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7/13/2015 Calculating Capacitor Current in Transformerless Power Supplies | Homemade Circuit Projects
http://www.homemadecircuits.com/2015/01/calculatingcapacitorcurrentin.html 1/9
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Monday, January 26, 2015
Posted by Swagatam Majumdar
Calculating Capacitor Current in Transformerless PowerSupplies
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You may have studied countless transformerless power supplies in this blog and in theweb, however calculating the crucial mains capacitor in such circuits has always remainedan issue for the many constructors.
Before we learn the formula for calculating and optimizing the mains capacitor in atransformerless power supply, it would be important to first summarize a standardtransformerless power supply design.
The following diagram shows a classic transformerless power supply design:
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7/13/2015 Calculating Capacitor Current in Transformerless Power Supplies | Homemade Circuit Projects
http://www.homemadecircuits.com/2015/01/calculatingcapacitorcurrentin.html 2/9
Referring to the diagram, the various components involved are assigned with the followingspecific functions:
C1 is the nonopolar high voltage capacitor which is introduced for dropping the lethalmains current to the desired limits as per the load specification. This component thusbecomes extremely crucial due to the assigned mains current limiting function.
D1 to D4 are configured as a bridge rectifier network for rectifying the stepped down ACfrom C1, in order to make the output suitable to any intended DC load.
Z1 is positioned for stabilizing the output to the required safe voltage limits.
C2 is installed to filter out any ripple in the DC and to create a perfectly clean DC for theconnected load.
R2 may be optional but is recommended for tackling a switch ON surge from mains,although preferably this component must be replaced with a NTC thermistor.
In the entire transformerless design discussed above, C1 is the one crucial componentwhich must be dimensioned correctly so that the current output from it is optimizedoptimally as per the load specification.
Selecting a high value capacitor for a relatively smaller load may increase the risk ofexcessive surge current entering the load and damaging it sooner.
A properly calculated capacitor on the contrary ensures a controlled surge inrush andnominal dissipation maintaining adequate safety for the connected load.
The magnitude of current that may be optimally permissible through a transformerlesspower supply for a particular load may be calculated by using Ohm's law:
I = V/R
where I = current, V = Voltage, R = Resistance
However as we can see, in the above formula R is an odd parameter since we are dealingwith a capacitor as the current limiting member.
In order to crack this we need to derive a method which will translate the capacitor's currentlimiting value in terms of Ohms or resistance unit, so that the Ohm's law formula could besolved.
To do this we first find out the reactance of the capacitor which may be considered as theresistance equivalent of a resistor.
The formula for reactance is:
Xc = 1/2(pi) fC
where Xc = reactance,
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7/13/2015 Calculating Capacitor Current in Transformerless Power Supplies | Homemade Circuit Projects
http://www.homemadecircuits.com/2015/01/calculatingcapacitorcurrentin.html 3/9
pi = 22/7
f = frequency
C = capacitor value in Farads
The result obtained from the above formula is in Ohms which can be directly substituted inour previously mentioned Ohm's law.
Let's solve an example for understanding the implementation of the above formulas:
Let's see how much current a 1uF capacitor can deliver to a particular load:
We have the following data in our hand:
pi = 22/7 = 3.14
f = 50 Hz (mains AC frequency)
and C= 1uF or 0.000001F
Solving the reactance equation using the above data gives:
Xc = 1 / (2 x 3.14 x 50 x 0.000001)
= 3184 ohms approximately
Substituting this equivalent resistance value in our Ohm's law formula, we get:
R = V/I
or I = V/R
Assuming V = 220V (since the capacitor is intended to work with the mains voltage.)
We get:
I = 220/3184
= 0.069 amps or 69 mA approximately
Similarly other capacitors can be calculated for knowing their maximum current deliveringcapacity or rating.
The above discussion comprehensively explains how a capacitor current may becalculated in any relevant circuit, particularly in transformerless capacitive power supplies.
