LED Circuit

[Enough!Participant @enough]

[Enough!Participant @enough]

OK, mechanical engineer type question:

I have a PCB containing an array of LEDs all connected in parallel, powered by 3 AA batteries, and fed by a 1.2 Ohm resistor (maybe about 5W – larger than the typical 1/2-W anyway).

I want to cut the board into two halves an mount them spaced on a 3D-Printed part then hard-wire them up again. That part's trivial in this case.

I would also like to feed them from a 5V USB supply. So what should I change the dropper-resistor to (Ohms and Watts) to do this?

I don't know anything specific about the LEDs other than they are the typical high-brightness blue/white LEDs used in many Far-Eastern flashlight-type devices these days. These happen to be 5mm dia if it matters.

[john swift 1Participant @johnswift1]

how many LED's in the array ???

 

the volt drop across white LED's will be about 3.3V at 20mA

so you need to drop (1.4 x 3) – 3.3 = 0.9V [ assuming your using alkaline batteries]

for one LED

R = 0.9/0.02 = 45 ohms

 

 

with a 5V power supply

R for one LED at 20 mA = 1.7 / 0.02 = 85 ohms

 

 

using the 5V power supply

running the array of LED's in parallel with one resistor

divide the 85 ohms by the number of LED's in your array

for example

with 10 LED's R = 8.5 ohms

or 20 LED's R= 4.25 ohms

 

 

John

 

 

 

 

Edited By john swift 1 on 24/11/2017 20:19:16

[Les Jones 1Participant @lesjones1]

White LEDs typically have a voltage of about 3.3 volts across them at their rated current, As 3 x AA primary cells in series give 4.5 volts then there will be about 1.2 volts across the 1.2 ohm resistor which means the current for the full array is 1 amp. So for a 5 volt supply you would need to add another 0.5 ohm resistor in series, Either add a 0.47 ohm resistor in series or just use a 1.8 ohm resistor in place of the 1.2 ohm resistor. (At least 2 watts rating.). It is not good practice to connect LEDs in parallel as the voltage of individual LEDs varies slightly. If the brightness of the LEDs is uniform enough then use the above values. If not work out the resistor value to have a current limiting resistor for each LED.

Les.

[Alan VosParticipant @alanvos39612]

Do you have an ammeter to measure the current when running off 3 x AA ? Les Jones's maths looks sound, but AA batteries would only last around an hour supplying 1A. You also need to ensure your USB device can supply the required current. They can all manage 500mA. For anything more than that you need to check the specificiation.

[Enough!Participant @enough]

There are 24 LEDs so based on John's analysis a 3.5 Ohm resistor would be required.

… vs 1.8 Ohm from Les

… see this is why I became a Mechanical Engineer ….

I take the point about the single current limiting resistor Les but this is an el-cheapo LED flash light from China which perhaps explains why it is this way. Rewiring the PCB for individual resistors would be vastly more trouble than it's worth – I might just as well hand wire the whole thing in-situ in its final use. Plus, visually the LEDs appear equally bright and the end use is not rocket science.

[This is to go into a 3D printed part to mount to the side of my DaVinci 3D printer and throw light on the platen.
Also mounted will be an ip-camera so that I can check progress from my phone (something that I never thought I'd hear myself say – oh well, at least it will get used once in a while …. the phone that is). Anyway, it's not a super-critical application.]

Alan, I wouldn't doubt that the AA batteries do drop off in an hour. The flash light is obviously only designed for intermittent use and who keeps count?

As for USB power supplies, my experience is that that lower than 1 Amp supplies went the way of the Dodo a few years ago when Apple introduces later versions of the iPad which demanded 1A supplies, minimum, and wouldn't even start to charge from anything lower. Certainly, everything that I've got will do at least an Amp; most will give quite a bit more.

[Ady1Participant @ady1]

Something to throw into the mix

I got half a dozen of these from Morrisons for three quid a pop each because they go for up to 6 months 24/7 on 3xAA batteries and make handy night lights, especially in winter

12 LEDs and with the dimming rheostat set to the minimum possible setting gets you 6months of 24/7 light

Edited By Ady1 on 25/11/2017 07:48:00

[Michael GilliganParticipant @michaelgilligan61133]

For the 3D printer-specific lighting; something like this looks useful

https://www.ebay.co.uk/itm/USB-LED-Hard-Strip-Light-Tube-Lamp-Bulb-On-Off-Switch-5V-For-Home-Reading-as07/252990683812

**LINK**

That one is from a U.K. supplier, so I presume they will be available even cheaper from China.

MichaelG.

[Author]

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