#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.
Here’s a quick demo..
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
- 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)
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:
|IC1||QX5252 (or YX805)|
|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.
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.
Carefully put together so that it forms a clip over the battery: