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Questions relating to designing PCBs
By smdFan
#50544
Hi there,

Another question:

Suppose a user by mistake put a battery in reverse in the battery holder of your circuit. Can your circuit be damaged? If yes, how to protect your circuit against such mishaps.



Regards
User avatar
By leon_heller
#50548
Use a diode.

Leon
By smdFan
#50549
leon_heller wrote:Use a diode.

Leon

Leon,

Thanks. I understand what you mean. But do you know of an image of a circuit depicting diode's use for the same purpose? Thanks

Regards
User avatar
By leon_heller
#50550
+ve ---->|-----


-ve ------------


Leon
User avatar
By bigglez
#50560
Greetings smdFan,
smdFan wrote:Suppose a user by mistake put a battery in reverse in the battery holder of your circuit. Can your circuit be damaged? If yes, how to protect your circuit against such mishaps.
A power diode is series with the positive lead will block
accidental reverse connections. However, a silicon diode
drops a half volt or so, and wastes power.

A power diode in parallel with the supply (cathode of
diode to positive supply) will "short out" the supply
and protect the circuit from reverse polarity. No power
is wasted or voltage dropped for correct connections.

A zener diode in parallel with the supply will protect
the circuit from both reverse polarity connection and
over-voltage. The zener voltage should be slightly higher
than the worst case normal supply voltage.

If the circuit draws a high current, or the suppply is
capable of high currents, a fuse should be included
before the diode.

If the circuit is isolated from the ground of the supply
a common bridge rectifier in series with the supply will
work correctly for either polarity connection, but there
will be two diode voltage drops (allow 1.5V).

Comments Welcome!
User avatar
By Chupa
#50573
using a FET is another way you can do it. I used a FET in combination with a zener here.

Image
By Andrew02E
#50574
A zener diode in parallel with the supply will protect
the circuit from both reverse polarity connection and
over-voltage. The zener voltage should be slightly higher
than the worst case normal supply voltage.

If the circuit draws a high current, or the suppply is
capable of high currents, a fuse should be included
before the diode.
IMHO that setup is a great balance of simplicity and effectiveness.

Image
User avatar
By leon_heller
#50576
A resettable fuse could be used.

Leon
By emf
#50580
This app note from Maxim has a few other variations.
By riden
#50582
Andrew02E wrote:
A zener diode in parallel with the supply will protect
the circuit from both reverse polarity connection and
over-voltage. The zener voltage should be slightly higher
than the worst case normal supply voltage.

If the circuit draws a high current, or the suppply is
capable of high currents, a fuse should be included
before the diode.
IMHO that setup is a great balance of simplicity and effectiveness.
I don't disagree, but know that on an over voltage condition, you will almost certainly need to replace the zener since it will be toasted. Leon's suggestion of a resettable fuse is a good one as well, especially given the current requirements for the examples (i.e., okay for lower current, not quite so good for high currents).
By smdFan
#50586
Thanks a lot folks. Your responses were insightful and led me in the proper direction.


I think for the simple design I am doing, I will settle for a Schottky diode in series (see below). The forward voltage drop of Schottky diodes are small and they are cheap. Thanks for your help.


My battery is 3V

The load voltage requirement range is: 1.8V- 3.6V


Bat ---->|-----


GND ------------


Regards


smdFan
By smdFan
#50592
smdFan wrote:Thanks a lot folks. Your responses were insightful and led me in the proper direction.


I think for the simple design I am doing, I will settle for a Schottky diode in series (see below). The forward voltage drop of Schottky diodes are small and they are cheap. Thanks for your help.


My battery is 3V

The load voltage requirement range is: 1.8V- 3.6V


Bat ---->|-----


GND ------------


Regards


smdFan

Also another issue about it.

The battery I have is 3V. But the operating range of the device, as you can see above, is in the range of 1.8v to 3.6v. The supply voltage falls in between 1.8v and 3.6v . Would this be a problem? Thanks

Regards
By riden
#50594
You'll be dropping about 0.2v across the Schottky so your battery limit is around 2v. Many batteries' output drop off rapidly near the end of the useful life, so I'm not sure that the extra 0.2v will give you much more time.
By smdFan
#50596
riden wrote:You'll be dropping about 0.2v across the Schottky so your battery limit is around 2v. Many batteries' output drop off rapidly near the end of the useful life, so I'm not sure that the extra 0.2v will give you much more time.
I thought it would be like this:


3.0-0.2=2.8v


some Schottky diodes have even lower Vf like 0.1v

How long of a battery life I might get?



.
By emf
#50597
smdFan wrote:How long of a battery life I might get?
It should last quite a long time if you put the batteries in backwards :-)
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