Capacitive dropper power supply design review - Page 1
Forum section: electronics / projects

author: shkhamd
author_rank: Contributor
timestamp: May 30, 2017, 09:43:55 am
content: Hi all,I am using this attached capacitive dropper power supply scheme on a BLE based AC dimmer project.My AC power is 220V rms and I am aware of the safety concerns for using such power supply.A BTA16 triac is switched through an opto triac from the ble and there are two other indicator LEDs in the circuit in total.The opto triac needs 15mA current and total current consumption is probably below 40mA (I am estimating this, not measured absolutely yet).I have setup the circuit in bread board to test without the MOV, C5, C6 and U5.I am using R2 value as 3W 330Ohm .I have noticed that,when the 4.7V zener diode is connected in the circuit,- R2 over heats.- the total circuit draws 74mA even without any load.If the 4.7V Zener is removed,- the current consumption and overheating stops.- but the voltage across the DC+ and DC- terminal of the rectifier bridge becomes 212V.So far I can deduce that the 74mA is passed through the 4.7V Zener diode and causes R2 to over heat.I am not sure if this is a normal behavior of such circuit.The overheating of R2 makes me worried to use it as a valid sub-circuit in my project.And also a no-load current of 74mA is no good since the target of such minimization is to reduce consumption and BOM cost.I am out of ideas.Could anyone please review the circuit for possible corrections and modifications?ThanksShaikh

author: oldway
author_rank: Super Contributor
timestamp: May 30, 2017, 01:42:31 pm
content: There is nothing wrong with your circuit....But C4 must be a X2 capacitor for safety.C4 combined with R2 have a total impedance of 3.184K at 50 Hz.At higher mains voltage (220V + 10%), current will be: {240V - (4.7 + 1.2V) }/3.184 K = 73.527 mAAt lower mains voltage (220V - 10%), current will be : {200 - (4.7 + 1.2V)}/3.184K = 60.961 mAMax power dissipation of R2 : (73.527mA)² x 100 = 540.32 mW or 0.540W...perhaps better to choose a 2W resistor instead of a 1W resistor.3.3V is a little at the low side to drive the opto triac, use 4.7V instead....Be careful, use an isolation transformer to test such a circuit.NB: why did you increased value of R2 ? 100R is ok....it is only limiting peak current at power on to protect bridge rectifier...max peak current of 3.4A seems ok.NB2: with no load, max power dissipation of 4.7V zener diode reach 345.58 mW...must be a 1W zener diode !

author: shkhamd
author_rank: Contributor
timestamp: May 30, 2017, 02:37:05 pm
content: I first prepared the schematic with 100R resistor, later I found out that I do not have any 3W 100R, but i have 330R.So, if I am not wrong, total dissipation at no load would be the sum of dissipation on the zener and R2, which is 885.90 mW right?This seems to me a huge waste considering a dimmer circuit silently always sucking 1W power.

author: oldway
author_rank: Super Contributor
timestamp: May 30, 2017, 03:00:53 pm
content: Quote from: shkhamd on May 30, 2017, 02:37:05 pm

author: DBecker
author_rank: Frequent Contributor
timestamp: May 30, 2017, 04:16:11 pm
content: Quote from: shkhamd on May 30, 2017, 02:37:05 pmCapacitive dropper supplies are close to obsolete, or at least have much narrower applicability. They are still simple and inexpensive, but potentially very inefficient. That inefficiency runs afoul of increasingly tight efficiency mandates (Energy Star, etc.)For a system that uses unpredictable burst power, look at the Linkswitch-tn (LNK302) and many similar competitive parts.

author: SeanB
author_rank: Super Contributor
timestamp: May 30, 2017, 05:25:16 pm
content: R1 should be either a 470k 0.5W high voltage resistor, or 2 470k 0.25W resistors in series, as most common 0.25W resistors are limited in voltage to 250VDC.As you are driving a triac, I would go for a slightly different approach and drive the triac in quadrant 4, sinking current to the negative rail with the positive rail being neutral. Most SCr and triacs are a bit more sensitive in this regime, so allowing you to have a lower standing current in the dropper, and as well use an interrupt to get a zero cross of the mains, and then use a few 1ms pulses of drive current to trigger the power device. This just requires a common positive rail, and a single NPN transistor and base resistor ( and a gate shunt resistor to keep from false triggering) to drive it, and you can then go from 1uF down to 220nF of capacitor.Make the negative supply 12V, and burn the rest in the regulator, so there is more ripple allowance, and use a 470uF 35V capacitor across the zener diode. 100uf will lead to brownouts, especially with a regulator running so close to dropout.

