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Author Topic: Surface Mount Diodes/LEDs  (Read 812 times)

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Offline springwood

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Surface Mount Diodes/LEDs
« on: September 01, 2019, 02:02:43 PM »
I have recently purchased some 0805-type SMD's and plan to instal them either to the side walls or floor of some inspection pits in my service depot shed. These are very tiny but I have successfully wired one up in a simple 12v dc circuit, using a resistor.

That's fine, but I want to have three (possibly four) SMD's per pit. Would be grateful for any assistance on how to wire multiple LEDs in series such that they all come on at the flick of a SPST (on/off) switch. I have attached my simple circuit, based on wiring one only.

These SMDs have a forward voltage of 3.2 to 3.8v and forward current of 20mA to 30mA (max)

On the real thing, are these pit lights mounted at intervals along the side walls of the pit?

Thank you

Robin


Online jpendle

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Re: Surface Mount Diodes/LEDs
« Reply #1 on: September 01, 2019, 04:06:29 PM »
For a start you will need to wire them in parallel, NOT series. Then you just repeat your circuit N times. As long as your power supply can provide enough current there shouldn't be any problems.

Regards,

John P
Check out my layout thread.

Contemporary NW (Wigan Wallgate and North Western)

https://www.ngaugeforum.co.uk/SMFN/index.php?topic=39501.msg476247#msg476247

Offline springwood

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Re: Surface Mount Diodes/LEDs
« Reply #2 on: September 01, 2019, 06:15:35 PM »
Thanks for your response John P

Not sure why you are saying in parallel rather than series? Looks like I will have 8 SMDs in total (4 for each pit). I am using a nominal 12v dc supply (from my guagemaster model D controller). Can anyone guide me as to what the value of each resistor should be? Does my attached wiring diagram look correct?

Offline ntpntpntp

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Re: Surface Mount Diodes/LEDs
« Reply #3 on: September 01, 2019, 06:29:35 PM »
You certainly CAN wire them in series as long as the total forward voltage is less than the supply voltage, it's more economical as they will share the current.  Leave a couple of volts "clearance" to allow for tolerance in the forward voltages and in your supply voltage. You MUST have a current limiting resistor in each series string, so just recalculate it based on  the total forward voltage.

I find SMD LEDs really don't need to be driven anywhere near that much current: when I built the lighting for an Abnormal Load haulage model I found I could run them quite happily at less than 5mA.


Here's where I was testing out the head/tail lights for one of the lorries. 4 LEDs in series with a single resistor.  Actually these lights all came pre-wired like this from a bunch of cheap N gauge cars from ebay, and saved me having to fiddle about wiring up the SMDs (which I did have to do for the other flashing lights etc :-) )



[edit] Here's the LED resistor calculator I always use

http://led.linear1.org/1led.wiz
« Last Edit: September 01, 2019, 06:39:15 PM by ntpntpntp »
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline Malc

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Re: Surface Mount Diodes/LEDs
« Reply #4 on: September 01, 2019, 07:44:33 PM »
As ntpntpntp says, you can run them in series, but if your supply is 12 volts and the forward voltage drop is 3.2 to 3.8 volts, you won’t be able to run all 4.
I would use a 1k resistor in series with each led and wire the 4 resistor/led assemblies in parallel. You could get away with one resistor, but I prefer not to as if one led blows, it will affect the rest. If the led is too bright, increase the resistance. In inspection pits, as you correctly say, the lights are embedded in the walls near the top.
The years have been good to me, it was the weekends that did the damage.

Online jpendle

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Re: Surface Mount Diodes/LEDs
« Reply #5 on: September 01, 2019, 07:58:17 PM »
Thanks for your response John P

Not sure why you are saying in parallel rather than series? Looks like I will have 8 SMDs in total (4 for each pit). I am using a nominal 12v dc supply (from my guagemaster model D controller). Can anyone guide me as to what the value of each resistor should be? Does my attached wiring diagram look correct?

I suggested wiring in parallel as it is simpler.  Your diagram is correct and the value of the each resistor is exactly the same as the value of the single resistor that you have used.

Regards,

John P
Check out my layout thread.

Contemporary NW (Wigan Wallgate and North Western)

https://www.ngaugeforum.co.uk/SMFN/index.php?topic=39501.msg476247#msg476247

Offline themadhippy

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Re: Surface Mount Diodes/LEDs
« Reply #6 on: September 01, 2019, 08:26:05 PM »
Quote
but if your supply is 12 volts and the forward voltage drop is 3.2 to 3.8 volts, you won’t be able to run all 4.
Bet you can,the stated forward voltage will be for maximum lumen output,try running 1 led on 2 AA batterys and see what you think.

Offline ntpntpntp

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Re: Surface Mount Diodes/LEDs
« Reply #7 on: September 01, 2019, 08:32:25 PM »
Don't be surprised if the "nominal 12V dc" output of the Gaugemaster is actually noticeably higher, even under load.

