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Author Topic: Help for a dunce please  (Read 470 times)

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Online Philip.

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Help for a dunce please
« on: May 30, 2020, 02:34:44 PM »
Can any kind person give me some advice on how to wire up the street lights plus each house has an LED. The LEDs are 3v and I have a 3v power supply available.

Do I wire in series, parallel [as if i know what that means] do I need resistors?

Any help or very simple diagram would be gratefully recieved.




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Offline ntpntpntp

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Re: Help for a dunce please
« Reply #1 on: May 30, 2020, 02:38:09 PM »
LEDs are current driven devices (not voltage) so without a resistor or a current limiter they could potentially draw way too much current from the supply.

Another problem is white LEDs as used in model street lights etc. can have a typical minimum forward voltage of at least 3 volts, so if your supply is only 3V that may not be high enough.  Worth putting a meter across your supply and checking the actual voltage.

How much current is your 3V supply rated for?  It's usually unwise to run a supply up to its maximum power.

With only a 3V supply they will have to be wired in parallel, however you should still use a series resistor with each LED to limit the current, even if the resistors are only a few ohms.

Otherwise you could consider a constant current driver and run the LEDs in series.   You'd need a driver that can limit to say 10-15mA, and I'm not sure if it will work with only 3V supply  (I'm used to using resistors).



https://www.ledsupply.com/blog/wiring-leds-correctly-series-parallel-circuits-explained/





My preferred approach is to use a higher voltage supply, eg. 12-15V  and run LEDs in series groups of 3 with a single resistor,  and then each group then in parallel across the supply.  That way the current in each parallel group  is shared across 3 LEDs for greater efficiency.
« Last Edit: May 30, 2020, 02:59:38 PM by ntpntpntp »
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Online Philip.

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Re: Help for a dunce please
« Reply #2 on: June 01, 2020, 01:55:01 PM »


How much current is your 3V supply rated for?  It's usually unwise to run a supply up to its maximum power.

It's a variable power supply rated at 2.1A
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Offline ntpntpntp

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Re: Help for a dunce please
« Reply #3 on: June 01, 2020, 02:23:17 PM »


How much current is your 3V supply rated for?  It's usually unwise to run a supply up to its maximum power.

It's a variable power supply rated at 2.1A

Continuously variable or discrete steps?

As I mentioned, white LEDs can be more than 3V forward voltage so I'd be looking to run at the next available voltage step, and preferably higher if I want to run multiple LEDs on each parallel strand to share the current. 

For example, let's say there are 51 LEDs each running at 15mA. Lets assume each LED has a forward voltage of 3.2V ( to illustrate it being greater than 3V)

With a 5V supply each LED has to be on its own parallel strand with its own 120 ohm resistor.  Total current draw 765mA

With a 12V supply there is enough voltage (with spare headroom) to run 3 LEDs in series with a single 180hm resistor so that 3 LEDs can share 15mA. 17 parallel strands of 3 LEDs (ie. our total 51 LEDs) still only draws 255mA.
If you wanted to have 51 parallel strands each with 1 LED you can do that, but then each would need a 680 ohm resistor and the current draw would be 765mA.

So you can see why I prefer a higher source voltage to let me run a few LEDs in series (and then as parallel strands of multiple LEDs).  Far less current moving around, better for the power supply and better for the wiring too.

There is also the "constant current driver" approach to running LEDs  but I'll be honest I've not played with them so I'm not able to comment on how they're used.

[edit] PS - 15mA is a lot for a miniature street lamp with an SMD LED.  I run mine at 5mA or less, so again saving on total power draw  :D
« Last Edit: June 01, 2020, 02:25:16 PM by ntpntpntp »
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Re: Help for a dunce please
« Reply #4 on: June 01, 2020, 02:29:22 PM »
Power supply has 7 different settings ranging from 3v to 12v

So I can run the lights in parallel at 5v, each with a 120 ohm resistor?
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Offline ntpntpntp

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Re: Help for a dunce please
« Reply #5 on: June 01, 2020, 02:42:31 PM »
Power supply has 7 different settings ranging from 3v to 12v

So I can run the lights in parallel at 5v, each with a 120 ohm resistor?


Well I'm assuming 15mA in the examples above.  It would be better to check any instructions you have.  It would also be worth trying a few higher resistor values and see what they look like with less current. Resistors cost pennies and it's useful to have a few values in your bits box.  As I say, the tiny LEDs in street lights usually don't need anywhere near that much current, but it also depends on whether they look the right brightness for you?