author: oldway
author_rank: Super Contributor
timestamp: May 30, 2017, 05:28:21 pm
content: Quote from: DBecker on May 30, 2017, 04:16:11 pmYou do not seem to have any knowledge or experience in the field of household appliances.I have been and I am still a volunteer in repair cafes for more than 5 years, I have repaired hundreds, if not thousands of gears with capacitive power supplies and I can tell you that they are far from being absolete ....There are millions of them in operation in all kinds of household appliances, vacuum cleaners, Coffee machines, mixers, etc ...And all these devices continue to be manufactured.https://repaircafe.org/en/Indeed, there are not many alternatives .... for powers as low as 231 mW (70mA 3.3V), a micro power SMPS power is of exaggerated cost and sophistication.The reliability of such micro power SMPS also leaves something to be desired.The losses in a capacitive supply is not so high ... there is above all a consumption of reactive power (capacitive) which is not detrimentalThe advantage is the extreme simplicity of the capacitive power supply which makes it easily repairable.With micro power SMPS, one of the greatest problem is that you rarely find the same integrated circuits in different devices and there are a lot of "exotic" integrated circuits hard to find.

author: oldway
author_rank: Super Contributor
timestamp: May 30, 2017, 05:44:17 pm
content: The advantages and security provided by snubberless triacs can hardly be denied ... especially with inductive loads.These triacs accept a very high dv/dt almost equivalent to scrs and is a huge advancement in triac technology.But these triacs MAY NOT WORK in quadrant 4 !!!!shkhamd intend to use an opto triac, this make the main triac works in quadrants 1 and 3.He already need 25mA for the opto triac, no way to reduce C4 from 1µf to 0.22µf.A 3.3V power supply seem to show that he intend to use a µc.His question is about the capacitive power supply, not about how to make a dimmer.QuoteAs input voltage is very high comparing to the supply voltage (only 4.7V + 1.2V drop of bridge rectifier = 5.9V) , time without feeding the capacitor is very short ( 122µs) and voltage drop during this time (40mA) is only 53mV so 100µf is enough.

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2017, 06:28:48 pm
content: Quote from: shkhamd on May 30, 2017, 09:43:55 amIt's possible you don't need an opto-TRIAC because it isn't isolated. You can probably drive a TRIAC directly, as mentioned above.Another thing to bear in mind is that the TRIAC only needs to be triggered for a very short length of time, just after the mains voltage has passed through the zero crossing point, so the average current consumption can be well under that required to trigger the TRIAC. All you need is a large enough reservoir capacitor to store the energy for long enough to trigger the TRIAC without the supply voltage dropping too low.

author: Seekonk
author_rank: Super Contributor
timestamp: May 30, 2017, 08:39:01 pm
content: Quote from: oldway on May 30, 2017, 01:42:31 pmJim Lewis would disagree. He says X2 capacitors are born to fail, but fail safe. Use X2 capacitors and the circuit will fail in a couple of years.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 30, 2017, 08:52:27 pm
content: There are several opto-Triacs that are reliably triggered with only 5 mA.Using one of those would cut your power dissipation by about 1/3.

author: bktemp
author_rank: Super Contributor
timestamp: May 30, 2017, 08:55:46 pm
content: Quote from: Seekonk on May 30, 2017, 08:39:01 pmIt is a bit like saying don't use a fuse, because then the circuit will stop working when there is a too high load current.If you use a good quality X2 capacitor it can last quite long.Most of the failed X2 capacitors I have seen were some cheap brands with a smaller case size than the replacement capacitors I could get. They probably used a thinner foil, so it will lose capacity much faster than good quality ones.

author: Gyro
author_rank: Super Contributor
timestamp: May 30, 2017, 09:03:21 pm
content: Quote from: Seekonk on May 30, 2017, 08:39:01 pmI don't know who Jim Lewis is but X2 capacitors are designed to present a low impedance path to high frequency currents between Live and Neutral, for the life of the equipmnent they are designed into. Just because a part is designed to fail safely under overload conditions doesn't mean that it's designed to fail early, any more than a fusible resistor is designed to fail in a properly functioning circuit.

author: Benta
author_rank: Super Contributor
timestamp: May 30, 2017, 11:12:46 pm
content: Who is Jim Lewis?This guy? :https://en.wikipedia.org/wiki/James_Paul_Lewis,_Jr.

