Electronics - advice on circuits and LEDs

dbov22

dbov22

dbov22

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Hello, haven't posted for ages but still read a good few times a week.

I recently got some entusiasm to start making an LED light panel.

I have done some research on internet and youtube but am confused by different pieces of advice.

I used a DC transformer from a garden light set which has the following info stamped on it.

INPUT 220 - 240V ~50Hz 5W 0.002A
OUTPUT: 12V COS (CIRCLE WITH LINE THROUGH SYMBOL) = 0.98
MAX 5W MIN 0.2W 0.8A 250V

Right, now maybe wrong here and don't fully understand the theory behind all of this (working on it) but I bought some LED's from Ebay which were 0.022A.... I worked out I could put 40 in my makeshift panel, I wired them all up in what I beleived to be correct way and then plugged into a small 3V watch battery - they all worked, then wired up to mains - they all worked.

I dont get how they work on a 3V battery and also with 9V mains. Also, I placed no resistors in the circuit but should I have done and why?

Could I put more LED's in the circuit or less and what would happen?

Thanks guys!!

Dan
 
My guess would be that the 12v COS DC supply is a constant current power supply intended for LEDs. A 3v watch battery is current limited by it's own internal resistance so while it may provide more than 20mA it's not enough to instantly fry the LED.

LEDs are current devices so provided you control the current to them they'll work happily.

Still you'd imagine that wiring 40 LEDs up to a battery in parallel wouldn't work too well.

Normally resistors are required to control the current to LEDs. If you don't use them and you provide a high current source to them they'll attempt to use all the current and fry themselves almost instantly. An alternative to resistors to control the current is a constant current supply.

Depending on how you've wired those LEDs you'd probably experience LED failures pretty quickly as the one with the lowest forward voltage steals all the current and soon you'd get a runaway failure as they'd all pop in turn.

Wiring LEDs is simple but you have to make sure you don't have too many in series with each other, each series must be protected by a resistor (or constant current supply) and never wired in parallel to a single resistor.
 
Where did the '9V mains' come from? Do you mean 12V as you specified earlier?

Perhaps you could tell us your final goal with these LEDs, and then maybe draw up a circuit diagram of what you've achieved so far as a starting point.

EDIT: Just to add, the 'COS ϕ' you saw on the transformer is related to efficiency (put simply). This is not a constant current power source, and if you use it to power LEDs without resistors, you will destroy all of them very quickly.
 
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tim80bwi said:
EDIT: Just to add, the 'COS ϕ' you saw on the transformer is related to efficiency (put simply).
Click to expand...
Well, it is Power Factor really :)

The circle with a line though it is actually Phi, Cos Phi is how Power Factor is expressed


@12v, Fv 2v, 20mA, series resistor for each LED required would be 560ohms, 1/2W


P/V=I
5W/12V=0.4Amp max output of PSU

that's enough for 20 LEDs @ 20mA each



(unless I made an error, am tired... I was sure Vf was nominally .7v but it seems it is 2v ?)


example circuit, change values to suit
http://chow****cheong.com/Electronics/LED%20Circuit.jpg

**** = f o o k :rolleyes:
 
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bitslice said:
Well, it is Power Factor really :)

The circle with a line though it is actually Phi, Cos Phi is how Power Factor is expressed
Click to expand...

Clearly. I was merely trying to avoid adding any more confusion to the thread :)

bitslice said:
(unless I made an error, am tired... I was sure Vf was nominally .7v but it seems it is 2v ?)
Click to expand...

Forward voltage drop can vary wildly across semi-conductors. Bog standard diodes will be closer to 0.7V, whereas LEDs can be significantly higher, largely dependent on the colour of light emitted.
 
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No doubt I will have done something wrong, the LED's have been on for 4 hours now without any sign of dimming, blowing out etc, I will draw a circuit diagram and take a pic of what i've done - give me 12 hours, lol...Bed time!

zzz zzz zzz
 
Well LEDs are dirt cheap to replace, so no harm in finding out what you can and can't do the fun way :p
 
here are the pics - all i'll say is, I know cardboard is not the best for a curcuit board - I just wanted to make sure everything worked!

