3
\$\begingroup\$

I have a LED strip (5050) and a sine-wave AC power supply of a sufficient wattage and the voltage regulated in a sufficient scale. What would be a good design of driving the LED strip from this supply? I want to keep it simple.

My idea was to:

  • setup the voltage of the power supply to the level required by the output luminosity.
  • include a rectifier to keep "no-power" time as short as possible.
  • connect a capacitor parallel to the LED strip to prevent flicker and not to shorten LED strip life (because it's 50 Hz AC power). Is there a way how to calculate the capacity of the capacitor based on the LED strip power consumption? I know it has to withstand the max. voltage it connects to.

Is there anything else to make it better or more sustainable?

Edit:
This is a minor importance, but the final goal is connecting several LED strips in series and the power supply would be 220V AC electrical grid. The circuit will be with circuit breaker and an Earth leakage circuit breaker.

\$\endgroup\$
  • 1
    \$\begingroup\$ Be more specific than "AC power supply of a sufficient wattage and the voltage regulated in a sufficient scale". What does your LED strip need in voltage and current and what is your power supply rated for? \$\endgroup\$ – jippie Dec 3 '13 at 8:25
  • \$\begingroup\$ @jippie: See my edit. \$\endgroup\$ – Peter Ivan Dec 3 '13 at 10:44
  • \$\begingroup\$ I think the question was about the voltage required by the LED stripes, not the input voltage of your power supply. It's also important to know what is the power supply method that is recommended or sold by the stripe manufacturer. \$\endgroup\$ – Blup1980 Dec 3 '13 at 11:30
  • \$\begingroup\$ @Blup1980: The strips are rated 24V DC, which is not so important (I can connect 10, 11 or even more pieces is series, which is all OK - tested on DC). There are 3 questions, all specific about driving those strips from AC. \$\endgroup\$ – Peter Ivan Dec 3 '13 at 11:55
  • 2
    \$\begingroup\$ It may be worth reminding the casual observer that if the final situation involves a Mains Electrical supply then even with a circuit breaker and an ELCB in the supply the circuit must not be allowed to come into contact with an extraneous conducting item, like say a human body. At best the current will leak out of the circuit, thereby operating that ELCB, at worse it could kill someone. Good insulation will be required and the average strip does not have the sort of insulation needed at Mains voltages! \$\endgroup\$ – SlySven Sep 27 '16 at 21:18
3
\$\begingroup\$

There are more than a few ways to drive LEDs directly from AC mains, among them:

enter image description here

In 1 and 2, half of the LEDs will be driven during the mains' positive half-cycle, and the other half during the negative half-cycle.

With 50 Hz mains, each string will half-wave rectify the mains and flash at 50Hz (i.e. be on for around half the time), which is visually detectable and annoying, so that approach will be abandoned.

In 3 and 4, the mains is full-wave rectified, so the string will flash at 100Hz, which is visually undetectable and, therefore. acceptable.

For 24V 5050 strips to be connected across 240V mains, no current limiters will be needed and all that'll be required is to connect 10 of then in series across the 240V mains like this:

enter image description here

Likewise, for 12V strips to be connected across the main, 20 will need to be connected in series then connected across the 240V mains.

\$\endgroup\$
3
\$\begingroup\$

This is the same problem as any "how do I get DC from AC?" question. You rectify it:

schematic

simulate this circuit – Schematic created using CircuitLab

You can perform the rectification with four discrete diodes, as shown here, or you can purchase pre-made bridge rectifiers that are exactly this, in an integrated package.

C1 is optional, and serves to reduce the output ripple. Without it, the output bounces between 0V and some positive peak value, close to the peak-to-peak voltage of the AC input but reduced slightly by the forward voltage of the diodes in the rectifier.

ripple from hyperphysics

If you omit C1, then your LEDs will flicker at 100Hz (twice your AC input frequency), but you probably won't be able to see this.

If you want to include C1, to calculate its value, decide how much voltage decrease is permissible between cycles. Say that we decide the output voltage can ripple between 12V and 10V: C1 must then be able to supply the current necessary to run the LEDs between each half-cycle without decreasing the voltage by more than 12V-10V=2V. We can make some simplifying assumptions: that the LEDs are a constant current sink, and that the capacitor will fully recharge at each half cycle, and must supply all the current between cycles. Start with the fundamental behavior of a capacitor:

$$ i = C\frac{dv}{dt} $$

Replace the LED current (let's say 20mA) for \$i\$. Our peaks happen 100 times per second (twice the input frequency) so the capacitor must go 0.01s between recharges (\$t_{dis}\$), and we decided the allowable voltage drop in this period was 2V. That gives us a value for \$\frac{dv}{dt}\$:

$$ \begin{align} 20mA &= C \frac{2V}{0.01s} \\ \frac{20mA \cdot 0.01s}{2V} &= C \end{align}$$

We can check that a farad is equal to an ampere-second per volt:

$$ F = \frac{A\cdot s}{V} $$

and these are the units we have above, so we can conclude:

$$ C = 0.1mF $$

\$\endgroup\$
0
\$\begingroup\$

setup the voltage of the power supply to the level required by the output luminosity.

From what I can see, the required voltage of commercial strips of 5050 SMD LEDs is generally 12 V. Example. You do not alter luminosity by varying the voltage. You need a 12 V DC supply designed for driving LED strips.

include a rectifier to keep "no-power" time as short as possible.

DC power supplies generally contain a full-wave rectifier (four diode rectifies in a bridge arrangement).

connect a capacitor parallel to the LED strip to prevent flicker

You should provide the LED strip with DC. A DC supply generally contains a smoothing capacitor.


If your question is about designing your own PSU, you'd need to give some more details of the LED strip (or a link to a data sheet for it). You might want a switched-mode or linear PSU with a 240 V AC 50 Hz input and a 12 V DC output of (say) 150 W.

I'd just go out and buy one, but if you are experienced and aware of the safety issues, it shouldn't be difficult to design and build one for the purpose.

\$\endgroup\$
  • \$\begingroup\$ Yes, most 5050 LED strips are 12V, but mine are 24V (half the current needed). I'd like to avoid any low voltage DC sources (otherwise I wouldn't ask this question :). \$\endgroup\$ – Peter Ivan Dec 4 '13 at 8:23
  • \$\begingroup\$ For equal brilliance from the LEDs in a 5050 strip rated for 12V or one rated for 24V, the current through the LEDs must be identical. \$\endgroup\$ – EM Fields Jun 5 '14 at 11:43
  • \$\begingroup\$ EMFields: All the LED strips are rated for 24V, so there is no problem there. \$\endgroup\$ – Peter Ivan Jun 5 '14 at 13:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.