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README.md

README.md

#437 LED Drivers: QX5252 Joule Thief

Using a QX5252/YX805 LED driver chip as a joule thief for driving an LED from a low-voltage battery.

Build

Here's a quick demo..

clip

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Notes

I recently "discovered" solar LED driver ICs that are mainly designed to drive solar-powered lights, such as the QX5252 and YX805. They come in TO-94 and DIP-8 packages, and most applications only require an additional external inductor.

I've since used these chips in some circuits like the LEAP#436 BEAM Vibrabot.

The QX5252 and YX805 turn out to be identical in form and function, but since I was more successful in finding resources for the QX5252, I will mainly refer to that product.

The core features include:

  • a switching circuit for voltage boost from 0.9-1.5V to the 2-3V required to drive an LED (similar to a joule thief circuit)
  • charging a rechargeable battery from a solor cell, with integrated Schottky Diode for reverse polarity protection
  • over-discharge protection
  • light control switch, only available in DIP-8 package - requires external photocell to achieve a similar result with TO-94 package

Key specifications:

  • Operating voltage: 0.9V-1.5V
  • Output current: 3mA-300mA

The primary configuration is as follows:

  • rechargeable battery e.g. NiMh is the main power source for the LEDs
  • a fast oscillation (100kHz +) on the LX pin turn the inductor into a boost converter to drive the LEDs at sufficient voltage
  • a solar cell will charge the battery (no fancy charging circuitry)

primary_configuration

The Simple "Joule Thief" Configuration

As a test and demonstration, I'm ignoring the solar charging aspect here, and just use the chip as a "joule thief" to drive an LED with a battery that has a lower voltage than the forward voltage of the LED.

To make things a little interesting, I'll build this as a free-standing circuit that just clips onto the battery.

The battery I'm using here is an AG8 (aka LR1120, 191) rated at 1.55V, and I confirmed it cannot directly drive the LED I'm using - a nice blue 0805 SMD (and blue typically has much higher forward voltage than other colours).

Only 4 parts are required:

Item Description
IC1 QX5252 (or YX805)
L1 inductor
LED1 The LED - blue 0805 SMD
BAT Battery - AG8 1.55V

The only question to answer is the size of the inductor:

  • some versions of the datasheet mention 68µH as a guide. That works.
  • or it can be calculated: LED Power = 2 * VBAT / L * 10^-6 (formula from QX5252 datasheet)
  • or one can experiment..

In truth, a wide range of inductance will work, with the considerations:

  • too low, and it may not generate sufficient voltage to drive the LED
  • too high, and it may fry the LED (if the battery can deliver enough power)

Here are some examples at both ends of the spectrum.

First, with a larger inductor of 100µH, voltage output from the boost converter (at LX, driving the LED) looks like this. This is a good curve, peaking at just over 3V which is ample to drive the LEDs (Vf = 2.7V). The switching frequency is around 130kHz, and duty cycle over Vf is perhaps 20%. This results in a nice strong LED illumination.

scope_100uH

Dropping the inductance to 1µH and the result is quite anaemic. Sufficient voltage to drive the LED is only generated for a very short period. So while it does still manage to illuminate the LED, it does so quite weakly.

scope_1uH

Construction

Breadboard

Schematic

Carefully put together so that it forms a clip over the battery:

build_01

build_02

build_03

Credits and References