author: oldway
author_rank: Super Contributor
timestamp: May 31, 2017, 10:19:27 am
content: If shkhamd want help for his project of BLE based AC dimmer, I suggest he open a new topic with all the informations we need to help him.About his capacitive power supply, I think he has all the informations needed.I don't know if he intend to project a commercial product, but if he don't use a X2 capacitor, he will never sell it in Europe.

author: Seekonk
author_rank: Super Contributor
timestamp: May 31, 2017, 02:33:12 pm
content: Capacitive dropping is not across the line. Cite the reg.This debunks the myth that only X2 capacitors should be used in capacitive dropping supplies. There was a post where X2 capacitors kept failing in a series application. This could be why. I contacted James Lewis, KEMET, about why the data sheet said not to use X2 capacitors in series with mains applications. The following is his response:"The actual issue is related to how typical X2 Capacitors lose capacitance over time. A typical film capacitor loses capacitance, over time, because of two effects: 1) corona and 2) corrosion. The voltage applied to most X2 capacitors tends to be in the 120-277 range. Corona occurs around 300 Vac and higher. So, while some capacitance might be lost because of corona effects, this isn’t the main concern. The real concern is that moisture will eventually egress into the package and oxidize the metallization. Some plastics, like polyester, more readily absorb moisture and accelerate this effect.In typical X2 applications where the capacitor is across-the-line, the capacitance loss isn’t a concern. First, the capacitance value isn’t critical and second, most people don’t care if the end-product passes EMI tests 4-5 years into its life.When used in a series-coupling or series supply application, however, the loss can be critical. Even a 10% loss in capacitance can result in not enough current for the supply.This comes down to (and this applies across the industry), most X2s were not designed with “stable capacitance” in mind. So the R46, our most popular, can easily lose up to 30% of capacitance in a high humidity environment after only a couple of years. Which is why we made series with “better” epoxies, thicker metallization, and series-electrode constructions. Their capacitance loss will be much less than a typical X2, making them more suitable."Not my idea, I just bought 2,000 X2 capacitors. Hope they are still good three years from now!

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 02:51:47 pm
content: I don't see the need for X2 capacitors here. Make sure the circuit has adequate over current protection, such as a fusible resistor and it should be fine. As the ballast capacitor has a fairly high value, it can be a couple of electrolytic capacitors connected back-to-back.

author: oldway
author_rank: Super Contributor
timestamp: May 31, 2017, 04:06:46 pm
content: I agree, if you don't use a X2 capacitor,you must provide an adequate over current protectionBut it is obvious that the circuit of shkhamdhas no over current protectionand therefore, he must use a X2 capacitor.

author: DBecker
author_rank: Frequent Contributor
timestamp: May 31, 2017, 06:44:28 pm
content: Quote from: oldway on May 30, 2017, 05:28:21 pmSure, they still exist. But so do tube radios. And plenty of equipment still uses mains frequency transformers, when a design refresh would certainly use a more complex SMPS.Capacitive supplies are obsolescent, not obsolete. They work well for limited input voltage ranges powering fixed loads. And they are still cheaper for certain low power applications when that's not the case. But energy efficiency mandates and the dropping cost of direct-switching supplies results in most new designs using a modern approach.Quote from: oldway on May 30, 2017, 05:28:21 pmReliability is tough to guess at. There are lots of examples of old electronics still working after decades of use. What you don't see is the utter crap that quickly failed, and the otherwise-robust eqiupment that had a single bad part or bad assembly technique.As for power supply repairability... that has moved into the realm of a hobby interest. As a design consideration it's a distant second to reliability (which could well be a distant second to cost and marketability/regulatory compliance).

