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I've been learning about using zener diodes to generate a reference voltage for a small comparator circuit, the intention is to detect when a motorcycle battery is charging (i.e more than 13v), and use that to activate an Arduino circuit with a relatively high draw of 2A)

I've been fortunate enough to be given an example I can work from here on SX. However, something occured to me whilst learning about Zener diodes.

Once they are electrically closed, the input voltage has to be removed, before the open again. (speaking in terms of electrical continuity). To put that another way, once more than 12v (for a 12v 5A Zener) is seen, and the reverse bias breaks down, I (apparently) have to completely remove the input voltage in order to sever the connection.

Is that accurate, and is this expected, normal behaviour? I need to be able to power down my device when the voltage at the battery drops back down below 12v.

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  • \$\begingroup\$ Any chance you can use the analog comparator in the Arduino to turn it off? \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 30 '13 at 7:20
  • \$\begingroup\$ Very possibly, but what would I be turning off? Cutting the input to the zener, and turning myself off? I suppose then I could watch the voltage after the zener with the Arduino to detect the opposite effect, such as low voltage. \$\endgroup\$ – Lee Hambley Sep 30 '13 at 7:42
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    \$\begingroup\$ That's not true about zeners - if you go back below the threshold where they appear to regulate/conduct you are back to square 1 unless of course you damaged the zener with too much current and it has gone short circuit due to melt-down. \$\endgroup\$ – Andy aka Sep 30 '13 at 7:47
  • \$\begingroup\$ Connect the Arduino through the NC contact on a relay, and energize it when you detect the decrease. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 30 '13 at 7:51
  • \$\begingroup\$ Andy, perfect, if you can post as an answer (with citation?) I'd be happy to accept. \$\endgroup\$ – Lee Hambley Sep 30 '13 at 8:04
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It occurs to me that you're thinking of a zener diode as a thyristor or a diac.

For example, the IV curve for a diac:

enter image description here

Once either threshold voltage is reached, the diac "turns on" and stays on until the current drops below the holding current value. Importantly, note that there are two values of current associated with any voltage between \$V_F\$ and \$V_{BO}\$ since the diac can be "on" or "off".

However, the IV curve for a zener (or avalanche) diode:

enter image description here

is qualitatively different. Note that, for the zener, for any voltage, there is only one value of current associated so the zener isn't a two-state device. Yes, it does have regions of operation (forward biased, reverse biased, and breakdown) but these simply refer to different regions of the IV curve.

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That's not true about zener diodes - if you go back below the threshold where they appear to regulate/conduct you are back to square 1 unless of course you damaged the zener with too much current and it has gone short circuit due to melt-down.

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