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Automotive reverse polarity

Rajinder

Jan 30, 2016
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Hi all,
I am using 12V from a car battery to power electronics. The current consumption of my circuit is 1A max, however it will run closer to around 850mA.

I need to add reverse polarity protection to my circuit.
I have opted for a simple Schottky diode solution, as I am limited for PCB space and it has a forward voltage of around 0.5V.

A car battery can spike to negative and positive voltages due to load dump and inductive loads. The only requirement is that my surge current selection of 30A is high enough.

I am looking to select a diode with a VRRM of 60-100V, forward surge current of 30A. Average current of 2A.

Is there anything else that I need to consider or have overlooked? Do I need to consider any power dissipation? The circuitry can operate at 7V so the 0.5V drop should not be an issue.

Any help would be appreciated.
 

Harald Kapp

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Nov 17, 2011
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You will need an additional circuit to protect your circuit from the overvoltage due to load dump.
Otherwise where would the forward surge current go if not into your circuit and destroy it?
 

Rajinder

Jan 30, 2016
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I have a fuse, then TVs before the reverse polarity diode.
 
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Kiwi

Jan 28, 2013
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I have had very good success using the buck/boost, step up/down, DC-DC convertors from the likes of Aliexpress and Bangood etc.
They have a potentiometer to set the output voltage, eg 12.0v.
This voltage will be constant even when the battery voltage swings from about 10v during engine cranking up to 14-15v when the engine is running.
 

Harald Kapp

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I have a fuse, then TVs before the reverse polarity diode.
This may protect your circuit, but you'll have to replace fuses now and then. Imho not a practical solution. look up e.g. "load dump protection circuit" for more elegant solutions.
 

davenn

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I have had very good success using the buck/boost, step up/down, DC-DC convertors from the likes of Aliexpress and Bangood etc.
They have a potentiometer to set the output voltage, eg 12.0v.
This voltage will be constant even when the battery voltage swings from about 10v during engine cranking up to 14-15v when the engine is running.

great for the overvoltage protection he needs, but wont help with the reverse polarity protection he needs ;)
 

Kiwi

Jan 28, 2013
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Reverse polarity protection shouldn't be an issue in a vehicle as the battery will prevent this happening.
You only have to worry about excessive positive spikes.
 

Rajinder

Jan 30, 2016
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The fuse I am using is a PTC type, so it will reset after fault condition is removed. The TVS is for transient conditions. The reverse polarity is recommended in the DCDC buck datasheet, for example if the car battery requires changing then it is possible for leads to be connected incorrectly. I will post up a schematic later today.
I just want to ensure that the diode is not going to get damaged by high current surges, or high reverse voltages i.e. load dump surges.
 

Martaine2005

May 12, 2015
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If the battery requires changing and somebody reverses the leads, they’ll have more problems to worry about than your 1A circuit. The diode will be fine.
I am intrigued to know how you think you’ll get a load dump?. These are caused by disconnecting the battery when it’s being charged, ie - disconnected while the engine is running. So again, you’ll have much more to worry about if changing a battery with the engine running and reverse the polarity of the leads.
Hmm, where on earth would somebody do this?


Martin
 

Rajinder

Jan 30, 2016
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If the battery requires changing and somebody reverses the leads, they’ll have more problems to worry about than your 1A circuit. The diode will be fine.
I am intrigued to know how you think you’ll get a load dump?. These are caused by disconnecting the battery when it’s being charged, ie - disconnected while the engine is running. So again, you’ll have much more to worry about if changing a battery with the engine running and reverse the polarity of the leads.
Hmm, where on earth would somebody do this?


Martin
I was thinking if the battery was being charged and then the charger removed? My other slight concern was that I have the VRRM of the diode at 60V, I have read that the battery can spike negative to -150V. I do have a TVS which would clamp in the input to 39V, the input max for the DCDC is 42V, hence the selection of the TVS to clamp it at a safer voltage. Do you think my diode would be ok for that condition?
 

Martaine2005

May 12, 2015
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So this battery is not in a car? You’re using a 12V car battery to power devices away from a car?
Your diode won’t see repetitive reverse voltages. If it does, you’re doing it wrong. You can’t make a ‘fail safe’ for every eventuality if you only have PCB space for a diode!.


Martin
 

Rajinder

Jan 30, 2016
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Hi
I have attached the circuit protection.
The fuse is rated at 3.7A, MF-LSMF185/33X2 hold current of 1.85A, which drops to 0.85A at 85C.
The TVS is bidirectional SMC3K24CAHM5/57 this has a working voltage of 24V, clamping at 38.9V.
The diode is SBR2M60S1-F, has low VF.
The idea was that the TVS clamps any voltage higher or lower than 38.9V to protect my DCDC which operates at max input of 42V.

I want to test this actually is the case. Would I be able to put 60V into just this circuit and reverse it (with no DCDC) connected? Or is there a better way of testing the operation of the Fuse/TVS and diode circuitry?
 

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