Favorite reverse bias protection for battery circuits [Design]

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From: krw on 16 Mar 2010 19:48

On Tue, 16 Mar 2010 10:56:46 -0700, Charlie E. <edmondson(a)ieee.org> wrote:

>Hi All,
>Still working on my color reader, and getting close. Biggest problem
>is that now I have almost TOO much drive on my LEDs, but that I can
>deal with!
>
>Now, to the latest problem. My system is for the visually impaired. I
>just put the batteries in one of my prototypes backwards, and it
>didn't last very long. While I expect this device to have very long
>battery life, since it normally doesn't operate but for a second or
>two, it looks like the MC1253's don't like getting reversed biased
>very long. So, for my design question of the day - What is your
>favorite circuit for preventing reverse bias from the batteries? The
>old diode trick drops too much voltage, so wondered if there are any
>clever FET tricks to block backwards voltages.

A plastic slot that the '+' end of the battery fits nicely into but such that
the '-' end doesn't make contact?

From: Jan Panteltje on 16 Mar 2010 19:49

On a sunny day (Tue, 16 Mar 2010 14:32:50 -0700) it happened Joerg
<invalid(a)invalid.invalid> wrote in <80abo8Fd3pU1(a)mid.individual.net>:

>Very easy: Oscillator that is connected via a diode so it works only
>when batteries are reversed, drives piezo. Battery inserted the wrong
>way -> beep ... beep ... beep ...

Clever.

>If you want to also cover the case where only one of the batteries is
>reversed use two of those, can drive the same piezo.

Use a bigger batteriy and a bridge rectifier,
then it does not matter how they are put in.

From: Joerg on 16 Mar 2010 20:00

D Yuniskis wrote:
> Joerg wrote:
>> D Yuniskis wrote:
>>> Hi Joerg,
>>>
>>> Joerg wrote:
>>>> D Yuniskis wrote:
>>>>>>> "Key" the battery holder so the user can feel the correct
>>>>>>> orientation for the batteries *and* so it prevents the
>>>>>>> cells from making electrical contact if not oriented
>>>>>>> properly.
>>>>>>>
>>>>>>> Think of how the *user* is going to interact with your device.
>>>>>>> Close your eyes and "figure out" why it doesn't work :-/
>>>>>>
>>>>>> Would be the best result, but these are standard AA cells, no way to
>>>>>> key them without using expensive custom battery packs. Not the
>>>>>> way to
>>>>>> go for inexpensive...
>>>>>
>>>>> I take it you aren't making a custom (molded) case?
>>>>> Why not purchase COTS battery holder that *is* "keyed"?
>>>>
>>>> Anything other than AA cells is expensive. And as Charlie wrote AA
>>>> cells are difficult to key without going to precision molding or
>>>> milling.
>>>
>>> Yes. Find a battery holder that someone *else* has incurred
>>> the cost of that molding! Preferably, something that lets
>>> the user access the "battery compartment" without having
>>> to disassemble the device itself.
>>
>> Very tough. I had tried that at times in the past and there ain't much
>> out there. Even the ready-to-go enclosures with built-in compartment
>> that I came across aren't really good in that respect.
>
> I've been eying the Bulgin products. MPD and Keystone
> tend to be bottom feeder designs which haven't impressed
> me. But, I'm currently designing for much higher power
> requirements so batteries (at least standard cells)
> aren't an option.
>

Their battery holders don't look particularly well keyed either:

http://www.bulgin.co.uk/Products/BatteryHolders/BatteryHolders.html

>>> OTOH, if he is making "one off", a bit of plastic glued on either
>>> side of the '+' holder terminal can do the trick.
>>>
>>> I'd also look for those (camera?) batteries (two cells side
>>> by side... 3V?) as I think that is keyed (at the very
>>> least, it eliminates the problem of putting one cell in
>>> correct and one backwards)
>>
>> I have one, in case of a trip way into the boonies (those are Li
>> batteries so they'll last longer). Very expensive, not really an
>> option for everyday use.
>
> Are you sure? ...

Yup:

http://www.atbatt.com/product/14514.asp

> ... The ones I have are made by Duracell (though
> none of them are in convenient places for me to examine).
> I would imagine a "colorimeter" sees infrequent use so
> you would want a battery that's going to be "there for you"...
> (My pet peeve re: flashlights is you use them so infrequently
> that the batteries are *always* dead when you need them)
>

That's why everyone here knows where the spares are. There are also
spare flashlights.

