I am working on a project that will have around 200 LED's, a little less. I purchesed a power supply 12vdc 60w. I am doing some testing before I put everything together (so I dont end up with a big disaster) as I am an extreme newbie at these projects and electricity and all. so my white LED's ar rated 3.0v forward 20 ma, which tells me on 12v I can use 4 with a 1 ohm resistor. only when I I do that the LED's burned instantly. I put all my resistors in the >100 value in and they all started burning eventually. I placed 10 LEDS with a resistor again all >100 and the same outcome happened. Is this happening because I am only powering so few LEDS or will this happen when I have everythig completed with all the LED's?
Question about a project
- Thread starter SillyBoy
- Start date
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AnAppleSnail
Flashlight Enthusiast
Your power supply is probably causing the problem. It would probably like to deliver at least two amps at twelve volts. Ten little LEDs will take just about an amp for a second or two before they fail as you've seen.
There are two types of 'ideal' power supplies in the world: Constant current and constant voltage. A constant-current power supply will deliver whatever voltage it takes to run some number of amps. There is usually a sane limit on this range, although it might be inconveniently high. Some constant-current drivers float up to 18v when they cannot deliver their 'full' current. This destroys LEDs when you can't take the full power.
Constant-voltage power supplies calmly sit at a near-fixed voltage level. All that budges them is a huge load that will lower the voltage. Ohm's Law is always followed, so a 60W-rated 12V power supply will not deliver much more than five amps without dropping out of voltage regulation. Some cheap "fixed voltage" power supplies drift in output with low loads, again up towards 18v.
LEDs, especially little ones, are sensitive to voltage changes. Increasing a 3v LED's voltage by 1v can easily double the current it will pass, easily exceeding its maximum rating. So to summarize:
It might go away. Why don't you use an ammeter to check current next time? Make the connection 'momentary' and if you see something higher than you expect, pull the ammeter out to open the circuit. This project shows one reason I don't like those low-power LEDs. They just aren't robust enough for interesting projects.
Edit: Here are some diagnostics you can do.
First; I assume, but do not know: Are you wiring your LEDs in SERIES or in PARALLEL? This is important for our purposes. You could have one "string" of 4 LEDs in a row for a forward voltage of 12V, or four "strings" of 1 LED in a row for a forward voltage of 3V. One will work better than the other here.
1. Go to Wal Mart or a tool store and get a 12v halogen lamp. This will be in the lighting section, and say "MR11" or "MR16" on it. GU10 is no good for this. You want a 10W and a 50W or so. Plug the 50W into your power supply, it should glow BRIGHTLY. Unplug it and put the bulb on a fire-proof surface. Plug it in again and check voltage at the bulb. You should see about 12v. If it's above 14v, take note: Your voltage creeps up under light loads. Next check the current. You should see about 5A. Now try the 10W. Does it immediately burn out? If so, your power supply does not take kindly to light loads. Eew.
There are two types of 'ideal' power supplies in the world: Constant current and constant voltage. A constant-current power supply will deliver whatever voltage it takes to run some number of amps. There is usually a sane limit on this range, although it might be inconveniently high. Some constant-current drivers float up to 18v when they cannot deliver their 'full' current. This destroys LEDs when you can't take the full power.
Constant-voltage power supplies calmly sit at a near-fixed voltage level. All that budges them is a huge load that will lower the voltage. Ohm's Law is always followed, so a 60W-rated 12V power supply will not deliver much more than five amps without dropping out of voltage regulation. Some cheap "fixed voltage" power supplies drift in output with low loads, again up towards 18v.
LEDs, especially little ones, are sensitive to voltage changes. Increasing a 3v LED's voltage by 1v can easily double the current it will pass, easily exceeding its maximum rating. So to summarize:
It might go away. Why don't you use an ammeter to check current next time? Make the connection 'momentary' and if you see something higher than you expect, pull the ammeter out to open the circuit. This project shows one reason I don't like those low-power LEDs. They just aren't robust enough for interesting projects.
Edit: Here are some diagnostics you can do.
First; I assume, but do not know: Are you wiring your LEDs in SERIES or in PARALLEL? This is important for our purposes. You could have one "string" of 4 LEDs in a row for a forward voltage of 12V, or four "strings" of 1 LED in a row for a forward voltage of 3V. One will work better than the other here.
1. Go to Wal Mart or a tool store and get a 12v halogen lamp. This will be in the lighting section, and say "MR11" or "MR16" on it. GU10 is no good for this. You want a 10W and a 50W or so. Plug the 50W into your power supply, it should glow BRIGHTLY. Unplug it and put the bulb on a fire-proof surface. Plug it in again and check voltage at the bulb. You should see about 12v. If it's above 14v, take note: Your voltage creeps up under light loads. Next check the current. You should see about 5A. Now try the 10W. Does it immediately burn out? If so, your power supply does not take kindly to light loads. Eew.
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