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I found some articles talking about transformerless power supplies, and I have uploaded the schematic of such a circuit. Does anybody have an idea about the pros and cons of such a power supply? Also how to calculate the value of this capacitor "C1" ?

enter image description here


marked as duplicate by winny, Enric Blanco, Voltage Spike, PeterJ, JRE Jun 25 '17 at 16:41

This question was marked as an exact duplicate of an existing question.

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    \$\begingroup\$ This is a non-isolated supply, not hobbiest friendly imho... \$\endgroup\$ – sstobbe Jun 21 '17 at 2:56
  • \$\begingroup\$ Typical design equations for transformerless supplies can be found in this app note: ww1.microchip.com/downloads/en/AppNotes/00954A.pdf \$\endgroup\$ – replete Jun 21 '17 at 3:08
  • \$\begingroup\$ @sstobbe Great thing to consider \$\endgroup\$ – Bodda Jun 21 '17 at 3:41

I've seen these done thusly


simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ C1 should be an X1. We've talked about this before. \$\endgroup\$ – Robert Endl Jun 21 '17 at 4:26
  • \$\begingroup\$ Was this answer wrong? @RobertEndl \$\endgroup\$ – Bodda Jun 21 '17 at 4:49
  • \$\begingroup\$ Sorry, X2. X1 probably overkill. I didn't see the X1 in the schematic cuz I'm getting old and careless. \$\endgroup\$ – Robert Endl Jun 22 '17 at 6:00

Polypropylene power supplies are a type of capacitive power supplies that supplies a limted current output. The value of the polypropylene capacitor determines the amount of current according to this equation:

X = 1 / (2 π f C)

I = V / X


X : capacitor reactance

π : Constant (pi) = 3.14

f : frequency of the main supply (50-60)

C : Capacitance value

I : output current (A)

V : main voltage (110 or 220)


  • constant current power supply is better in some applications like LED Lighting
  • wide output voltage range (typically any rated voltage device is able to connect -unless it stands the applied current- )
  • More efficient series connections are possible in some applications like LED lighting


  • Non-isolated power supply which means rush current can easily destroy your load
  • low output current values (Max ~ 350 mA) due to low available capacitance values. or huge number of parallel connected capacitors (10) to output a current of about 1 Amp.
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    \$\begingroup\$ Why was this answer accepted within a minute of being posted? This discourages competing answers, which improve the quality of the site. \$\endgroup\$ – replete Jun 21 '17 at 3:48
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    \$\begingroup\$ -1. Please explain why you think the circuit is a constant current supply. The circuit limits current by the reactance of C1 but it is not a constant current supply. Instead, it is a loosely voltage-regulated supply. \$\endgroup\$ – replete Jun 21 '17 at 3:54
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    \$\begingroup\$ The terminology is wrong. A constant current supply responds to varying load impedance by varying its output voltage. This supply does nothing of the sort, it loosely maintans a constant output voltage across C2 and will respond to varying load impedances by sourcing varying currents. \$\endgroup\$ – replete Jun 21 '17 at 4:04
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    \$\begingroup\$ been seeing a lot of these capacitor-based CC drivers recently being incorporated smack dab onto the COB's PCBs, thanks for the rundown on this interesting tech. \$\endgroup\$ – dandavis Jun 21 '17 at 4:16
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    \$\begingroup\$ -1 It isn't a constant current supply. It is a power supply with a limited current output. Look closely at the given diagram. It includes a zener diode to regulate the voltage. \$\endgroup\$ – JRE Jun 21 '17 at 5:34

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