author: oldway
author_rank: Super Contributor
timestamp: June 01, 2017, 04:57:49 am
content: For example, more than 25 millions Senséo coffee machines have been sold and all with a capacitive power supply .... they are still manufactured and sell by thousands .... And you compare that with tube radios?With regard to reparability, it is a problem of difference of point of view between Americans and Europeans.Europeans consider that waste and pollution must be limited because natural resources are not inexhaustible.That is why Europeans place more emphasis on sustainability and reparability than Americans.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 01, 2017, 07:12:39 am
content: The issue with capacitive dropper is that it's simplicity can be misleading; by removing isolation, it moves the complexity elsewhere, and does it in a potentially very dangerous way, across different design people in the team. (That is why you mostly see it in large-scale, simple products sold by millions by established companies.)So, usage of a capacitive dropper is an engineering decision which mandates total product-level evaluation; the end result may be cheaper, or more expensive.Simply put: when you power the coffee machine electronics with a capacitive dropper, the whole keypad design must conform to safety isolation specs (which may increase the cost, especially because the keypad may be used with wet hands). Also, products that require any type of I/O connectors will see increased cost.Of course, this works in products that are produced in millions, when they can optimize the product as the whole, and have enough resources and time to do that safely. Naturally, the capacitive dropper is used when it's cheaper. This is simply calculation and fact based engineering.Repairability is often a red herring. Certain "repair" people seem to be extremely happy when they have a product that fails all the time for some silly reason, simply because it's easy to repair (exactly because the reason for the fail was silly!). Then, another manufacturer makes a product that fails 100x more rarely because the trivial reason for the fail is fixed in design, but now the remaining issues are more difficult to repair, and the "environmentalist repairers" go all over the place about the unsustainability; when in reality, the "repairable" product ends up filling the landfill, because only a miniscule percentage ends up in the hands of the repair people.I'm all for repairing stuff, and always repair my broken electronics, but the reality is that repairability is thelastthing to do; designing for reliability is paramount if you care about environmental waste. Design a product thatvery rarelyfails. Simple differences in design may easily cause orders of magnitude difference in reliability.

author: oldway
author_rank: Super Contributor
timestamp: June 01, 2017, 08:12:20 am
content: In a household electrical appliance, there are many circuits that are connected to the mains network and the fact that the capacitive supply is not isolated does not change the "dangerousness" of the circuit ....The microcontroller controls the gates of 2 triacs directly (one for the pump, another for the boiler), without insulation ... The whole control circuit is therefore at the potential of the electrical network.Safety is ensured by the fact that the device complies with the safety standards for insulation class 1.There is simply no touch pad.The 3 plastic pushbuttons press on micro switches on the printed circuit board, assuring a secure insulation, even with wet fingers.In a dimmer based on BLE, the problem seems even easier since there is probably no command button.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 01, 2017, 08:23:38 am
content: Quote from: oldway on June 01, 2017, 08:12:20 amNo one said that. Capacitive supply expands the unisolated mains area, compared to using an isolated supply, and does it by making the control section live - which is against the (wrong) expectations of many people.Whether this matters or not depends on actual design. In designs with no interaction to outside world at all, like small LED light bulbs, this is a no-brainer.There is, however, a risk of inexperienced designers mostly working with isolated supplies overlooking this issue. Isolated supplies make most things easier to design, and more safe, so I'd always recommend an isolated supply for a hobbyist, or a small company with little experience on mass producing cheap household items safely.Not going to continue this offtopic any further.

author: oldway
author_rank: Super Contributor
timestamp: June 01, 2017, 08:34:57 am
content: In terms of reliability of micro-power SMPS, to deal with ultraminiatures ferrite transformers is a major problem because the required insulation safety distance remains the same as on large transformers but become incompatible with the dimensions of the transformer.I note that the manufacturers adopt the SMPS solution when the consumption of the control circuit is high and no longer allows to use a capacitive power supply.At 220V230V, the maximum value of the current limiting capacitor found in home appliances rarely exceeds 1µf.QuoteAn inexperienced designer SHOULD NEVER BE ALLOWED TO WORK WITH CIRCUITS AT MAINS VOLTAGE !!!!!..This is looking for troubles and accidents....

author: Zero999
author_rank: Super Contributor
timestamp: June 01, 2017, 08:59:29 am
content: If I remember rightly, in the EU standby power consumption for most appliances should be under 1VA. If there's no power switch, the maximum current for a capacitive dropper can be no more than 4mA.Most appliance manufacturers don't bother designing their own switch mode power supplies. They buy them from another supplier.

author: bktemp
author_rank: Super Contributor
timestamp: June 01, 2017, 09:05:46 am
content: Quote from: Hero999 on June 01, 2017, 08:59:29 amI couldn't find any reference to that, only 0.5W since 2013. 1VA sounds too low, because that would be almost impossible for most SMPS because of the required filtering at the input.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 01, 2017, 02:21:59 pm
content: Quote from: Siwastaja on June 01, 2017, 07:12:39 amWise words!I fully agree with you.Unfortunately, nowadays the trend or expectation is that many appliances should only last a couple of years.For the simple reason that planned obsolescence will render many consumer electronics to be discarded, even if it they are still operating.So...if your gadget is going to be replaced anyways, what is your paramount design objective?Low cost of course!That not only includes the design of the product itself, but also the design on how it is going to be manufactured and assembled. Most disposable consumer products no longer utilize screws. Ultrasonic welding, snap-in parts or adhesives are the norm. Taking apart such a product requires damaging its exterior.