Design high power LEDs and LM3914 in parallel - Page 1
Forum section: electronics / projects

author: dethlore
author_rank: Newbie
timestamp: June 04, 2014, 04:11:59 pm
content: I have been trying to design and prototype a circuit that has a voltage indicator (LM3914), and a switch that turns on 3 high power LEDs. 2 LEDs are constant on and one pulses via a 555 timer.I am really just a beginning hobbyist. I have built constant current laser drivers and lots of simpler LED circuits, but haven't done anything quite this complex... so this is really funbut I have a problem and some questions.Problem: when the LEDs are switched on, the volt indicator drops about 1.5V (From 10 lit LEDs down to 3, which is 8.4 down to 7ish volts).Question: The LEDs are in parallel so I would expect them to have the same current draw, but they are different. One has a 1 Ohm resistor to get 650mA and the other has .5 Ohm to get 450mA out of it.This points to my circuit having some obvious flaws... See attached diagram.Circuit Description:Power Supply: 2x 18650 lithium ion batteries in series (7.4V, 8.4V max)LM3914Measures 6-8.4V and displays on 10x 3mm LEDsLM1185 Voltage Regulator to 4.5VIC Switch(3x parallel to each other) to base of 2N3904 TransistorsAllows regulated 4.5V to (3x parallel to each other) HUF76423P3 Logic Mosfet2 mosfets allow regulated 4.5V through diode 1N4001 in series with LED (2.6V @ 700mA typ)1 mosfet allows regulated 4.5V through 2 diodes 1N4001 in series with LED555 Timeroutput to transistor basesaturates mosfet gatemosfet allows higher power pulse(555) to 1 LEDmeasurements of voltage across a constant LED and driving mosfet:Vgs = 3.3VVds when g is low = 3.03VVds when g is high = 0.03VVf-led = 3.06VAny suggestions?

author: Pillager
author_rank: Unknown
timestamp: June 04, 2014, 05:51:30 pm
content: First thing that comes to mind: you drew your MOSFETs like bipolar transistors. Maybe you thought it through like that, too.A MOSFET needs a Voltage to switch, but (practically) no current, a bipolar transistor needs some current and only a small voltage (think diode)Keep that in mind, it looks like you needed some current flowing into the gate, but that's not going to happen...

author: dethlore
author_rank: Newbie
timestamp: June 04, 2014, 06:33:35 pm
content: You're right, those are drawn wrong... but I did intend them to only use the voltage on the gate, which is why I use transistors (low current) to take the lower volt signal and allow the higher voltage to pull the gate high. Do mosfets pull current into the gate if allowed?The switch actually outputs a lower voltage as well. It's not really just a switch... but for intents and purposes, just know that it is a lower voltage output switch.Thanks.EDIT: I just realized... when I was prototyping initially, I was using a constant power supply... had slipped my mind that the battery would certainly show a lower level of voltage under load.The other wonder I have though... is why would 2 parallel circuits with identical parts show a fairly substantial difference in current flowing through it? I mean the constant on LEDs: 1 draws 600mA with a 1 ohm resistor. the other requires .5 ohm just to get 500mA. What would cause this?

author: georges80
author_rank: Frequent Contributor
timestamp: June 05, 2014, 04:37:12 am
content: Quote from: dethlore on June 04, 2014, 06:33:35 pmBecause the Vf or your LEDs is different.... which is why you want a current regulated driver versus just a resistor, especially for higher power LEDs.cheers,george.

author: dethlore
author_rank: Newbie
timestamp: June 05, 2014, 10:42:30 pm
content: They probably are! It's better practice to current regulate them anyways I suppose. Thanks Georges!