[Current Regulator] Final Prototype and ESR considerations


Introduction

First news the final prototype is working, with all modifications done. 2 days before receiving my Chinese LED the light bulb of my desktop magnifier died in some mysterious ways, simple coincidence or an Enigma of the Mystical… We will never know ;)

By the way my LED current regulator seems to be right in time. I began by removing the light bulb and his mounting, I kept the heat-sink which protect plastic to put the LED on the metal with some thermal paste.

The Problem

The regulator worked nicely but there was too much ripple in the output. The problem was not present in the Spice simulation. In fact, on the first round I didn »t have low-ESR capacitors I just put two E-Cap of 10uF and forget it. It tooks me One week to understand the problem:

High ESR Capacitors

This measure is done with two Chemical Caps 2x 10uF, Yellow line is the output voltage, Blue line is the Voltage accross the sense resistor of 1 ohm, so it’s exactly the current flowing through the load.

As you can see the Ripple is really High, about 350mA. Before testing with ceramic capacitors (in fact I didn’t have them at that moment ;)) let’s take a look to the LTSpice simulation. Following The Wikipedia ESR article chemical capacitor have an ESR in a range of 0.1 to 3 Ω

Spice Simulation

R2 = 30Ω

C2 = 20uF We will modify the Equ. Series Resistors to validate the ESR idea ;)

When ESR = 0Ω

Ripple current = 4mA

ESR = 1Ω

Ripple Current = 27mA

ESR = 3Ω

Ripple Current = 65mA

Testing

I finally got my ceramic 1210 10uF capacitors time to measure the Ripple and validate Spice simulation ;) The Ripple current is ten times less important with these capacitors.

Next step

Next step will be to correct the kicad schematic to add the Totem-Pole configuration and etch a PCB to see if I can reduce the ripple again with clean soldering.

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