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Reverse Polarity and Over Voltage Protection Design 
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It's not just about voltage linearity and regulation though. These caps are upsized to provide some short term energy buffer during the rapid fluctuations of battery voltage during cranking on each compression stroke. For that reason the series resistor and largish electros are there. The tant and ceramics cover different frequency bands and reduce the ESR further.

Perhaps we should KISS with the power supplies and simply have something close to the original setup without the zener clamp or equivialent. IE :

reverse diode 1N5408
1R low pass resistor
normal polarised electro cap of suitable largish size and 35 - 60 volts rating
tant cap in size/spec as per the schem explanation
ceramic cap
reg
same caps again except 10v rating on the electro and tant

That provides reverse protection for cheap and compact, *very* clean power, simple design and layout, common parts, etc

This is only a first cut after all.

Thoughts?

Fred.

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Sat Aug 16, 2008 8:13 pm
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QFP80 - Contributor

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Sounds good to me. The 1R resistor will need to be about 2.5W to save it getting too hot, asssuming current to be approx 500mA. Or, to put it another way, if the resistor is rated at 5 times the required power it won't get too warm. Keeping the temp down will increase the reliability.


Sat Aug 16, 2008 9:30 pm
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Brian wrote:
Sounds good to me. The 1R resistor will need to be about 2.5W to save it getting too hot, asssuming current to be approx 500mA. Or, to put it another way, if the resistor is rated at 5 times the required power it won't get too warm. Keeping the temp down will increase the reliability.

Isn't that 10 times?


Sat Aug 16, 2008 9:40 pm
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QFP80 - Contributor

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I think it's 5 times....now, where is the resident EE? :?:

Or, do you mean the resistor should be rated at 10 times not 5 times?


Sat Aug 16, 2008 9:47 pm
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Power = R x I x I = 1 Ohm x 0.5 A x 0.5 A = 0.25 W.


Sat Aug 16, 2008 9:59 pm
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QFP80 - Contributor

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Ooops, don't you love it when you make a simple mistake? :shock:


Sat Aug 16, 2008 10:12 pm
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LQFP144 - On Top Of The Game
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:lol:


Sat Aug 16, 2008 10:25 pm
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It becomes really simple if you assume 1 Ohm and 1 Amp :-) = 1 Volt and 1 Watt :-) Good to over rate slightly anyway :-)

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Sat Aug 16, 2008 10:54 pm
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Sounds good to me, what about a foot print? Does Newark's 65K3871 look good? I think that's going to be a thru hole only part, so I'll plan on using KICAD's R7 foot print.


Sun Aug 17, 2008 2:32 am
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Also note, the above newark resistor is wirewound, so slight inductance, but I don't think it matter in this case.


Sun Aug 17, 2008 3:20 am
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