Hi,

I'm looking for a simple solution to best protect my 12 volt "Power Pulse Modulator' circuit from being inadvertently connected to a reverse polarity 12V battery supply. I did this recently to a circuit and as a result fried a capacitor.

I read recently that using a MOSFET N-channel ( Si4838DY ) on the ( V- ) side would wok fine, but would really like to be assured and more so to know a little more about how it would work?

The maximum current drawn from the 12 volt car battery is measured at 13 Amps per pulse / second on a multi-meter.

Any help would be appreciate

 

Google images: mosfet reverse polarity protection

Ken

 

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Hi,

I'm looking for a simple solution to best protect my 12 volt "Power Pulse Modulator' circuit from being inadvertently connected to a reverse polarity 12V battery supply. I did this recently to a circuit and as a result fried a capacitor.

I read recently that using a MOSFET N-channel ( Si4838DY ) on the ( V- ) side would wok fine, but would really like to be assured and more so to know a little more about how it would work?

The maximum current drawn from the 12 volt car battery is measured at 13 Amps per pulse / second on a multi-meter.

Any help would be appreciate

Check out this excellent video dealing with what you'd requested:

 

Why not use a cheap 10A diode, which could certainly withstand pulse currents well above its continuous rating:-

https://www.ebay.co.uk/itm/5-x-10A1...=item3ac64098d8:g:1zAAAOSwkRhZVisa:rk:10:pf:0

 

Why not use a cheap 10A diode, which could certainly withstand pulse currents well above its continuous rating:-

https://www.ebay.co.uk/itm/5-x-10A1...=item3ac64098d8:g:1zAAAOSwkRhZVisa:rk:10:pf:0

Voltage drop is something I'd consider before choosing a diode for this sort of application.

 

Why not use a cheap 10A diode, which could certainly withstand pulse currents well above its continuous rating:-

Depending upon the duty-cycle of the pulse, the power dissipation could be problematic.

Also the diode forward drop will reduce the battery lifetime (if that's a concern).

 

Many thanks for all for your guidance.

I've chosen the Mosfet and Zener diode option in the youtube attachment, with thanks to CMartinez.

 

Now I am really wondering about "my 12 volt "Power Pulse Modulator' circuit" that draws 13 amps off of a 12 volt car battery. Certainly a series diode drop would greatly reduce the power, so your choice is a good one, although you may well need a heat sink on the transistor.
So could you please explain what your circuit is and what it does, since that sounds quite interesting. And I am always wanting to learn stuff.

 

I've chosen the Mosfet and Zener diode option in the youtube attachment

For 12V operation you don't need the Zener.

although you may well need a heat sink on the transistor.

If you use a MOSFET with an ON resistance low enough that the average I²Ron power dissipated is no more than about a watt you won't need a heatsink.

 

Check, if this comes handy, i had previously done this

 

Check, if this comes handy, i had previously done this

That works fine as long as you don't mind the 2-diode voltage drop the bridge generates.
Using a Schottky bridge would reduce that drop.

 

That works fine as long as you don't mind the 2-diode voltage drop the bridge generates.
Using a Schottky bridge would reduce that drop.

Crutschow is exactly right. And with battery power that is often important. And there is one more consideration as well, which is that using a rectifier bridge puts the common side of the protected device a whole diode drop above the supply negative. In some situations that can be very inconvenient, and in all situations you are losing almost a whole cell's worth of power in the diode drops. The MOSFET method is far better.

 

... not to mention that Schottky diodes normally don't have high current capability.

 

... not to mention that Schottky diodes normally don't have high current capability.

High-current Schottky rectifier diodes are VERY common; Digi-Key lists thousands of 'em, some with current ratings of several hundred amps.

 

High-current Schottky rectifier diodes are VERY common; Digi-Key lists thousands of 'em, some with current ratings of several hundred amps.

THAT ... I didn't know ... thanks for sharing.

 

THAT ... I didn't know ... thanks for sharing.

Silly me ... I've just gone through the diodes I have with me, and sure enough, they're all tiny SMT devices with small current capability. That's why I had assumed all of them had that restriction.

 

Silly me ... I've just gone through the diodes I have with me, and sure enough, they're all tiny SMT devices with small current capability. That's why I had assumed all of them had that restriction.

Those are probably similar to the 1N6263 small-signal Schottkys in my goodie box. I also have 1 amp 1N5819 and MBR160 rectifiers, and 250V, 40A MBR40250 rectifiers on hand.

Like MOSFETs, there's a Schottky diode for every purpose. Digi-Key had one rated at 300A and 200V. Yikes!

 

Now I am really wondering about "my 12 volt "Power Pulse Modulator' circuit" that draws 13 amps off of a 12 volt car battery. Certainly a series diode drop would greatly reduce the power, so your choice is a good one, although you may well need a heat sink on the transistor.
So could you please explain what your circuit is and what it does, since that sounds quite interesting. And I am always wanting to learn stuff.

Hi MisterBill2
It's a "Power Pulse Modulator" that drives a step up transformer, the output from which delivers a high voltage pulse which I use to fence livestock, or commonly known as an "electronic fencer".

My most recent fencer died and as there was nothing obviously wrong or burnt I was unable to find the problem so this option was a cheaper alternative to a new fencer which can be quite expensive.

Hope this helps

 

OK, that is indeed an interesting application. In the past I have had folks bring me fence chargers to see if they could be repaired. In all but one case, out of about 7, the failure was multiple opens in the secondary of the pulse step-up transformer. Lightning is evidently a common destroyer of these devices. The other unit that failed had suffered major failure of both power supply fuses and damage to the semiconductors as well.
So, for protection against reverse supply connection, I suggest a simple series diode to protect the low powered timing and trigger circuit, since it only draws a very small current, and a driven MOSFET device to switch the pulse power. But you will need to be aware of the internal intrinsic diode in some devices. Using a bipolar transistor, if the polarity is reversed then the driver would not run with the polarity reversed and so there would be no damage, unless I have overlooked something.

If you are really ambitious you can check and see if it is "just" an open in the pulse transformer secondary, and then unwind it to where the breaks are and repair it. Evidently most of the transformers are not varnished and so can be unwound. Quite a tedious task, but not hard. It is possible to unwind the wire onto a spool and then wind it back on after the breaks are repaired.