Zener Regulator Series Resistance - Page 1
Forum section: electronics / projects

author: Ashraf Almubarak
author_rank: Contributor
timestamp: July 31, 2017, 07:19:22 am
content: I am designing a Capacitive power supply, I found someone who did the circuit shown in the figure. I don't know what is the usage of the 47-ohm (R2) resistor in series with the Zener diode (12V MELF package). Also, what is the purpose of the 100nF (C3) ceramic capacitor in parallel with Zener diode? I didn't find any one who suggests that. In fact, this product is known as the most robust one among its peers.if this is used to protect the zener from excessive transient current, I didn't find any information about that in the data sheets.The question is:What is the use of this resistor and 100nF capacitor?

author: MK
author_rank: Regular Contributor
timestamp: July 31, 2017, 07:50:52 am
content: D2 stops the voltage from climbing too high if there is no load attached. the capacitor across D2 does not do very much in that circuit.

author: Ashraf Almubarak
author_rank: Contributor
timestamp: July 31, 2017, 10:44:32 am
content: Yes, it is a voltage regulating diode, but what is the purpose of resistor R2?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 31, 2017, 11:16:02 am
content: My guess would be these parts are used to reduce the modulated noise injection back to the mains, essentially for the EMC compliance. Many Zeners are very noisy up to RF and 100nF capacitors reduce the noise voltage. It is generally a good practice to include these caps if a Zener is used. The 47 Ohm resistors reduce the current ripple through Zeners (but increase the output voltage ripple) so would reduce the noise modulation and create a filter to reduce the amount of noise from Zeners entering the mains.CheersAlex

author: Ashraf Almubarak
author_rank: Contributor
timestamp: July 31, 2017, 12:30:18 pm
content: What is the level of the Zener noise? I don't find any information in the data sheets of Zener.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 31, 2017, 01:25:55 pm
content: That is not something to advertise. Do a search on "zener diode rf noise" .CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2017, 06:08:06 pm
content: Quote from: Alex Nikitin on July 31, 2017, 11:16:02 amThat's an interesting theory, although I wouldn't have thought 47R would have made much difference, when R3 and R4 are taken into account. I suppose C2 and C5 form a low pass filter with R2 & R2 which might help.I did think it might be something to do with reducing the surge current through the zeners but R3 and R4 would do that.Perhaps the resistors and capacitors are there to stop the noise from entering the circuit being powered?Another possibility is the person who designed the circuit, saw a similar thing somewhere else and just blindly copied it.

author: bson
author_rank: Unknown
timestamp: July 31, 2017, 06:47:16 pm
content: R1 and R2 make the zener current (and hence zener voltage) vary with voltage across C1, creating negative feedback. Not unlike an emitter resistor on a BJT.Edit: sorry, the zener voltage is somewhat unrelated; the resistor caps the zener current and hence the zener voltage, while providing feedback (along with the varying zener voltage, to a lesser extent).

author: MK
author_rank: Regular Contributor
timestamp: July 31, 2017, 07:01:10 pm
content: well it does depend a lot on what the zener voltage actually is, presumably a zener between 10 and 13 Volts, as it is meant to deliver 12 volts when loaded, then you can work out how much current passes through the 47 ohm resistor. I could see the C3 cap reducing the rf, but there is a lot of rf hash delivered down the mains anyway. so still not sure it has much purpose.

author: DBecker
author_rank: Frequent Contributor
timestamp: July 31, 2017, 10:08:28 pm
content: What type of circuit does the original power supply feed?It may be tolerant of line frequency ripple, but susceptible to the higher frequency noise from the zener clamp. This would be common with audio circuits, where feedback (and bandpass filters) eliminates hum but not audio frequency noise.

author: Ashraf Almubarak
author_rank: Contributor
timestamp: August 01, 2017, 05:21:25 am
content: Quote from: Alex Nikitin on July 31, 2017, 01:25:55 pmOk, I will, thank you a lot.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 01, 2017, 07:24:27 am
content: Quote from: Hero999 on July 31, 2017, 06:08:06 pmIt is not straightforward, however these resistors completely change the circuit behaviour. Without them the current through Zeners modulated 100% from zero to about 50mA peak. The Zener RF noise is near maximum at small currents, so on each transition from/to zero current Zeners generate a burst of RF, nicely modulated with twice the mains frequency. With resistors in, 220uF capacitors are charged to a higher voltage and provide a continuous current through Zeners at the same time reducing the peak current, so the current modulated less and does not go into the high noise area (providing the load current does not exceed about 5-6mA). At the same time the resistors act (together with 220uF caps) as filters for that noise going back to the circuit. The obvious drawback is the higher voltage ripple on the output somewhat moderated by a lower level of high order components in the ripple voltage.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 02, 2017, 09:51:16 am
content: Here is the simulation result for the Zener current with and without these resistors.CheersAlex

author: Ashraf Almubarak
author_rank: Contributor
timestamp: August 03, 2017, 05:15:19 am
content: Quote from: Alex Nikitin on August 02, 2017, 09:51:16 amAppreciate your effort in the simulation. I didn't do my homework in Zener Rf noise yet, but I will do in the next few days.Is this what you mean by the rf noise? because it has a frequency of 50Hz? The load in our circuit is 1.5K ohm.

author: Ashraf Almubarak
author_rank: Contributor
timestamp: August 03, 2017, 05:38:09 am
content: I did the simulation too, and I got your results, I never expect that. I will read more about this phenomena.Is this to pass the EMC tests?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 03, 2017, 08:31:08 am
content: Quote from: Ashraf Almubarak on August 03, 2017, 05:15:19 amQuote from: Ashraf Almubarak on August 03, 2017, 05:38:09 amRF noise is generated by Zeners in avalanche mode and it extends into hundreds of MHz, AFAIK. An AM radio in the vicinity of this circuit would pick up the modulated noise emission. So yes, the resistors should help to pass the EMC tests.CheersAlex

author: Ashraf Almubarak
author_rank: Contributor
timestamp: August 03, 2017, 08:46:21 am
content: Quote from: Alex Nikitin on August 03, 2017, 08:31:08 amThat is a precious information, I know that EMC tests cost a lot of money. If the designer knows the best practices to pass that tests, would make differents in the budget.thanks a lot.