charge my 12v battery and provide a good 12v source for my load
 
The connection shown in the attachment here is often used when the diode drop cannot be tolerated on the supply.

JimB
 
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