zener diode

[Agrum]

Hello, I need to charge a capacitor to its maximum allowed voltage which is 11V.
There is 11V zener diode to protect it. Can I use a AC 12V 2A supply? Will the voltage be clamped by the zener?
Thank you!

[Manul]

You can not charge capacitor from AC source, you need at least one series diode to rectify AC (half wave rectifier) or better - diode bridge (full wave rectifier). Also, to use zenner as a clamp, you need to limit current, because otherwise the zenner can get very hot and burn. To limit current you may add series resistor.

Draw a schematic of what you are planning to do. It will be much more clear.

[Agrum]

Thanks for replying.
Sorry, is a DC 12V 2A supply, not AC.
Attached the schematic. Is it ok just like this? Thanks!

[Zero999]

Zener diodes aren't very accurate. You're better off using a programmable shunt regualtor such as the LM431.

[exe]

Actually, in this case a normal shunt regulator like lm317n might be better in terms of power efficiency as it won't dissipate heat when capacitor is not charging. Just I'd check there is not overshooting on the output. So, an lm317, and a trimpot will do the job. Add small input capacitor if you want to get fancy . Although, that's approximately complexity of tl431...

Zenner will work too, but they are annoyingly drifty. As they heat-up the voltage will drift. Also, initial voltage is not accurate. It is like +-5%. It also changes with current.

[Zero999]

Actually, in this case a normal shunt regulator like lm317n might be better in terms of power efficiency as it won't dissipate heat when capacitor is not charging. Just I'd check there is not overshooting on the output. So, an lm317, and a trimpot will do the job. Add small input capacitor if you want to get fancy . Although, that's approximately complexity of tl431...

The LM317 is not a shunt regulator. It's a series linear regulator. It's no good for this application, because the output voltage is 11V, which is too high for the LM317 with just a 12V supply. A low dropout regulator such as the LT3080 is required for 11V out, with 12V in.

[MathWizard]

What type of capacitor is it? an electrolytic ?? I've heard there's not much room for over-voltage at all with them. Yeah I would not trust a 11V zener to save a 11V cap for very long, especially if it's an older, or cheap cap.

[Agrum]

Thank you all for helping.

The cap and the zener are already on a pcb.
I need a quick and easy way to charge this capacitor which is used to feed a dcdc and a sensor in a small energy harvesting system.
That's why I was thinking to simply use a dc source (of course I don't have lab equipment at the moment).
Using a regulator is definitely the best option but is not something that I can do in 1-2 days.... I'll think about it 

ps: it is a supercapacitor and is quite new and definitely not cheap

[exe]

because the output voltage is 11V, which is too high for the LM317 with just a 12V supply. A low dropout regulator such as the LT3080 is required for 11V out, with 12V in.

Ha-ha, good catch! LT3080 won't probably work as well as it:
1) requires minimum load, or it will not regulate
2) The dropout is actually 1.2V, unless there is a separate supply for Vcontrol.

[Agrum]

Just one last question...
Is there a way to test if the zener isn't broken (without unsoldering it from the pcb) ?

Thanks

[ledtester]

ps: it is a supercapacitor and is quite new and definitely not cheap

Is it made of a bunch of lower voltage super capacitors in series? You might need to balance the individual caps, e.g.: https://www.powerelectronictips.com/fundamentals-supercapacitor-balancing/

[Zero999]

because the output voltage is 11V, which is too high for the LM317 with just a 12V supply. A low dropout regulator such as the LT3080 is required for 11V out, with 12V in.

Ha-ha, good catch! LT3080 won't probably work as well as it:
1) requires minimum load, or it will not regulate
2) The dropout is actually 1.2V, unless there is a separate supply for Vcontrol.

1) The minimum load is no problem, just add a resistor.
2) Oh yes, that is an issue. The MIC29302 is better.

[Agrum]

no, it's just one  I don't know how supercap are done

[exe]

1) The minimum load is no problem, just add a resistor.

I guess this depends on the application because this resistor would start discharging the supercap. Although, min current is only 0.5mA or so. Also, if supercap is to provide power for the circuit while not powered, then it's better to check if a regulator tolerate voltage difference between in and out. So, I'd say, unless large supply current is needed, shunt solution is safer in this regard.

[Jwillis]

Just one last question...
Is there a way to test if the zener isn't broken (without unsoldering it from the pcb) ?

Thanks

Measure the voltage across the zener . Provided your not exceeding the rated current of the zener or current across the Limiter Resistor the voltage should be close to the Zener Value .

[Zero999]

1) The minimum load is no problem, just add a resistor.

I guess this depends on the application because this resistor would start discharging the supercap. Although, min current is only 0.5mA or so. Also, if supercap is to provide power for the circuit while not powered, then it's better to check if a regulator tolerate voltage difference between in and out. So, I'd say, unless large supply current is needed, shunt solution is safer in this regard.

The capacitor will need to be disconnected from the power supply when it's not being charged, irrespective of whether there's a resistor to provide the minimum load, or not, otherwise it'll discharge back into it. Most regulators can be damaged by voltages on the output, when unpowered, hence why a diode between the output and input is normally recommended.

[Capernicus]

All just to charge one capacitor its just this damn complicated.

[amishasingh]

I have one small doubt on this discussion. Which is the best zener diode can I go with ?

[Capernicus]

shouldnt u rate the voltage to the capacitor?

[Zero999]

I have one small doubt on this discussion. Which is the best zener diode can I go with ?

The general advice is don't use a zener diode.

If you must, then choose one with a maximum zener volage of 11V, with a test current of 10mA, since that's the current passing through, in your circuit. I had a quick look at the RS Components website and found the PLZ11B, which has a maximum breakdown voltage of 11.05V and a minimum of 10.5V, with 10mA flowing through it.
https://docs.rs-online.com/28dd/A700000007647287.pdf

[amishasingh]

Thanks for the information and it really looks very useful. I have seen the content over the link you have provided. One small doubt, why it is not advised to go for zener diode. What is the major drawback they really posses.

[Zero999]

Thanks for the information and it really looks very useful. I have seen the content over the link you have provided. One small doubt, why it is not advised to go for zener diode. What is the major drawback they really posses.

A zener diode's reverse voltage, depends on the current and temperature.

[Jwillis]

Zener diodes used as clamps have a wide range of error in breakover voltage due to voltage drift caused by temperature and current . Generally zener diode rated below 5.1V will have a negative temperature coefficient (voltage drifts down as temperature increases )and diodes above 5.1 will have a positive coefficient (voltage drifts upward with temperature ) . That would have to be confirmed by the data sheet for the specific zener diode being used.
 Also Zener diodes rated voltage can be ±5% . So the higher the rated voltage of the zener , the greater the voltage difference. This requires careful selection of components .

They're OK if used with a  constant current like a voltage reference. But in OP's case the current changes as the capacitor charges. So it becomes difficult to predict how the zener will react . This depends on how quickly the charge and discharge needs to be .
Zener voltage clamps are normally used as a last defence against catastrophic failure not as an accurate voltage regulator.

The Op can either change the type of charging system or widen the tolerances by lowering the capacitor maximum charge voltage .  Electrolytic capacitors can be tricky when charged to the maximum rated voltage . This is why electrolytic capacitors are selected with a higher rated voltage than what is required  for an application . The higher the energy potential of the capacitor , the bigger the bang is when catastrophic failure occurs . Super Caps are expensive and not something a person wants to have to replace .