LEDs … The Good, The Bad, and The Ugly

[Anonymous]

Posted by Gordon W on 06/10/2014 12:05:54:

Thanks Andrew, So if I buy 6 LEDs I wire them in series and run at 12Volts? Seems too easy.

Yes, it is too easy! You really need a series resistor as well; even more so if your voltage source isn't particularly well regulated. Let's consider the white LED I am planning to use for my traction engine lamps, as I have the figures in my head. The voltage drop for the typical operating current of 20mA is 3.4V, but worst case the voltage drop can be 4V. However, the maximum DC current is specified at 25mA, no far above the 20mA operating current. So you really need to be sure that the LED is operating at the current you think it is! Hence the series resistor. A series resistor along with a voltage source simulates a current source.

I didn't follow the comment by Neil on resistor values. If you're simulating a current source with a voltage source and series resistor then the larger the resistor the better the compliance of the simulated current source. So larger value resistors are better. Of course if you're using the resistor to simply sense the current and using that to control the voltage by external means then the resistor can be small. Usually limited by noise performance and the amplification needed. Common-mode can be a problem too, if the current sense resistor isn't referenced to 0V.

Regards,

Andrew

[Gordon WParticipant @gordonw]

Thanks Andrew, seems I don't know as much about electricity as I thought, and that's not much. Simulating a current source with a voltage source ?. All new to me, and I got a national certificate in electrical eng. about 50 years ago! Anyway- I've just taken my 6 LED headlight to bits- it has 3 dry cells total voltage 3.5v ( cells a bit old ) can't get at the LEDs or electronics, if any. These must be wired in parallel to give 3v at each LED ? Think I'll give up and buy something ready made. Starting to rebuild battery drill pack, think I can manage that.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 06/10/2014 12:09:31:

Posted by Gordon W on 06/10/2014 12:05:54:

Thanks Andrew, So if I buy 6 LEDs I wire them in series and run at 12Volts? Seems too easy.

.

Gordon,

Be aware that different colours of LED have different forward voltages.

White ones are more typically about 3.2v

MichaelG.

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P.S. …. This site has useful calculators and info.

Edited By Michael Gilligan on 06/10/2014 12:14:14

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Gordon,

I'm re-posting this, in case you missed it

… I'm sure you will find it helpful.

MichaelG.

[Gordon WParticipant @gordonw]

Thanks I did look at the site, may well use it later. Just had a head banging moment and realised that these things are diodes, and not simple filament lamps.

[Neil WyattModerator @neilwyatt]

>I didn't follow the comment by Neil on resistor values.

because the Vf for the LEDs ensures the voltage to be dropped is quite small, you only need a fairly small resistor to limit the current, making the approach relative efficient.

You will often encounter a single, small white LED run off a stack of three button cells, or 20 or so in parallel run off three alkaline cells, no dropper resistor. Despite breaking the rules the internal resistance of the cells is sufficient to limit the current to (Vbat-Vf)/Rbat.

Well, I know what I mean, anyway!

Neil

[WALLACEParticipant @wallace]

A few pics of two dissected 12v MR16 bulbs in my album.

Not a very thorough investigation as my meter is somewhere else ! I’ll do a better test when I have it back, both on ac and dc – the meter is true rms reading and I’m interested in seeing the current difference between the two – plus any 50 or 100 hz ripple on the output.

The scope pic is the output of the 61 led bulb pcb,0.01v and 2us per division – ac coupled with the bulb running on 12v dc.

The blue, red and black wires in the photos were added by me – they’re not original.

W.

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for the update, Wallece

That lamp with the three 1W beads looks tidy.

I must profess to being a little concerned by the spikes on your 'scope trace … Could you try a faster timebase? the detail might be interesting.

MichaelG.

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 Let's hear it for these Guys 

Edited By Michael Gilligan on 07/10/2014 18:54:58

[jason udallParticipant @jasonudall57142]

The larger the resitor the better the regulation. .when using the norton equivalent of voltage source and series resistor…similarly the higher the voltage source the better too since this leads to bigger resistors…
Infact…infinite voltage infinite resistor..is the equivalent of a constant current supply.
The same equivalence for a voltage source…constant current with resistor shorting it…just read as infinite current and zero resistance. ..

Now back to the real world

[Michael GilliganParticipant @michaelgilligan61133]

NewsFlash : eMail from Screwfix, this morning

I haven't tried any of their LEDs, but Screwfix' price, service, and returns policy, are generally good.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Regarding the EU legislation on efficacy:

Here is a useful and reaonably intelligible leflet from Sylvania.

MichaelG.

Edited By Michael Gilligan on 13/10/2014 09:43:37

[CirclipParticipant @circlip]

Just replaced 4 GU10 Halogens with 3W leds. Although 24months and 20,000 hrs warranty, after 15 secs., half leds (30 out of 60 )in one unit go out. Turn off and let the afterglow run down, switch on again and fully lit for 15 secs and drops out again. Rotated unit through 180deg. and same half drops out.

Regards Ian

[Michael GilliganParticipant @michaelgilligan61133]

Ian,

It would be helpful if you could "Name 'n' Shame"

MichaelG.

