Life expectancy of a Capacitive Dropper ?

[Swaroop 21]

How much life expectancy can I get from a Capacitive dropper using a X2 Capacitor, the circuit will be always powered on and only be off in case of a power outage. The circuit draws about 2mA at 5V.

[Phil1977]

X2 caps are intended to be directly connected to mains, so a good X2 cap in a dropper will live longer than the components around it.

If I had to repair capacitive droppers then mostly the inrush limiting resistors failed, followed by any rectifier or zener diodes.

If you can size the resistor in a way that it resists double the peak voltage of the input voltage then you should be fine.

[coppice]

The main problem with cap drop supplies is people only designing and testing them for a clean sine wave mains. Get some serious distortion from a UPS, or in some industrial area, and the low impedance at the harmonics can really get things toasty. If the resistor is not way oversize, and raised from the PCB, the designer probably didn't take their job seriously.

[Benta]

Depends on how clean/dirty your mains line is.
If there are lots of spikes, you'll experience punch-through in the X2 cap. That's not catastrophic, but will reduce its capacitance over time.

[ejeffrey]

In normal conditions it should last basically forever.  Similar capacitive dropper supplies are used in smoke alarms where long life and reliability are needed.  They still can fail if there is a surge so include a fusible resistor or fuse.

[SeanB]

I would suggest, as you only need 2mA, to forget about the capacitive dropper, and instead simply go for a resistive dropper instead. Use 2 10K 5W ceramic resistors in series, and a simple bridge rectifier and 12V 1W zener diode, along with a 470uF 25V capacitor, to give a roughly regulated 12V supply, that then uses a low current 5V regulator to give 5V. Cheap, long lasting, and probably the same volume overall, and will last decades in service. Yes has around 3W of power dissipation in use, not exactly the best, but will work over the full range of your nominal 230VAC mains, all the way from around 150VAC to around 270VAC with no problems.

Otherwise with a capacitive dropper you will need a series current limiting resistor, around 470R 5W, and a 275VAC VDR after the resistor, and between the capacitor and the common rail, to limit voltage spikes that will damage it, and the resistor will limit energy fed into the capacitor to increase life. Might do 2 years with a cheap X2 capacitor that is not a fake, and up to 10 years with a good brand non fake one.

[Swaroop 21]

Well, I didn't think about the Resistive method I will have to try it out first as how hot does it get. I don't have a scope to look at the mains but it is not very smooth and as everyone said it may affect the life of the cap itself. If I plan to use the capacitive dropper I will add the inrush limiting resistor and also MOV to reduce the effect of the spikes. 

[wasedadoc]

I've had cases of commercial equipment (ie not designed nor constructed by me) with those "yellow box" film caps used as droppers which have lost half capacitance in about 5 years.

[Phil1977]

I just took apart one 230V-AC noise filter embedded in a warm water circulation pump. It was installed for around 20years and had its own timer/regulator so that it was always on.

It contained two 330nF EPCOS X2 caps. I tested them first with 2500V DC then with the LCR-meter. HV-Test was okay, capacity was in the 10%-tolerance, ESR was negligible.

Okay, I see, we now have two extreme examples. Anyhow, a good capacitor should do the job, though a resistive dropper may be more reliable in total.

[Gyro]

Strictly speaking, you don't need to use an X cap. X rated caps are designed not to cause a fire risk in an across the mains situation, where there may be no 'useful' upstream protection (eg. just a 13A fuse or 16A breaker). They may also need 'across the mains' fault energy to be available in order to self-heal (depending on value).

X caps tend to be used in dropper applications because they are cheap and voltage rated, but their thin metalization does make them prone to losing capacity on transients. You can use a standard high voltage rated film capacitor, but under following restrictions...

1) You must have a decently rated low voltage overvoltage clamp after the cap, which means bigger zener or TVS.
2) You must include suitable overcurrent protection in series (mains side) with the capacitor to protect against transients. This can be a current limiting resistor and suitably rated fuse, or preferably, a fusible resistor. These should limit the prospective fault current to less than the zener / TVS can handle.
3) A mains VDR is helpful in preventing the series protection device from failing. Not essential but highly desirable. It also needs to be after the fuse.

This approach is typically a bit more expensive, but will give longer capacitor life. Another compromise is to use an X1 rated cap (I would always specify an X1 rather than X2 in a dropper). The higher voltage ratings will make it last longer but it will be physically larger than an X2, you still need to include the series transient limiting resistor, as SeanB says, preferably fusible.

[Wolfram]

X2 caps can be problematic, mainly through tempereature-humidity-bias (THB) induced capacity degradation. X caps didn't traditionally have any lifetime requirement, but were optimized for size, cost and good self healing properties, so thin zinc metallization was used, making them susceptible to corona damage over time in the presence of atmospheric moisture.