>>>>>> Will be looking for a good PMOS FET that will still conduct with two
>>>>>> low AA batteries, probably around 1.8VDC, if they make one! ;-)
>>>>>
>>>>> If you go that route, consider how your design can tell the user
>>>>> "yes, I am working" vs. absence of that indication so the user
>>>>> can deduce "something is wrong with the batteries or their
>>>>> installation" (or the device)
>>>>
>>>> Very easy: Oscillator that is connected via a diode so it works only
>>>> when batteries are reversed, drives piezo. Battery inserted the
>>>> wrong way -> beep ... beep ... beep ...
>>>>
>>>> If you want to also cover the case where only one of the batteries
>>>> is reversed use two of those, can drive the same piezo.
>>>
>>> You don't need to be that extravagant. Just something in the
>>> *normal* usage of the device that people can count on for
>>> feedback/confirmation. E.g., if the device uses audio
>>> output (synthetic speech or just "tones"), then design
>>> the device so that immediately on turning on, it "chirps"
>>> once.
>>>
>>> [I don't recall what he has chosen for his output medium;
>>> personally, I would opt for limited vocabulary speech as
>>> it can be *very* small]
>>>
>>> This gives a positive indication to the user in *normal*
>>> operation (i.e., those times when you HAVEN'T just
>>> changed batteries) that the device is, in fact, "on".
>>> And, knowing to expect this, it can be used when replacing
>>> batteries to act as a confirmation that the batteries
>>> are, in fact, installed correctly, aren't "dead", etc.
>>
>> Yes, that's usually good enough. No audio feedback -> something must
>> be wrong with the batteries.
>
> *If* the audio feedback in normal operation is noticeable
> enough. E.g., audio annunciators in most *cars* are
> ineffective as: 1) there are too many of them 2) they
> don't "complain" enough.
>
> I.e., make the "chirp" something distinctive (a little
> tune) so it's noticeable without being obnoxious -- and
> "missed" when absent.
>
> E.g., you could power up a Windows PC with your eyes closed
> and would "notice" if their silly little tune didn't play...

It's the first thing I disable when I get a new PC :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

From: D Yuniskis on 16 Mar 2010 20:13

Hi Jan,

Jan Panteltje wrote:
> Use a bigger batteriy and a bridge rectifier,
> then it does not matter how they are put in.

The device ends up larger. And weighs more, etc.

For *one* device, this is no big deal. But, folks with
disabilities tend to have *lots* of "special devices".
Each is typically larger than it's counterpart (for
the mainstream population).

From: D Yuniskis on 16 Mar 2010 20:28

Hi Joerg,

Joerg wrote:
>>>>>> I take it you aren't making a custom (molded) case?
>>>>>> Why not purchase COTS battery holder that *is* "keyed"?
>>>>>
>>>>> Anything other than AA cells is expensive. And as Charlie wrote AA
>>>>> cells are difficult to key without going to precision molding or
>>>>> milling.
>>>>
>>>> Yes. Find a battery holder that someone *else* has incurred
>>>> the cost of that molding! Preferably, something that lets
>>>> the user access the "battery compartment" without having
>>>> to disassemble the device itself.
>>>
>>> Very tough. I had tried that at times in the past and there ain't
>>> much out there. Even the ready-to-go enclosures with built-in
>>> compartment that I came across aren't really good in that respect.
>>
>> I've been eying the Bulgin products. MPD and Keystone
>> tend to be bottom feeder designs which haven't impressed
>> me. But, I'm currently designing for much higher power
>> requirements so batteries (at least standard cells)
>> aren't an option.
>
> Their battery holders don't look particularly well keyed either:
>
> http://www.bulgin.co.uk/Products/BatteryHolders/BatteryHolders.html

I've been through their site and haven't been able to find
any mechanical drawings *or* photos that show enough detail
to decide. I have a few on my next Digikey order just to
"evaluate" -- though it would be hard to come to a conclusive
decision for *all* of their products based on a sampling of
a few... (if I like the quality -- touch and feel -- I will
followup with a call to the manufacturer)

>>>> OTOH, if he is making "one off", a bit of plastic glued on either
>>>> side of the '+' holder terminal can do the trick.
>>>>
>>>> I'd also look for those (camera?) batteries (two cells side
>>>> by side... 3V?) as I think that is keyed (at the very
>>>> least, it eliminates the problem of putting one cell in
>>>> correct and one backwards)
>>>
>>> I have one, in case of a trip way into the boonies (those are Li
>>> batteries so they'll last longer). Very expensive, not really an
>>> option for everyday use.
>>
>> Are you sure? ...
>
> Yup:
>
> http://www.atbatt.com/product/14514.asp

No, that's not the same battery. I'll have to find one of mine
and see what size it claims to be...

>> ... The ones I have are made by Duracell (though
>> none of them are in convenient places for me to examine).
>> I would imagine a "colorimeter" sees infrequent use so
>> you would want a battery that's going to be "there for you"...
>> (My pet peeve re: flashlights is you use them so infrequently
>> that the batteries are *always* dead when you need them)
>
> That's why everyone here knows where the spares are. There are also
> spare flashlights.

We now use "emergency flashlights". I've disciplined myself
to give each one a "quick wind" whenever I come across one
just to keep the battery fresh.

I recently came across one of these "emergency" devices
that stores energy *mechanically* -- no more dependence
on Li cells (going bad from lack of use). I need to
see what other offerings they have.