First time user of LED's just need some help...

[bachamp]

I have decided to make a better looking fish Tank by using some LED’s with a small light angle to work like spot lights.

Well this is the first time I have made a circuit using leads, I have played around with normal light globes and motors but I have realized this is slightly more technical.

I can read circuit diagrams, and know the basics of the technical side of electricity.

So I have a few questions.

If I use the 12 V DC 1000mA power supply I can power 50 20mA LED’s.
Say (for the point of learning) each LED is 3V than I would put 4 LED’s in series and the link the series up in a parallel circuit. Now am I right, would this be a complete circuit? I would not need any restores or anything, right?

If I’m wrong please tell me, or for that matter if I’m right also tell me.

Another question is that for the fish tank it is currently running of a 20W florescent light, I would like to produce around the same amount of light, so roughly would the 12V DC 1000mA power supply be enough, even If I was using standard brightness led’s, or should a get a bigger power supply just in case.

 

[bananasiong]

If I use the 12 V DC 1000mA power supply I can power 50 20mA LED’s.
Say (for the point of learning) each LED is 3V than I would put 4 LED’s in series and the link the series up in a parallel circuit. Now am I right, would this be a complete circuit? I would not need any restores or anything, right?

I will do it in this way:
R=V/I, 12 V/20 mA=600 hm:
Each LED is connected series with a 680 hm: resistor and 50 sets of them are connected up in parallal. But it's not a good idea that driving the circuit to the maximum supported rating.
If you connect up LEDs in series, I think the light intensity of each LED might be different due to the voltage drop. Anyhow, you need current limiting resistors.

 

[audioguru]

Banana,
You are wasting most of the voltage of the 12V supply by using only a single LED. Also, you calculated the wrong value for its series resistor.
The resistor has 12V - 3V (for the LED)= 9V across it. For 20mA then the resistor needs to be 9V/20mA= 450 ohms. 470 ohms gives a current of 19.1mA.

If 3 LEDs are in series then their current-limiting resistor has 3V across it. Then its value is 3V/20mA= 150 ohms.

Bachamp,
LEDs are diodes, not light bulbs. They operate with current so they need a current-limiting resistor in series. Use 3 in series then connect a 150 ohm current-limiting resistor in series with them. Each string of 3 LEDs needs its own resistor.

The LEDs will use 9V and 1A which is 9W. They will not be as bright as a 20W fluorescent light tube.

 

[bananasiong]

Arhh.. I always forget about the voltage drop of the diode. Thanks
Is 3 V for all of types of the LEDs?

EDIT: What do you mena by this:

You are wasting most of the voltage of the 12V supply by using only a single LED.

Do you mean all the series of the resistors and LEDs are connectec in parallel is not a good idea?

 

[bachamp]

The LEDs will use 9V and 1A which is 9W. They will not be as bright as a 20W fluorescent light tube.

So 50 Leds will not produce as much light as 1 20W Flrorescent light tube, I would have necer have guessed that.

Ok.

So since the leds work on Current not Voltage than there will not be any difference in brightness in a series circuit.

Tell me if i'm wrong but if I put 4 Leds (3V each) in a series circit than I dont need a current-limiting resistor at all?

Also that means that that series is using 12V right? or am I still mixed up?

 

[bachamp]

tell me if I have 1000mA and each led is 20mA, than how many leds can I run

 

[Norlin]

Taking a whack at it using my overly rusty knowledge.
If you set things up like audioguru described, 3 LEDs in a chain with a 150ohm resistor and assuming the LEDs have a 3V drop across each of them.
You end up having 3V across your 150 ohm resistor, current equals voltage over resistance (I=V/R), so your current draw per 3-LED chain is 3/150 A
Or, 20mA, meaning if you max out the suppy you should be able to run 50 chains (150 LEDs) but as was said, it's not recommended that you max things out.

Of course, you also said they're 20mA LEDs, so you'd set up the circuit to put close to 20mA through each LED, which means you'll always have 50 chains whether you go with a 1, 2 or 3 LED chain (changing the size of the inline resistor with each chain size)

 

[audioguru]

LEDs have a range for their voltage. Some will be 2.5V and others with the same part number and same manufacturer will be 3.5V.
If you connect four 2.5V LEDs in series and use 12.0V then the current would be extremely high and they would all burn out instantly.

If you connect four 3.5V LEDs in series to a 12.0V supply then they will not light.

A 3V light bulb is a resistor. If the voltage is 3.0V then it draws 20mA. If the voltage is 2.5V then it draws about 17ma. If the voltage is 3.5V then the current is about 23mA.