[Neil WyattModerator @neilwyatt]

Hi Ian,

Unlike GU10s, worth taking back for a refund/exchange. Hope you have your receipt!

When I was a boy and helped out in my Dad's shop, every time I sold a bulb (any voltage or rating) I had to test it and show it was working on a big board he had made covered with sockets and rails etc.

I always assumed that was why they used to be in packing with the contacts ex[posed, so you could test them without taking them out of the packet!

Neil

[CirclipParticipant @circlip]

Posted by Michael Gilligan on 13/10/2014 11:37:31:

Ian,

It would be helpful if you could "Name 'n' Shame"

MichaelG.

 

THG GU10 3W DayWhjite 60 SMD 240V   Bought via T'internet, E-mail out waiting for reply, dispatched from Scotland.

 

Regards Ian.

Edited By Circlip on 14/10/2014 13:37:59

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Circlip on 14/10/2014 13:36:20:

THG GU10 3W DayWhjite 60 SMD 240V Bought via T'internet, E-mail out waiting for reply, dispatched from Scotland.

.

Thanks Ian

Hope the outcome is good.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Here's an odd one …

Plugged a little USB 'gooseneck' LED light into the Mac, just to check it.

Mac popped-up a warning message, telling me that the device required too much current, and was therefore being switched off … Odd, because the USB port on the MacBook Pro is capable of delivering One Amp, and the single LED should only draw a few milliAmps.

Thankful that the Mac had protected itself … I had a look inside the lamp:

Note that the insulating sleeve is just a little shorter than it needs to be.

[Michael GilliganParticipant @michaelgilligan61133]

< continued >

Sorry, the Red lead broke at the solder joint before I could cut it.

Removing the outer sleeve revealed a very nicely executed Heat-Shrink sleeve inside, covering the dropper resistor:

… Which makes me think they bought-in the LEDs with the resistors attached.

It certainly looks like two different standards of workmanship.

MichaelG.

[HowiParticipant @howi]

Just to let everyone know, B &Q have boxes of 5 x 4 watt led lamps gu10 fitting for £16.00, that is £3.20 each.

They seem to use 1 x 4 watt led rather than multiple led of lower rating.

Supposed to be equiv to 35 watt halogen but both the wife and myself think they are brighter than the 50 watt halogens they have replaced..

Energy saved = 4 watt x 10 lamps replacing 10 x 50 watt = 460 watt saving

[Neil WyattModerator @neilwyatt]

Hi Michael,

The USB limit is only 150mA for USB1 and 500mA for USB2.

A proper USB controller will only let each device use an allocation of 100mA 'units' if it is 'asked' for permission.

I imagine that your MAC implements this strictly and the LED is not an intelligent device.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 16/10/2014 09:43:27:

Hi Michael,

The USB limit is only 150mA for USB1 and 500mA for USB2.

A proper USB controller will only let each device use an allocation of 100mA 'units' if it is 'asked' for permission.

I imagine that your MAC implements this strictly and the LED is not an intelligent device.

Neil

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Neil,

May I suggest that you check Apple's specifications … The USB2 ports on the MacBook Pro [and several of the other machines] were upgraded to deliver up to 1000mA, so that they were capable of charging the iPad.

Agreed, the LED lamp is not an "intelligent device" and so the USB port should limit the current … but the point I was making was that the LED lamp had an internal short … and [as an aside] I was pleased to find that the Mac not only protected itself, but informed me how and why it had done so.

MichaelG.

[Michael HornerParticipant @michaelhorner54327]

I have made up 2x 4 foot LED strip lights double row from a 5 meter flexi strip. I have stuck them on to a strip of aluminium flat bar to act as a heat sink. When I first turned the one above the lathe it didn't seem that bright and the heat sink didn't get warm. The power supply was an old PC unit. The 12 volts was rated at 8amp. When I measured the voltage under the load of the lamp it was 9.5v! Bought a 20amp PSU now the heat sink gets warm and the LEDs are bright. The garage temperature is 13.6C and after 45 mins the heat sink is 34.7C. Something to warm my fingers on in the cold!

Cheers Michael.

[Les Jones 1Participant @lesjones1]

Hi Michael,
PC power supplies need a reasonable amount of current to be drawn from the 5 volt output (And possibly the 3.3 volt output on modern ones.) before the 12 volt output will give the correct voltage.

Les.

[Les Jones 1Participant @lesjones1]

I have added a way to drive a string of LEDs that require between about 13 volts and 30 volts with constant current drive from a 12 volt supply to my Weebly website. It uses one of the cheap LM2577 based step up regulators that are sold on Ebay.

Les.

[jason udallParticipant @jasonudall57142]

Feeling idol just now..
Any one got a comparison of THERMAL resistance for surface mount lead free solder joint and through-hole lead-tin solder joint…because it strikes me that the thermal managment of the production lamps is significantly off

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Horner on 26/10/2014 08:46:16:

… Bought a 20amp PSU now the heat sink gets warm and the LEDs are bright. The garage temperature is 13.6C and after 45 mins the heat sink is 34.7C. Something to warm my fingers on in the cold!

.

Very interesting, Michael

If you have the opportunity … Could you please check what current these strips are now drawing at 12volts.

Also maybe the dimensions of your heatsink.

Many thanks

MichaelG.

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