Simple Way to Light a String of LEDs

Had been struggling to come up with a power supply to light a string of LEDs an agarose gel transilluminator. The approach in this instructable really worked well. It requires a few capacitors, two diodes, and a power resistor and that is it. You get a constant current, full wave rectified power supply for approx. 100 LED in series. The video summarizes the construction.

The circuit diagram shows a voltage doubler where AC voltage (110V in US) is split into two paths to charge two capacitors. The left capacitor is charged to the peak voltage of ~ 150V on the positive cycle of the AC voltage as the diode above this capacitor only allows the positive voltage to flow through. The capacitor on the right is charged to ~ 150 V on the negative cycle of the AC voltage. The diode in series with the right capacitor only allows the negative voltage to flow through. The difference in voltage between the two capacitors becomes 300V (actually lower when LEDs are connected).

By connecting a string of LEDs through a quenching resistor (approximately 100 ohm 3 W) between the two capacitors a constant current will flow lighting the LEDs. The amount of constant current is defined by the value of the capacitors. 1 uF gives ~ 20-25 mA. Larger capacitors will give more current. 1 uF has a resistance of ~ 2.6 k ohms at 60 Hz. 10 uF has a resistance of ~ 265 ohms. So by ohms law (V=IR), for a 1 uf capacitor, the maximum current will be - 110 V/2650 ohms = 41 mA. For a 10 uF capacitor, the maximum current will be 415 mA (110/265), great for high power star LEDs!

Note - you cannot use electrolytic capacitors as they will burst when connected to AC. You will have to use X2 capacitors that are rated for AC use. These are typically motor start capacitors and in some cases high quality audio capacitors (non-polarized high voltage).

I connected it to a bread board with the LEDs in series as shown in the closeup. The breadboard wiring diagram is a bit more clear. And then hooked it up to AC power. Breadboards are not designed for 110 Vs so only use them for a quick check. Power on and the LEDs come on.

Have to attach an ammeter in series with the LEDs to see how much current actually goes through but other than that it is time to make this into a proper LED illuminator in a suitable insulated enclosure with fuses etc that will protect the user from the live AC.

Again, remember the circuit is connected to live AC. Be very careful at what you connect and how you connect. You can seriously hurt yourself.