An LED is a diode. If it is actually 3V then with 3.0V it draws 20mA. With 2.8V it draws 5mA and is dim. With 2.5V it draws nothing and therefore does not light.
With 3.5V it will try to draw 100mA and it will burn out.
LEDs must have a current-limiting resistor in series. A cheap key-chain LED flashlight uses the high internal resistance of the tiny battery as the current-limiting resistor.

Some LEDs are much brighter than others. LEDs focus their beam in a single direction so bright LEDs would appear brighter when on-axis than a fluorescent tube that shines its beam in about a 90 degrees angle.

 

[justDIY]

you need some expensive and hard to get LEDs to beat "gas in glass" type light sources (fluorescent, hid, etc)

they do exist, but they're not for the hobbiest and not easy to obtain in small quantities

 

[bachamp]

OK, OK, I starting to get it...

Thanks for the advice, I have been doing some reading since my last post and I now know what I'm doing.

If I get a bigger power supply, say 4A but only have say 50 series (3led's and a resistor) is that ok? Will the power supply only draw as much electricity as it needs rather or will it turn the rest of the amps into waste heat?

If I have LED's that are 3.9V each, and a 12V circuit can I have 3 LED's running in a series circuit with a resistor, or will the resistor take up some volts as well and there for there will not be enough volts for the led's?

If this is the case than how do I work out how much volts I need for the resistor?

My understanding of resistors are not the best, but if I select the right one, for this circuit (12-11.7)*1000/20=15, so 15ohm resistor required. Than will the circuit be fine and really the resistor is taking to 0.3V of leftover voltage?

 

[bachamp]

Also with a fish tank, a "gas in glass" type light sources currently using a fluorescent means the light is not really natural, it makes the whole tank light up, and cost AUD$90, but leds can but used like point lights, and this creates a nice ripple affect, it looks good, and also doesn't cost that much in comparison, I am spending a fair bit but i'm getting a good 500 LED's but will only use around 100 of those, so taking that into account it will only cost around AUD$80. So it is not that bad.

 

[Speakerguy]

look for luxeon emmitters. that is what is used in high end fish tank lighting fixture (google PFO solaris). then google the luxeon. They are expensive in single unit quantities.

 

[HarveyH42]

Just out of curiousity, how big is the tank? About how deep? A cheap LED flashlight should give you some idea how it might look. I think even 1 watt luxeons would need some sort of diffusion (don't they warn you not to look directly into the emitter?). You wouldn't want to blind the fish.

 

[audioguru]

You cannot buy "3.9V" LEDs. LEDs have a range of voltages like from 3.5V to 4.5V for the same ones. So allow plenty of voltage drop across the current-limiting resistor so they won't burn out or not light.

You also cannot buy a "12V" power supply. It also has a range of voltages for the same one. It might be 12V +-5% which is from 11.4V to 12.6V.

The current-limiting resistor's value could be 5%low or 5% high.

You should consider all these things.

 

[justDIY]

blind the fish?!

I hardly think that is a concern. Professional reef keepers use thousands of watts of HID lighting for their tanks, and don't seem to have a problem with fish blinding, let alone the real fish in the sea have the Sun to contend with, which is billions of times brighter than any led despite being millions of miles away!

 

[bachamp]

You cannot buy "3.9V" LEDs. LEDs have a range of voltages like from 3.5V to 4.5V for the same ones. So allow plenty of voltage drop across the current-limiting resistor so they won't burn out or not light.

You also cannot buy a "12V" power supply. It also has a range of voltages for the same one. It might be 12V +-5% which is from 11.4V to 12.6V.

The current-limiting resistor's value could be 5%low or 5% high.

You should consider all these things.

i will, but at the moment t'm trying to the the theroy right first.

 

[HarveyH42]

blind the fish?!

I hardly think that is a concern. Professional reef keepers use thousands of watts of HID lighting for their tanks, and don't seem to have a problem with fish blinding, let alone the real fish in the sea have the Sun to contend with, which is billions of times brighter than any led despite being millions of miles away!

True, but there is only one sun, not dozens. Also doesn't get turned on full brightness instantly and without warning...

 

[bananasiong]

Unlike human, fishes cannot close their eyes

 

[bachamp]

OK, the fish will not go blind...

Now back to LEDS...

How do I use dimmers?

How can I make it fade from one circuit to another?

 

[Speakerguy]

You would increase the resistance in series with the LED to lower the drive current to dim, and a microcontroller to get the fade effect from one bank of LED's to the next. Remember that we percieve brightness logarithmically. I would guess that LED's brightness would go linearly with drive current, but I could be wrong.