# How to use Zener Diode in Parallel RC circuit

What I'm trying to do is allowing the current to move through only one component. In the circuit shown, DC voltage is running two components (Z & C), also charging the capacitor

When voltage cut, then capacitor give current to Z & C. What I want to do is allowing the current to flow through only C but not Z. I tried to put zener diode on the line indicated by light blue but the current still flowing through Z (zener diode is not working).

How can I make this work? What can be the problem?

• why do you think you need a zener diode and not just a standard diode? – Trevor_G Jun 5 '17 at 15:33
• Well, I'm new to electronics, I thought zener would do the trick. Although I tried 6-8 different zener, it didn't work. What is your advice? What should I look for? – Alper Gündöken Jun 5 '17 at 15:39
• Zener diodes specifically allow current to flow in a reverse-biased direction if they are reverse-biased above their Zener voltage. – Chris M. Jun 5 '17 at 15:44
• Just a normal or shotky diode pointing right where your blue dot is will work. THough you may need to factor in the voltage offset across it. – Trevor_G Jun 5 '17 at 15:45
• My DC voltage is ~28V, Constant current 1.5A Capacitor 10000 mikrofarad, 35 V. What kind of component should I use? Edit: I guess (I hope), zener diodes are not working because my capacitor voltage is above zener voltage. As you mentioned, If I use normal diode, then it will work. I'll try & let u know – Alper Gündöken Jun 5 '17 at 15:49

No Zener diodes need to be abused:

When V1 is present, it will supply current for R1, R2, and charge cap C1.

When V1 is disconnected, the cap will supply current to R2 but not R1. Of course the energy in the cap is limited, and the voltage across R2 will decay exponentially towards zero over time.

There will also be a voltage drop across the diode, even when it is forward biased. For example, if V1 is 10 V, then there might only be 9.6 V or so across R2, depending on the current. If this is not acceptable, then a more complicated topology that switches a FET is needed.

• Well, thanks. After I bought A new "standard diode", I figured out that I connected "zener diode" incorrectly. Now I'm using a "standard diode" but "zener diode" would do the same thing if I could connect it right. – Alper Gündöken Jun 5 '17 at 16:42
• @Alp: A Zener diode is inappropriate here. A Schottky (as shown) would be best, assuming V1 is not too high. – Olin Lathrop Jun 5 '17 at 17:29

A Schottky diode as D1. Be careful of the voltage and current specifications of the Schottky you choose. In this case: at least 40V and 2A to be safely above maximum ratings.

• I implemented normal diode. I'm thinking of replacing that with mini led which can at least provide light that represent the current. Led could tell something while the circuit running & I can find all variant of this component everywhere – Alper Gündöken Jun 5 '17 at 22:11
• Do NOT use an LED in place of the schottky diode. This will limit the charge voltage of C1 to V1 minus the forward voltage of the LED. That is pretty much the same reason for not using a zener. – JRE Jun 6 '17 at 11:17
• Your led will burn immediately as soon as the capacitor discharges into the wrong direction of the led. If you apply positive current to the cathode of the led (-) then you destroy the led inevitably. – Fredled Jun 6 '17 at 22:45

Using a standard diode (eg in4007) should work for your application.

And the current in Z is because the Zener is operating in breakdown region. So either you need to select a Zener which has cut off voltage greater than 28V or just use one of the standard diodes as mentioned above. A standard diode should work perfectly for your circuit.

• Well, after some tests, I've seen that standard diode is not what I'm looking for. Because, although they say the reverse current of standard diode is on the order of nA, my tests show that some miliampers are flowing through reverse standard diode & Z component, too. This is not nA. it's what I don't want. I guess, My 50V capacitor which is filled by 28VDC look too much for this diode – Alper Gündöken Jun 6 '17 at 10:54
• is that mA continuous or just for moments when the capacitor starts conducting? I think you might be looking for fast switching diode. Since standard diodes conduct for a very small time when switched from FB to RB. – Mayank Jun 6 '17 at 12:35
• Please upvote the answer if it helps. – Mayank Jun 6 '17 at 12:36