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From: Grant on 17 Mar 2010 11:06
On Wed, 17 Mar 2010 06:49:44 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote:

>On Mar 16, 7:13 pm, Grant <o...(a)grrr.id.au> wrote:
>> On Tue, 16 Mar 2010 18:43:58 -0700 (PDT), MooseFET <kensm...(a)rahul.net> wrote:
>> >On Mar 16, 10:56 am, Charlie E. <edmond...(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.
>>
>> >I did a quick check and nobody mentioned the idea of a diode in
>> >series
>> >with a relay coil.
>>
>> That's because a relay coil likely draws more power than the circuit you're
>> trying to protect ;)
>
>I tend to be designing things that are called "portable" and run from
>a car
>battery or the like. Right now I am doing the lowest powered product
>ever
>and that is 2.5W.
>
>>
>> I like diode and polyswitch, or MOSFET circuit.
>
>I have found that polyswitches can't be trusted if you have a
>significant
>battery to get hooked up backwards. They take too long to go open.

Bigger diode? Where I've seen them used was on 24V traction battery,
lots heating power available.
>
>I have used the MOSFET trick
>
>I used a diode to supply power to a little circuit that pumps the gate
>of the MOSFET up so that the battery voltage doesn't have to be
>enough
>to work the MOSFET.
From: Charlie E. on 17 Mar 2010 11:42
On Tue, 16 Mar 2010 18:40:22 -0400, "Martin Riddle"
<martin_rid(a)verizon.net> wrote:

>
>
>"Joerg" <invalid(a)invalid.invalid> wrote in message
>news:80a0b7F5q1U2(a)mid.individual.net...
>> Hammy wrote:
>>> 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.
>>>>
>>>> Thanks!
>>>>
>>>> Charlie
>>> I usually use #2 on this page a shorting shottky.
>>>
>>> http://www.recom-international.com/press/Reverse%20Polarity%20Protection%20for%20DC/DC%20Converters.html
>>
>>
>> That renders the unit dead and a blind or almost blind person will
>> have a hard time finding or buying the proper fuse and installing it.
>>
>> Charlie: Solution #3 is the ticket (look on Hammy's link). But make
>> sure that the FET is guaranteed to be fully turned on at the lowest
>> allowed battery voltage, IOW the point where an UVLO comes on.
>>
>> --
>> Regards, Joerg
>>
>
>
>You can replace the fuse with a PTC, batteries might not like it, but it
>would still function when the batteries are inserted correctly.
>
>Cheers
>

Presently looking at http://www.micrel.com/_PDF/mic94052-53.pdf which
seems to fit the bill. Any one used these, and know the gotchas?

Thanks,
Charlie
From: Charlie E. on 17 Mar 2010 11:49
On Tue, 16 Mar 2010 17:57:01 -0500, Vladimir Vassilevsky
<nospam(a)nowhere.com> wrote:

>LTC4358

Yikes! $4.50 each at digikey! ouch...

Charlie
From: Vladimir Vassilevsky on 17 Mar 2010 11:51


Charlie E. wrote:

> On Tue, 16 Mar 2010 18:40:22 -0400, "Martin Riddle"
> <martin_rid(a)verizon.net> wrote:
>
>
>>
>>"Joerg" <invalid(a)invalid.invalid> wrote in message
>>news:80a0b7F5q1U2(a)mid.individual.net...
>>
>>>Hammy wrote:
>>>
>>>>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.
>>>>>
>>>>>Thanks!
>>>>>
>>>>>Charlie
>>>>
>>>>I usually use #2 on this page a shorting shottky.
>>>>
>>>>http://www.recom-international.com/press/Reverse%20Polarity%20Protection%20for%20DC/DC%20Converters.html
>>>
>>>
>>>That renders the unit dead and a blind or almost blind person will
>>>have a hard time finding or buying the proper fuse and installing it.
>>>
>>>Charlie: Solution #3 is the ticket (look on Hammy's link). But make
>>>sure that the FET is guaranteed to be fully turned on at the lowest
>>>allowed battery voltage, IOW the point where an UVLO comes on.
>>>
>>>--
>>>Regards, Joerg
>>>
>>
>>
>>You can replace the fuse with a PTC, batteries might not like it, but it
>>would still function when the batteries are inserted correctly.
>
>
> Presently looking at http://www.micrel.com/_PDF/mic94052-53.pdf which
> seems to fit the bill. Any one used these, and know the gotchas?

#3: If the input voltage terminals are shorted, this event is called
"psst" in language of zurgs. For the same reason, you can't make an OR
of two batteries on the load with a pair of FETs. The ESD while
inserting the bateries will do "psst" to FETs also, unless you take
precautions.

VLV





From: Joerg on 17 Mar 2010 12:02
Charlie E. wrote:
> On Tue, 16 Mar 2010 17:57:01 -0500, Vladimir Vassilevsky
> <nospam(a)nowhere.com> wrote:
>
>> LTC4358
>
> Yikes! $4.50 each at digikey! ouch...
>

And over $2 in production qties, which is why I never used a solution
like that and kept designing with discretes instead. LTC is a great
company but the pricing on the more mundane parts such as this one is
IMHO a wee bit out of touch.

--
Regards, Joerg

http://www.analogconsultants.com/

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