Reverse Voltage protection

westaust55

Moderator
For the simple diode approach using a 6 Amp schottky for example will have a Vf value around 0.5 volts are 3 amps.

Some lower current/signal Schottky diodes have Vf as low as around 0.35 V.

That will provide some reduction in power/heat dissipation while staying with a single component scheme.
 

erco

Senior Member
Nice video, but who's this drunk guy going around putting batteries in backwards everywhere?
 

Buzby

Senior Member
Eric,

Now don't knock those 'wrong way' battery boys.

We've both been here long enough to remember putting two 'AA's in upside down, and then cursing the TV.

Cheers,

Buzby
 

AllyCat

Senior Member
Hi,

Note that the FET is itself wired in "Upside Down" (Drain to "Supply", Source to "Load"). Otherwise the substrate diode (Source to Drain) gives a current path when the battery is reversed.

If you're only concerned with protecting against reversed connections to the battery terminals, then you could use an N-Channel FET in the negative battery lead. For lowish currents, something like a 2N7000/BS170 could be good enough (but not tested).

Or for lower currents/voltages (like a PICaxe supply) you might even use a bipolar transistor (e.g. BC548, BC337 or PNP equivalents, etc.) with "normal" polarity (because there is no internal substrate diode to the collector). A base resistor (to the opposite battery terminal) is needed of course, and the reverse base-emitter breakdown is usually (specified at) only 5 volts. So above this voltage, also connect a small-signal diode in series with the base.

Cheers, Alan.
 
Last edited:

Buzby

Senior Member
Regarding all these FETs, diodes, transistors, etc, why not just use a bridge ?.

Then it doesn't matter which way the battery is connected, it will still work.

Cheers,

Buzby
 

AllyCat

Senior Member
Hi,

Well, a bridge always has two diode forward drops, so approaching 1 volt even with four Schottkys (not particularly cheap). Rather a lot for a 2 x AA{A} or LiFePO4 (3.2v) supply for an 08M2. :(

Of course the majority of (AA type) battery holders use simple "mechanical" protection so that the negative "case" cannot touch the recessed positive contact. But I'm always worried that I might accidentally just touch the "PP3 style" battery connector the wrong way around.

Cheers, Alan.
 

AllyCat

Senior Member
Hi,

The LTC4365 is an interesting chip, but it appears to cost almost as much as the PICaxe that it might be protecting, and it uses two external FETs . ;)

Those two FETs are connected in series such that one is operating with "normal" polarity (I am carefully avoiding terms such as "wrong" or "incorrect") and the other FET is conducting current in the reverese direction, through its substrate diode. Thus it would appear to introduce a voltage drop of a normal (non-Schottky?) silicon diode and as far as the title of this thread is concerned, it is not doing anything different to a regular power diode (forward voltage drop included)?

However, the main purpose of this post is to ask if anybody "knows" the real significance of using a single FET in its "reverse" direction, as shown in the link from the OP? I could say a lot about using bipolar transistors in their reverse direction (NPN reversed is still NPN) but I won't here; suffice to say that they behave quite differently. However, I'm no expert on the applicaton of FETs, having hardly ever used them in practice.

There are of course published circuits (that do work) with FETs used (partially) in their reverse direction, such as the 3.3 - 5 volt bidirectional I2C bus level-shifters. But to what extent are power FETs usable in their reverse direction for "Power" control functions?

Cheers, Alan.
 

Buzby

Senior Member
Regarding the use of a bridge, a few years ago I came across a device that used 4 FETs arranged as a bridge, with the gates triggered by a microcontroller.
The supply into the bridge was a very 'dirty' AC current, and the intention was to regulate the DC power out of the bridge by phasing the FETs.
(One interesting aspect was that for the parts of the AC cycle where the device needed no more power, the FETs were switched to 'short' the input terminals !.)

Now I'm not an analogue expert, but could 4 low-rds FETs be hardwired to make a universal AC/DC input circuit ?.
 

Haku

Senior Member
Came across this, and although we all (I assume) try and avoid reverse voltage, thought it maybe handy for someone here

https://www.youtube.com/watch?v=IrB-FPcv1Dc
Perfect timing! Thanks for posting that, I'm midway through building a circuit to trigger the shutter of up to 16 DSLR cameras at once through optocouplers, with the idea of building more identical boards that will link up and trigger up to 160 cameras at once. I was just going to use a simple diode as reverse polarity protection but this is much better as I'll be adding in an input voltage LED display.

Just tested it with an AO4409 p-channel mosfet on my original 2-camera shutter trigger test board and the reverse protection works perfectly.
 

oracacle

Senior Member
No worries, glad it helped at least one person.

Also erco, pretty sure I should be drunk more often as it makes life, well my suffering of the less intellectually inclined (read dumb donkys) easier to stand.
 
Top