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7/13/2015 Calculating Capacitor Current in Transformerless Power Supplies | Homemade Circuit Projects
http://www.homemadecircuits.com/2015/01/calculatingcapacitorcurrentin.html 4/9
Zero Crossing ControlledSurge Free Transformerless
Power Supply
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Supply Circuit
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Based
More posts about Transformerless Power Supply
at Monday, January 26, 2015 Labels: Surge Suppressors, Transformerless Power Supply, Tutorial
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27 comments:
Hz021 February 2, 2015 at 6:53 PM
GREAT!.. thank you Mr Swagatam Majumdar. :)
Reply
Swagatam Majumdar February 2, 2015 at 8:27 PM
you are welcome Hz!
kostas March 22, 2015 at 12:58 AM
Firstly i would like to thank you for your schematic it was really usefull and yourexplanation is great ! But i would like to make a question about the current. I did make the circuit and iused a 7805 also in the output and i did supply an other pic microcontroller circuitwith voltage. BUT i measured the current betwen the AC power and the C1 and itwas always AC 220mA (it's maximum because i use 3,3uFcap at 250V) and wheni measure the DC current after the 7805 it was draining only DC 47mA.Can you explain me what happens ? Because if leave the device open for longtime the cap the diode bridge and the 7805 get hot.I would provide you with a schematic but i didn't know how to post the Picturehere.Regards, Kostas.
Swagatam Majumdar March 22, 2015 at 6:53 PM
Thank you kostas! If you ask me about your design I would suggest that it's notrecommended and is dangerous to use a capacitive power supply for sensitivecircuits, I would advise you go for any smps adapter instead.
as for the current difference, the issue may be due to the zener diode or the 7805which are itself draining all the current and allowing only 47mA to pass to thecircuit, try removing the zener and check, or you may want to try a transistorcommon collector stage instead of the zener and the IC for achieving the requiredregulation without dropping much current
(***)(impetuos)(***) February 17, 2015 at 12:01 AM
Hello Mr, I was reading your interesting post and I was wondering about the C1 value, I'mtrying to get work a 60led string (3,2V 150mA) but the simulation doesn't work, mycalculation is for 120V 60Hz and the C1 value is 3,3uF, also the C2 I use 15uf 250V but is
exclusively designed and developed by me.
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not working at all, I have a severe wave of VDC I mean is not continue , what could bewrong? Also I tried to get a voltage doubler but the signal on VDC is the same as yourcircuit simulation
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Swagatam Majumdar February 17, 2015 at 10:54 AM
Hello Mr. here are the calculations:
Since the supply is 120V you will need to make two strings of 30 LEDs each andconnect them in parallel, each having a 22 ohm series resistor
Therefore the total current requirement would become 300mA.
As per the calculations a 1uF caapcitor at 60Hz/120V would produce 45mAcurrent, therefore for achieving 300mA we would need 300/45 = 6.66uF capacitor
For Z1 you may include a 120V zener diode or simply eliminate it.
C2 could be a 15uF/250V no issues.
vasudevan v May 27, 2015 at 11:02 PM
Swagatham Majumdar,If I need output 3 V DC or 6 V DC what value of C!, C2 , R1 and R2 to have in thecircuit. Pl explain
RegardsVasudevan V
Swagatam Majumdar May 28, 2015 at 8:44 AM
Vasudevan, voltage can be controlled only by using zener diodes or volatgeregulator ICs, C1 decides the current not the volatge, 1uF/400V will allow you toget around 70mA current.
R1 can be 1M it's not relevant to the output, R2 is also not crucial can beremoved if the current specs of the output matches with the C1 max capacity.
An NTC or MOV is a must and should be considered.
Naresh Jain February 18, 2015 at 11:04 PM
sir please explain:Xc = 1/2 x 3.14 x 50 x 0.000001
= 3184 ohms approximatelyon solving above formula it is 0.0000785 not 3184
Reply
Swagatam Majumdar February 19, 2015 at 12:18 PM
Naresh,
the numerator 1 is for the entire denominator, calculate in the following manner:
1 / (2 x 3.14 x 50 x 0.000001)
Naresh Jain February 19, 2015 at 2:25 PM
Lot of thanks
Reply
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manpreet singh February 25, 2015 at 4:50 PM
hello sir I am using the same ckt but driving 24 led in series C1 is 2.2uf or R1 is 470E butcap C1 get blast after few time can u help me
Reply
Swagatam Majumdar February 25, 2015 at 8:39 PM
hello manpreet, what's the wattage of the LEDs?
