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  #1  
Old 05-02-2012, 03:54 AM
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JSV JSV is offline
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Default A question for electronic experts.

I studied electronics long ago, and I don't recall such a steup.

Why connect a diode and a capacitor in parallel with a battery?

I have this digital tire pressure meter that I really like. Only problem is that I must replace the cells every six months or so.

Today, I opened it, and saw a diode and a ceramic capacitor soldered across the battery output.

I wondered if this could cause a tiny leakage and slowly discharge the cells.

I know that the capacitor/diode pair can act as a rectifier/filter, but we are in pure DC here, no need for that...

Here's a crude schematic:

2x 3V lithium button cells.
1x Ceramic capacitor, labeled "104."
1x Diode, glass enclosure, color orange.



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  #2  
Old 05-02-2012, 03:43 PM
saratoga saratoga is offline
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The cap is probably to protect the battery from any voltage transients when the device is activated (button pressed, etc). I would guess the diode is to protect the circuit in case you've hooked up the batteries in backwards, but I guess if theres some big (inductive) load that might spike it could also be designed to clamp that.

Since that diode is reverse biased 6v, its current is probably very small. Likewise, caps usually don't have much DC leakage. If theres a part number on the diode, you can probably look up the leakage current.
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Old 05-05-2012, 01:23 AM
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JSV JSV is offline
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Thank you.

I never thought that batteries would need to be protected against transients in such a low power device.

Diode is certainly not for reverse polarity protection, because if one or both cells are placed the wrong way, the circuit would be opened.

The LCD of the device has an electroluminescent backlight ("Indiglo"), and I just read that these need over 100 volts AC to work. Maybe the "big inductive load" you mentioned is there... Does that makes sense?
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Old 05-05-2012, 10:02 AM
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yuki yuki is offline
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The freescale mems pressure sensor that I am sure that is in it (most of the cheap ones use one from them) has a fairly high inrush current to start sense.
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