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I have built a circuit which contains an integrated power supply stepping down mains voltage to 5v through a transformer, rectifier circuit and a voltage regulator. With a couple of smoothing caps the ripple is (virtually) non existent. I'm using this power supply to power an ATMega168 and a 16x2 LCD display, with a minimal amount of other small components, mainly a voltage divider with 2 x equal value resistors. All works fine, but when I connect the LCD backlight to my power supply, a large amount (1.3v) of ripple appears all over my DC supply which is obviously upsetting my voltage divider. I have searched on StackExchange and also other places to see if there is an easy solution to what I imagine is a common problem, but haven't found anything, so was wondering if you guys could help? What's the simplest way to get rid of this ripple?

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    \$\begingroup\$ We need datasheets for the main components, and some kind of schematic. \$\endgroup\$
    – Matt Young
    Commented May 7, 2013 at 23:10
  • \$\begingroup\$ a complete schematic would be nice \$\endgroup\$
    – David Norman
    Commented May 8, 2013 at 1:04
  • \$\begingroup\$ I have added a schematic. Please ignore where it says NC on pins 15 an 16 of the LCD. They should read Vcc and Gnd respectively. If I disconnect the supply from these pins, the ripple problem goes completely. \$\endgroup\$
    – Hoppo
    Commented May 8, 2013 at 9:20

2 Answers 2

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The ripple when switching on the LCD backlight leads to one suspicion: The backlight is an electro luminescent (EL) panel, driven by a high frequency inverter.

A typical EL inverter would consist of a switching circuit, producing a square wave load on the supply.

The common way of dealing with this is to decouple that load from the supply rail, at the source of the noise, i.e. right at the backlight driver. A large capacitance between supply and ground rails, placed as close as possible to the backlight driver, would be a start. If that too causes some ripple to remain at the supply, add an inductor between supply rail and backlight driver's supply input.

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More microfarads in the capacitors bwtween the rectifiers and your voltage regulator. That would be your first strategy. It is also possible that the voltage you have on these capacitors is too low and the voltage regulator is falling out of regulation when the current draw due to the backlight is introduced as part of the load. Addressing the voltage level would require using a transformer with higher voltage output or possibly using a voltage regulator that required less voltage headroom.

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