.....by the way the capacitor must be nonpolar and rated at 400V
manpreet singh February 26, 2015 at 10:42 AM
I am using .5 watt smd led and polyester film cap is 225j
Reply
Swagatam Majumdar February 26, 2015 at 6:18 PM
try a 1uF/400V PPC caapcitor
manpreet singh February 27, 2015 at 1:28 PM
Thank you sir but Iam using 5730 smd .5 watt led Forward Current is 150 mA and accordingto the above calculation 1uf/400V at 250V/50Hz able to provide 69 mA current.and the other issue I have LED heating to much for this I remove loading resistor and put 22ohm resistance in the series it work for 4 hrs.
Reply
Swagatam Majumdar February 27, 2015 at 5:04 PM
you may use 2.2uF capacitor but make sure its 400V rated and is nonpolar, andalso don't forget to put an NTC thermistor in series with the input of the capacitoras described here:
http://homemadecircuitsandschematics.blogspot.in/2013/02/usingntcresistorassurgesuppressor.html
R1 should be 1 Meg Ohm or any other higher value
manpreet singh February 27, 2015 at 5:56 PM
Thanks for ur respons sir 1 more think i want to know how to calculate the loading resistorvalue in the ckt.
Reply
Swagatam Majumdar February 28, 2015 at 5:04 PM
According to me you can use Ohm's law for this:
R = V/I, V = 330, and I = LED current
pradeep March 30, 2015 at 11:23 PM
sir is load resister and R2 are same ? if not how to calculate it ? how v=330 afterbridge rectifier ?
7/13/2015 Calculating Capacitor Current in Transformerless Power Supplies | Homemade Circuit Projects
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Swagatam Majumdar March 31, 2015 at 3:12 PM
pradeep the load resistance is not crucial, it should a low value resistor just torestrict the switch ON surge.
the peak value of 220V RMS is around 330V...after rectification the RMS turnsinto peak value in the form of 330V DC
pradeep March 31, 2015 at 7:54 PM
thanks sir for reply,my basic knowledge about analog electronics is not so strong, i want to know thatif i will get 220V RMS value after C1, then what is purpose of C1. how and whereac voltage is dropping.
Swagatam Majumdar April 1, 2015 at 10:06 AM
the capacitor restricts the 220V current to a very small level, when a load whichhas voltage requirement much lower than 220V but current much higher than thecapacitor output is connected, then the load acts like a shunts or shorts thecapacitor output until the 220V volts drops to the load voltage level.
Saqib Mehmood July 3, 2015 at 5:06 PM
Sir as I know that c1 control current then if current is 15 ma and my out putcurrent requirement is also 15ma then are u sure that at the out put we will get15ma
Swagatam Majumdar July 3, 2015 at 7:42 PM
yes that's a sure thing...the current control will be constant but during switch ON itcan increase in the form of a instantaneous spike.
shubham bansal July 7, 2015 at 2:20 PM
sir, can we simulate this circuit on multisim software?because i think, in this software the available capacitor is electrolytic capacitor.and we are using 105/440v PPC capacitor.if we cant simulate on multisim software so please guide me on which software we cansimulate this circuit.and how can we calculate the value of voltage and current after the bridge rectifier.
Reply
Swagatam Majumdar July 7, 2015 at 6:06 PM
Hi Shubam, I never use simulators so I won't be able to suggest much aboutthese softwares.
the voltage after the bridge is solely determined by the zener rating, if you removethe zener it will jump to 330V DC, although the meter would show the reading thatcould be equal to the filter capacitor's voltage rating.... but that would mean thecapacitor being subjected to a significant electrical stress....so the zener is acrucial part for controlling the output voltage to the required levels
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