![]() ![]() They may be used by Analog Devices to build a profile of your interests and show you relevant content on our site. Targeting Cookies: These cookies may be set through our site by Analog Devices and our service providers. If you do not allow these cookies we will not know when you have visited our site, and will not be able to monitor its performance. All information these cookies collect is aggregated and therefore anonymous. They help us to know which pages are the most and least popular and see how visitors move around the site. Performance Cookies: These cookies allow us to count visits and traffic sources so we can measure and improve the performance of our site. If you do not allow these cookies then some or all of these services may not function properly. They may be set by us or by third party providers whose services we have added to our pages. Functional Cookies: These cookies enable the website to provide enhanced functionality and personalization. ![]() These cookies do not store any personally identifiable information. You can set your browser to block or alert you about these cookies, but some parts of the site will not then work. They are usually only set in response to actions made by you which amount to a request for services, such as setting your privacy preferences, logging in or filling in forms. Strictly Necessary Cookies: (Always Active) These cookies are necessary for the website to function and cannot be switched off in our systems. After we finish updating our website, you will be able to set your cookie preferences. Replacing the EPOT of Figure 1 with an RC network reduces both the IC count and the required number of control lines.Ī similar article appeared as a Design Idea in the Februedition of EET.Īnalog Devices is in the process of updating our website. The circuit also includes a 6.65kΩ resistor across the integrating capacitor, which reduces the 5V PWM signal to 1.25V as required by the IC's ADJ input.įigure 2. An RC time constant at least 50 times the PWM period produces a smooth average voltage across the capacitor. The average voltage at ADJ (pin 4) varies with the PWM signal and enables the IC (a current regulator for LEDs) to control the LED brightness. Simply replace the EPOT with a simple RC integrator and program the port to output a PWM signal (Figure 2). You can control each LED driver directly with a single output port. ![]() There is an alternative that reduces the number of ICs and control lines. A common brightness-control circuit for LEDs uses an electrically controlled potentiometer (EPOT) to achieve digital control of the LED brightness. For large systems, however, that approach can require many EPOTs, each requiring two control lines, or address decoding, or both.įigure 1. A common approach for achieving this adjusts an electrically controlled potentiometer (EPOT), which in turn, controls the LED driver (Figure 1). Replacing the EPOT with an RC network, however, reduces both the IC count and the required number of control lines.Ī system in which a microcontroller or programmable logic controls one or more LEDs often requires the capability of controlling LED brightness. The maximum current would be with the pot set at about 40%, which would cause Q1 to be saturated, with 3V on the emitter and 60 mA through the LEDs.A common brightness-control circuit for LEDs uses an electrically controlled potentiometer (EPOT) to achieve digital control of the LED brightness. With the potentiometer set at 30%, the base of Q1 will be 3 volts, and the voltage on the emitter will be about 2.3 volts, corresponding to 46 mA through the LED string. Simulate this circuit – Schematic created using CircuitLab However, adding a single transistor and a current limiting reference resistor, a practical circuit can be made: To get 10% brightness at 1 mA, the shaft would be turned to get 3k resistance, or 80 degrees, and at minimum, you would have 300 uA, which is very dim. For a 10k pot with 270 degrees of rotation, this resistance comprises only 8 degrees. So for typical white LEDs with 3V forward voltage, only three can be connected in series, leaving 3 volts to be dropped by the potentiometer. So the 10k device shown in the image might be 1 watt, that means that its maximum current is:Īlso, there needs to be some "headroom" between the minimum supply voltage (in this case 12V) and the maximum voltage of the LED string for desired brightness (in this case limited by the 10mA maximum current). What must be understood about using just a potentiometer (or rheostat) for LED brightness control, is that its power rating and resistance determine the maximum current it can safely handle. ![]()
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