Please don't judge, just advise - I am not a pro and have little knowledge but want to learn!

20120827134551.jpg
By squeekybean at 2012-08-27[/IMG]

20120826165132.jpg

By squeekybean at 2012-08-27

20120826165210.jpg

By squeekybean at 2012-08-27
 
there are no resistors at all, as a youtube video advised me that if you put the correct amount of LED's in the circuit with the transformer then no resistors are needed?!
 
This isn't a good way to do this as LEDs in parallel rarely share current equally owing to forward voltage tolerances. You'll find some are brighter and some may barely even light at all.

The advice from Youtube is presumably trying to use the internal resistance of the transformer as an effective series resistor. This requires a particular specification of transformer to be safe and a SMPS type should be avoided.
 
the LED's are all completely even in brightness, and all very bright so I'm not sure how?

What would be the best method to wire up 40 LED's also where would each resistor have to go and which resistor. I keep being told different things by different people.

hmm (scratches head).

Dan
 
bitslice did the calculation for you in his post above with regard to the size of resistor needed. You need one resistor in series with every LED.

I hate to say it, but if you've been running your existing setup for any length of time already, you may find some of those LEDs start failing on you. It looks like a good start though, keep going! :)
 
Thanks for the encouragement Tim.

right - the rating on my transformer says 0.8A - am I correct in thinking that if each LED is 0.022A I can fit (0.8 divided by 0.022) 36.36 LED's in my circuit and if I have more then the LED's will be dimmer and if I use less then the LED's will need resistors or am I way off the mark?

In this circuit (new photo) where should the resistors Bitslice mentioned go - at which letter? also is there a better way of wiring up this circuit? layout wise etc?
201208271743401.jpg
By squeekybean at 2012-08-27[/IMG]
 
Once again buddy, bitslice has done all the maths for you. Your transformer is limited by its power output, and at 12V this equates to 0.4A of current, or 20 of your LEDs. Any more than this and your LEDs may get dimmer. Less than this, you won't get any increase in brightness. Regardless of how many LEDs you are using however, you need a resistor in series with each.

These won't go at any of the letters you've drawn. They will go in between each of your 'X' marks and the red lines to which they connect. ie. 20 LEDs will require 20 resistors.
 
Right, I have re-read bitslice post and this is starting to make sense...I think what has been confusing is the fact that I have had bad advice from people claiming to know what they're talking about...a good friend of mine said it would need a startup capacitor!

Currently doing more research and starting from the beginning!

Thanks mate, 1 for not just ****ging me off and 2 for your patience dealing with my apparent lack of any understanding. Lol

Dan
 
Well, the saga continues - there doesn't actually seem to be any solid advice on this subject...

I found a much bigger transformer which is 24V in the shed, it doesn't say what Watts it is though (confused)

Anyway can someone maybe just tell me how to make the following, maybe a quick drawing of the 20 panel (i'll make x2) and the 10 panel (i'll make x3).

Ideally all from the original transformer in the 8th post, or this new one..

201208281751171.jpg
By squeekybean at 2012-08-28[/IMG]

20120828171820.jpg
By squeekybean at 2012-08-28[/IMG]

or

20120828171854.jpg

By squeekybean at 2012-08-28
 
The one that says '60VA' on it is your best bet. It'll give you 2.5A output at 24V. So over 100 of your LEDs potentially. As before, you need a resistor in series with each LED, this time with a value of around 1.2 KiloOhms
 
Yes it does.

If you use LEDs with AC they'll basically flash at 50Hz which won't be nice.

You could build a simple full wave rectifying diode circuit with a smoothing cap suitable for the load though but you'll get 30v DC or more out of that 24v AC. (I forget exactly the conversion factor but I think it's 1.414 or something)

It's quite a simple circuit to build. With 30v you could have 10 or more LEDs in series with a single resistor and your power consumption would be 140mA at 30v which is really small and well below what those Christmas light transformers can manage.
 
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