I use this online calculator to work out the resistor value:

http://led.linear1.org/1led.wiz


So, for a 5V supply and a white LED at 3.2V forward voltage:
120 ohms gives you 15mA current flow
180 ohms gives you 10mA current flow
390 ohms gives you 5mA current flow

** Whatever you do, don't try without any resistor even with your supply set at 3V - you'll let the magic smoke out of the LED  :D (well, they just die you don't see any smoke)
« Last Edit: June 01, 2020, 02:48:38 PM by ntpntpntp »
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Re: Help for a dunce please
« Reply #6 on: June 01, 2020, 03:11:55 PM »
So even though the power supply is regulated at 3v or 5v, still use resistors? Sorry for being dim [pun intended] :D
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Offline ntpntpntp

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Re: Help for a dunce please
« Reply #7 on: June 01, 2020, 03:17:14 PM »
So even though the power supply is regulated at 3v or 5v, still use resistors? Sorry for being dim [pun intended] :D

Yes you must.

"regulated" refers to the output voltage of your supply being reasonably stabilised at whatever value you've selected, from 0 amps current through to the max 2.1 amps. With an unregulated supply the output voltage can be quite a bit higher when there's no load, and can drop down below the expected value as the load gets nearer the maximum current.

Going right back to the first thing I wrote in my first reply, you must limit the current flow to an LED (the voltage is not so important as long as it's above the minimum forward voltage and not stupidly high).  The LED itself has very little resistance, so without an external resistor your 3V/5V supply would happily allow 2.1 amps through the LED (for a nanosecond  :D until it failed )
« Last Edit: June 01, 2020, 03:34:58 PM by ntpntpntp »
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Re: Help for a dunce please
« Reply #8 on: June 01, 2020, 03:23:31 PM »
Thanks for your help, I think I'm getting there  ;)
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Re: Help for a dunce please
« Reply #9 on: June 01, 2020, 04:35:16 PM »
OK, very basic drawing  ::) but is this what I'm aiming for, or [in all probability] a disaster waiting to happen



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Offline ntpntpntp

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Re: Help for a dunce please
« Reply #10 on: June 01, 2020, 05:18:42 PM »
Yes, that is parallel wiring of the LEDs with a resistor.  Not the most efficient arrangement in terms of electrical load, but probably the simplest.

It doesn't matter whether the resistor is in the positive or negative side, but for your sanity and for neatness be consistent  :)

I presume you'll be soldering the resistor to the wire, so get yourself some heatshrink sleeve to cover the joint.  Some lights are sold with the resistor completely enclosed in heatshrink.  If the wire from the light is very fine, bare a few more mm than usual and wrap the fine wire around the resistor lead before soldering
« Last Edit: June 01, 2020, 05:33:50 PM by ntpntpntp »
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Be careful whose toes you step on, they might be attached to the foot that kicks your backside in the future!

Offline ntpntpntp

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Re: Help for a dunce please
« Reply #12 on: June 01, 2020, 09:20:14 PM »
@Philip.   Yeah they will do the job fine, but before you go and buy a big bunch of the same value, I suggest find a selection pack of various values, just so you can play around and check what resistance makes the lights look best for you.  Remember the values we've talked about above are just my general suggestions which should be safe to use.


This kind of thing. (note this is a far east seller so will take a little while to arrive. There may be UK sellers offering similar assortments)

https://www.ebay.co.uk/itm/10-1M-300PCS-30-Values-1-4W-0-25W-1-Metal-Film-Resistors-Assorted-Kit-Set/312785489218?hash=item48d377cd42:g:C1cAAOSw-ghb6O7p


You get 10 of each value in the range.    OK so there are some values you might never use, but for less than £3 you'll have a good range, and something to practice learning the resistor colour codes  :D   The first thing I do when I receive these "ammo belts" of resistors is write the value on the belt.

Once you've confirmed the value you want to use for your lights then buy a bunch.



Do you have a multimeter? Always check the resistance before you use a resistor in a circuit.
« Last Edit: June 01, 2020, 09:22:11 PM by ntpntpntp »
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Re: Help for a dunce please
« Reply #13 on: June 01, 2020, 09:33:26 PM »
@ntpntpntp I really appreciate all your help, many thanks  :thumbsup:
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Offline themadhippy

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Re: Help for a dunce please
« Reply #14 on: June 01, 2020, 09:44:01 PM »
Quote
I suggest find a selection pack of various values
I'd buy a bucket load of 100 ohm plus a  few 1k and 10k's,with series parallel combinations youll get close enough to the sort of values you need.
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