At the very least you could arrange the LEDS for each pit as two strings of 2.  If you try this with a 1k resistor that would restrict them to around 5mA, you might be pleasantly surprised at how good they look.  If there's one thing I hate it's over-bright LEDs on models.

It's always worth having a selection of resistor values in stock to play with.when experimenting, start with a higher value than you've calculated and see how it looks.  I recently bought a resistance box so I can dial in resistances to try in circuits, then when I'm happy with the circuit use the nearest equivalent resistor component.
https://www.ebay.co.uk/itm/Plastic-Variable-Decade-Resistor-Resistance-Box-0-to-9999ohm-1-ohm-step-Tool/323713908117

« Last Edit: September 01, 2019, 08:33:40 PM by ntpntpntp »
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline Miek

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Re: Surface Mount Diodes/LEDs
« Reply #8 on: September 01, 2019, 08:50:02 PM »
I tried them inside a small model house and the rated current was far too bright for that and that is with one LED. As far as the brightness in a confined inspection pit goes (I guess it will be the length and width of an engine) with 4 LEDs, I think the overall effect at e.g. 30mA through each LED will be extremely bright.

In the real world you would want to locate lights so that they provide a uniform light across the area and don't get obstructed by platforms / stairs etc. Maybe you could bring out some wires from the circuit so that you could try different resistors? (like ntpntpntp shows)

Also, if you are wiring in series, it's more important to check before you solder that all the LEDs in the chain are each the correct way round because if you get one wrong, it will stop the others working even if the others are correct. At least with parallel it will only affect the LED that is the wrong way round. With parallel you will need more components.

Thanks ntpnptntp for the resistor calculator link - very useful - particularly the page about the series / parallel version. It shows that it might be possible to connect all 4 in series with a 1ohm resistor, assuming a 12V supply (maybe check what it actually puts out), 3V per LED (I am assuming they are white) and a current of 2.5mA but it is right on the limits and the LEDs are at about a quarter of their brightness. I have used this data sheet https://www.vishay.com/docs/81602/vlmw11.pdf  on page 3.

Online Bealman

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Re: Surface Mount Diodes/LEDs
« Reply #9 on: September 02, 2019, 12:01:08 AM »
Thanks for the heads-up with that resistance box, ntp3!

It looks like a good bit of kit. I'll be ordering one!  :thumbsup:
Vision over visibility. Bono, U2.

Online Bealman

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Re: Surface Mount Diodes/LEDs
« Reply #10 on: September 30, 2019, 07:38:38 AM »
My substitution box arrived today. I've had a tinker with it, and some opinions which I will put into a review on this board.  :thumbsup:
Vision over visibility. Bono, U2.

Offline dannyboy

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Re: Surface Mount Diodes/LEDs
« Reply #11 on: September 30, 2019, 12:29:50 PM »
I've had a tinker with it, and some opinions which I will put into a review on this board.

I have one of these boxes but have not yet got round to 'playing' with it. Your opinions would be appreciated George.  :thumbsup:
David.
I used to be indecisive - now I'm not - I don't think.
If a friend seems distant, catch up with him.

Offline geoffc

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Re: Surface Mount Diodes/LEDs
« Reply #12 on: October 01, 2019, 07:48:32 AM »
I have recently purchased some 0805-type SMD's and plan to instal them either to the side walls or floor of some inspection pits in my service depot shed. These are very tiny but I have successfully wired one up in a simple 12v dc circuit, using a resistor.

That's fine, but I want to have three (possibly four) SMD's per pit. Would be grateful for any assistance on how to wire multiple LEDs in series such that they all come on at the flick of a SPST (on/off) switch. I have attached my simple circuit, based on wiring one only.

These SMDs have a forward voltage of 3.2 to 3.8v and forward current of 20mA to 30mA (max)

On the real thing, are these pit lights mounted at intervals along the side walls of the pit?

Thank you

Robin

If you want to wire these in parallel look at one of these, LaisDcc 2 Pack LED Resistor Board Part No.860027 DCC, they are a small board 20mm x 8mm with a solder pad for 12v input and 8 outputs each with a 1K smd. I would advise wiring the 12v through a variable resistor so you can set the led brightness to what you want.

Geoff            


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Re: Surface Mount Diodes/LEDs
« Reply #13 on: October 01, 2019, 08:09:36 AM »
Geoff, these look most interesting! Unfortunately, if that is a link, it does not show up as such in the quote.

I guess we Google it?

Running on 12V, this sounds like a godsend to railway modellers!
Vision over visibility. Bono, U2.

Offline geoffc

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Re: Surface Mount Diodes/LEDs
« Reply #14 on: October 01, 2019, 08:24:32 AM »
@Bealman, that was not intended to be a link, I am not that clever IT wise, if you Google it hopefully you can find a supplier from downunder and not have to have them sent from the UK.

Geoff

 

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