Protection Against Reverse Hookup

Eagle Eye

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Got to wondering what would happen if I were to accidently hook up my battery leads backwards. (did this once on a MS20 and it never worked again)
I'm using JoeD's chip for my timer and would hate to fry it.
My question is this.....Can I install a diode on positive lead from my battery and protect the circuit ?
And if so what diode should I use that would affect my voltage the least.
I'm using the Versa-Pak battery which is rated 3.6v but shows 4.5v fully charged. My fear is the diode might drop the voltage down enough to keep the 5v relay from triggering.
 



Passthru

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 Eagle eye,
 Think of a diode as a one way valve,All it does is allow voltage to flow in one direction,but not back in the other direction.If you hook up your battery backwards im just pretty sure you will fry something and a Diode will not save you from this happening.
 
 

shrtcirkt

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Eagle Eye,
Passthru is right, adding one diode to the input side of the timer is not going to help. but adding diodes to pin 1 & 8 of JoeD's timer might do the trick.
Tinhorn will probably give  a good idea onhow to protect this circuit.
I use Joes timer as well and have hooked one up reverse polarity to see how it would handle it.
Short term the chip got hot as He**, but it still functioned afterwards. But I do think it would fry if left that way for to long.
Also, I have found that running 4.5 volts is margional depending on what else is being powered by the battery. Joe recommends 4.5 volts, but I stepped it up to 6.0. If you do this it is at your own risk though.
HTH
 

Archilochus

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Hi guys,
Some diodes can't tolerate reverse voltages over 5 or 6 volts.  Others, like the general purpose 1N400x series can handle 70 volts or more.  These make fine reverse batt protection diodes.  But they will 'drop' about 0.6 volts under load.  A Schottky diode, such as the BATxx series, offers lower 'drop' of about 0.2 volts.

There is another way, which I use on my cams, that gives protection for reverse batts as well as short circuit fault protection. There is no 'drop' with this method.
I'll try to illustrate with an ASCII diagram:

Batt(+)--FUSE---*---------------*
                      |                   |
                  Diode           Protected
                      |                Circuit
                      |                   |
Batt(-)-----------* --------------*          

The Diode is hooked up so that it 'blocks' when batts are installed properly. Hook the banded end to 'Batt(+)'  *after* fuse.
If batts are installed reversed, the diode conducts fully and the fuse blows out.  Use a 1N400x diode and a 100 to 250mA fuse (1/10 to 1/4Amp)

This is a simple, cheap solution.  There are other really nice reversed batt *correction* circuits, but the parts can't be found at Radio Shack.

Good Luck!,
Archilochus
 

Eagle Eye

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Thanks Arch this is just what I was looking for. I'm running both my timer circuit and sensor off a single battery and didnt want to take a chance on frying one or the other, but I didnt want to lose any battery voltage either.
shrtcirkt, so far I've had good luck with this Versa-Pak battery at 4.5v. Its been in the woods about a month now. I'm scared to up the voltage, dont know if the X10 sensor could stand it, since it suppose to run off two AAA's.
I figure the relay draws the most current in my setup so it will be the first to quit functioning when the battery starts to get weak. I hope this will keep from burning up a roll of film which has happened on a different setup of mine where the sensor drew the most current.
 

shrtcirkt

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Arch, thats a great circuit, thanks!


Eagle, I have been using the PIR from Amazon and it seems to work better with the 6 volts. Either way the diode/fuse should work great. thanks again Arch!
 

Archilochus

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Hi wfontjr,
Using a "thermal" self-resetting fuse alone probably would not work.  By the time it heated up enough to go into its high resistance state, I'd guess the damage to your circuit would already be done.  Also note that these parts can only be overloaded a limited number of times - check manufacturers data sheets for specifics on each part#.

The diode in the circuit above acts as a low resistance 'shunt' - causing the damaging reversed voltage to be shorted before it reaches the protected circuit.  Having just a fuse alone would still allow the reversed voltage to reach the protected circuit.

Archilochus
 

Tinhorn

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Archy's way is probably the best (and it's what I do)

CB radios do the exact same thing.  Occasionally a diode will short so it may have to be replaced but it "Might" ahve protected your unit (I charged $17.50 to replace those shorted diodes when I had a CB repair shop  :eat-burger:

Tinhorn
 

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