Capacitors warming in mains power supply

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Sumsum

Joined Nov 10, 2012
4
I built a simple, direct-coupled mains power supply.
I wish to get on my load about 6V and 20mA DC. My mains is 220V AC.
I used a 1uf 250V capacitor, a bridge, a resistor, and two 100uF tantalum capacitors.
The 1uf /250V capacitor (NP) works well, and I got about 7VDC. However, I need a conderably smaller physical size capacitor.
I "built" a capacitor made of several 1uf/50V capacitors in series and parallel, so I got 1uf / 250V. Connecting it, the output was only 4.2V.
The caps temperature was about 45 degrees Celsius.
I replaced the 30(!) capacitors with 3 capacitors X 3uF / 100V in series. The temperature increased to approx 100*C! The voltage on the load also increased to 7.2 V DC.
I tested the capacitors, I replaced them with 3 new ones - but they are warming to more than 100*C.

Can someone suggest what happens? Why the 50V capacitors barely warm, and the 100V are warming as hell?
Thank you in advance for helping.
 

wayneh

Joined Sep 9, 2010
17,152
View attachment 227773
This is a simplified image. I need to get at the output 6-7V and 30mA.
The 1uF is my problem, as I have very limited space. Of course, I know about the X issue, however, I must use the smallest capacitor that I can find.
So, connecting 3 X 3uF 100V in series looked to be the best solution, however the temperature is very high.
I cannot understand why they behave this way.
I'll be very thankful for your explanations.
They're hot because they're small, have an internal resistance, and are thus dissipating power over a small surface area. Caps rated for low ESR may help but it really needs to be an X to be safer.
 
In a mains capacitive-dropper application, the capacitor's ripple current is extremely high, which is why they are heating up. Best to LTSpice simulate your circuit, you will see the problem.

Also you must use capacitors specially intended for capacitive-dropper applications or "series impedance applications" as Vishay calls it. Using X-capacitors is not enough- they will have a short lifetime. See Vishay App. Note "AC Film Capacitors in Connection with the Mains" see F1772 series.

You are missing a fuse, the 100R resistor must be flame-proof and fusible, if the LED open circuits the 100uF caps will get damaged. Sometimes it's just better to use a small power transformer.
 

neonstrobe

Joined May 15, 2009
145
Not only the above but also the output has to be isolated from any metal and potential contact with anyone with at least two levels of insulation.
Equipment has to be double isolated or earthed. The only capacitor coupled objects I've seen recently are LED night lights which have a plastic bulb (possibly glass) which prevents contact with the LEDs.
There are some 1W transformers available that would be safer and probably more reliable. I've had a 9V one with 50mA output that is still working 30 years later. Capacitors tend to blow - that is why you need a low ESR and suitably rated for AC mains.
 

Thread Starter

Sumsum

Joined Nov 10, 2012
4
Thank you all for replying.
I know about the potential dangers of using "simple" SMD capacitors.
I cannot use X capacitors, nor transformers, as the space is very limited.
However, the question was about the difference between temperatures. Using 30 x 0805 1uF / 50V capacitors - and resulting an 1uF /250V = relative low temperature, and using 3 x 3.3uF/100V 1206 in series resulting in very high temperatures.
All I need is a logical explanation...
The load is the same, in both cases. Using the 0805 I get ~4.2VDC. Using 1206 I get ~ 7VDC.
I measure the capacitors with a capacitance meter, and they are approx. 1uF.
Thank you in advance!
 

ericgibbs

Joined Jan 29, 2010
12,902
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