New requirements are in place now for applications where documented reliability is needed. The common requirement is 1000 hour life at 85 C / 85 % RH. When using caps that are not specified to meet this requirement, lifetime can be a lot shorter. You would need to investigate further if you want to see how the lifetime scales to more realistic operating conditions, but I have seen many cases of equipment fed by capacitive droppers failing due to loss of capacitance, and even more cases of salvaged X caps measuring a fraction of their marked capacitance, so this is not just an academic point.



https://www.signal.com.tr/pdf/cat/HJC-WP-XY.pdf
https://www.kemet.com/en/us/technical-resources/rfi-x2-capacitors-for-high-humidity-environments.html

[wraper]

X2 caps can be problematic, mainly through tempereature-humidity-bias (THB) induced capacity degradation.

Yes, but it mostly goes unnoticed because in normal use case their degradation does not impact device functionality, so people automatically think they're way more reliable tan they actually are.

[Hydron]

I've also fixed things where the failure has been a massive loss of capacitance from the yellow-box type X capacitors. One that springs to mind is a coffee machine - really does need a decent temperature/humidity rating in that use!

[Swaroop 21]

Thanks, my caps are not epcos braded, I just purchased them here in the local electronics market so mostly China based caps.

[Swaroop 21]

Hi there, I said x2 caps because they are the ones I currently have on hand. Interestingly thanks for the differentiation of the capacitor classes. Couple of years ago I used to teardown those rechargeable torches from China which used the "Brown" Polyester Film caps in the dropper circuit rated at 400V and typically ranging from 680n-1u in value but as many torches I have teared down I found those "Brown" caps bulged or deformed in just a year or two. So due to it I usually tend to avoid using those.

[Swaroop 21]

From the comments I read, the biggest culprit for loosing the capacitance of X1/X2 caps are transients. So as per recommended by many to use transient suppression in my circuit. Which means I can put a MOV and maybe a 250-270V Bi-TVS after the inrush resistor and fuse but before the cap ?

[wraper]

Hi there, I said x2 caps because they are the ones I currently have on hand. Interestingly thanks for the differentiation of the capacitor classes. Couple of years ago I used to teardown those rechargeable torches from China which used the "Brown" Polyester Film caps in the dropper circuit rated at 400V and typically ranging from 680n-1u in value but as many torches I have teared down I found those "Brown" caps bulged or deformed in just a year or two. So due to it I usually tend to avoid using those.

You need at least 630V DC rating for it to be reliable if using usual film capacitors.

[wraper]

250-270V Bi-TVS after the inrush resistor and fuse but before the cap ?

Guaranteed to fail with magic smoke.

[soldar]

I have had a few drop caps lose capacitance. See my thread
https://www.eevblog.com/forum/repair/repairing-panel-volt-amp-meter/msg2110735

[Doctorandus_P]

Some time ago (years?) Dave Jones did a video about an appliance with a capacitive dropper and a follow up with an capacitor expert about failure modes of capacitors. There are quite a lot of failure modes over time due to production quality issues of foil capacitors, such as oxidation of the metalization layer. That video is one of the best Dave ever made and really worth watching if you're interested in this sort of thing.

Also, during switch on, if a circuit is switched on at the peak of the voltage, then the capacitor is momentarily just a 0 Ohm resistor, and you have to design the circuit in such a way that this current peak can be handled. For example two LED's anti-parallel with a capacitive dropper does not work. It can mean instant death for a LED when switched on at the peak of mains voltage. You need to at least add a series resistor to limit the LED current to a few hundred mA to have them survive the first handful of milliseconds after the switching event.

It's a nice thing to build on a breadboard though (with the usual precautions for working with mains voltage). Just start with two leds, a capacitor and a switch, and every now and then a led will die, so make sure you have a bunch of spare LED's. Then add a series resistor and observe that the LED's don't die anymore.

[Swaroop 21]

Ohh yes I do remember, it was about a heater where the element was driven with a relay which had a capacitive dropper which the cap with time lost its capacitance which was then not able to power the relay.

[Swaroop 21]

Ohh no 
higher voltage rating tvs and mov or wrong placement ?

[wraper]

Ohh no 
higher voltage rating tvs and mov or wrong placement ?

When you see 250V AC rating on a varistor, it's a working AC Voltage. Nominal peak voltage for 230V AC mains is 325V, not counting tolerance and transients. And varistor withstand voltage is significantly higher than, also varistor has softer clamping than TVS. When you see 250V rating on TVS, it's a DC withstand or clamping voltage

[5U4GB]

If the resistor is not way oversize, and raised from the PCB, the designer probably didn't take their job seriously.

Ugh, flashbacks of 1990s Compaq desktop PC power supplies, which powered some sort of circuitry through an always-on cap dropper, and for which the first step when it wouldn't power on any more was to replace the 1/4W or whatever resistor sitting on the surface of the PCB with a 1-2W one with plenty of space around it for air flow.

E&OE, it was 30 years ago.

[elektryk]

I've seen a lot of these: