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 Post subject: Re: Calculating dropping resisitor for zener diodes
PostPosted: Apr Thu 25, 2019 6:08 pm 
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Joined: Nov Wed 08, 2017 11:52 pm
Posts: 260
Location: Phoenix AZ
Reading again the OP, his load is indeed 60uA and not 60mA. If that is the case then any 1W Zeners combination in series will work. The 60uA load will be insignificant relative to the current through the Zeners, so practically you can ignore it and not worry about the no load current and the Zeners power dissipation.

Looking at the Microsemi Zener datasheet for 1N4493 and 1N4489, 150V and 100V 1W 5% Zeners respectively, the test current of the 150V Zener is 1.7mA and 100V Zener is 2.5mA. I would set the current trough the Zeners to be 2.5mA. The power dissipation in each Zener would be 150V x 2.5mA=375mW and 100V x 2.5mA=250mW well withing the 1W limits. As far as the current limiting resistor, it would be (300V-250V)/2.5mA=20K, its power dissipation 2.5mA x 2.5mA x 20K=125mW so I would use a 20K 0.25W resistor.

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