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Re: [ST] Helmet cam - Zener diodes



The Tooth Fairy wrote:

> >I have found another DC-DC voltage regulator which supposedly
> >regulates at 12V although when tested I was getting 12.5V from the
> >regulator but 12.9V was going in.  I may try the solution with a 12V
> >Zener diode in series.
>ANG(- 
>ANG(- Sorry if you know this already but...the Zener diode would need to go
in
>ANG(- parallel with the camera, i.e. from camera +12 V input to ground.
You'd
>ANG(- need a resistor between the output of the regulator and the
camera/Zener
>ANG(- diode +12 V input.  The Zener and resistor can get rather warm, so
it's best
>ANG(- to check the power calculations.

>Current requirement is only 130mA

>What value resistor would I need?

Um...scratches head and tries to remember electronics through the haze of
drink at university...

The voltage across the resistor is 0.5 volts (input 12.5V from regulator,
output 12V at Zener/camera)
Current through the resistor is 130mA to the camera plus Iz, the current the
Zener takes
R = V/I = 0.5 / (0.130 + Iz) will give you the resistance, P = VI = 0.5 *
(0.130 + Iz) gives you the resistor power rating

You need to look at the voltage/current curves for the Zener to find Iz.
You also need to check that the power in the Zener P = VI = 12 * Iz isn't
going to blow up the Zener.  Note that the voltage is 12 here not 0.5 as
it's the voltage across the Zener.

So it all depends on the Zener you choose.  Being a gambler, I might just
try the camera to see if it's happy with 12.5 V anyway and forget the whole
Zener thing - but it's not my camera!

End of rusty electronics lesson!  You might get a better answer if you roll
home from the pub at closing time one night and find the right beardy
lecturer on Open University though!

Neil

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