Sl100
- Thread starter wdkh68
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It is not a Eagle schmatic - please go to http://www.electronicsforu.com/electronicsforu/lab/ click on S no. 143 for schmatic "LED-Based Message Display" Of course, I will not be using the transformer or bridge, but a wall 9V transformer with the usual filter caps on the 7806. Thanks!
I'm not very fond of that schematic due to the way they skimped on the current limiting resistors.
However, it might not be too bad because they aren't lit all of the time.
The problem you run into when paralleling a bunch of LEDs on one current limiting resistor is that the LEDs with the lowest Vf (forward voltage) get more of the current. If you want to skimp on resistors like that, you are going to have to match the forward voltages of your LEDs, or you will have uneven lighting from them for one (due to some of them getting more current, and some getting far less), and risk having the high-current LEDs burn out on you, causing a "domino effect" where the remaining LEDs parallel with them die due to overcurrent.
But if you want to build it as-is, instead of the transistors, use some of those logic-level n-channel MOSFETs I mentioned in the beginning. Pretty cheap, easy to use, have a very low on resistance, and can handle considerably more current than the transistors you're considering.
One limitation is that even though the transistor might be specified for 700mA current, the realistic capacity is roughly half that, or around 400mA.
Another reasin is that in order to saturate the transistor, you have to give it 1/10 the collector current as base current. This is known as "forced beta". If you try to use less than 1/10 the collector current in the base, the transistor Vce may be higher than expected. The 4017 just isn't capable of sourcing 40mA. You're lucky to get 18mA from it while going downhill with a tailwind, and that is really beyond its' specifications.
If you want to be conservative (always a good idea when designing something) then don't expect more than about 4mA maximum current from a 4017 output. So, that means a 40mA sink current from the transistor. If you're only planning on two 20mA LEDs in parallel, you'll be OK. However, sounds like you're planning on lots of 'em. They'll be pretty dim.
If you use the MOSFETs I suggested, they'll sink either 1.7A or 2.5A. De-rated 50%, that's 850mA and 1.25A respectively. Since they are essentially voltage controlled switches, your current requirements from the 4017 are very low.
However, it might not be too bad because they aren't lit all of the time.
The problem you run into when paralleling a bunch of LEDs on one current limiting resistor is that the LEDs with the lowest Vf (forward voltage) get more of the current. If you want to skimp on resistors like that, you are going to have to match the forward voltages of your LEDs, or you will have uneven lighting from them for one (due to some of them getting more current, and some getting far less), and risk having the high-current LEDs burn out on you, causing a "domino effect" where the remaining LEDs parallel with them die due to overcurrent.
But if you want to build it as-is, instead of the transistors, use some of those logic-level n-channel MOSFETs I mentioned in the beginning. Pretty cheap, easy to use, have a very low on resistance, and can handle considerably more current than the transistors you're considering.
One limitation is that even though the transistor might be specified for 700mA current, the realistic capacity is roughly half that, or around 400mA.
Another reasin is that in order to saturate the transistor, you have to give it 1/10 the collector current as base current. This is known as "forced beta". If you try to use less than 1/10 the collector current in the base, the transistor Vce may be higher than expected. The 4017 just isn't capable of sourcing 40mA. You're lucky to get 18mA from it while going downhill with a tailwind, and that is really beyond its' specifications.
If you want to be conservative (always a good idea when designing something) then don't expect more than about 4mA maximum current from a 4017 output. So, that means a 40mA sink current from the transistor. If you're only planning on two 20mA LEDs in parallel, you'll be OK. However, sounds like you're planning on lots of 'em. They'll be pretty dim.
If you use the MOSFETs I suggested, they'll sink either 1.7A or 2.5A. De-rated 50%, that's 850mA and 1.25A respectively. Since they are essentially voltage controlled switches, your current requirements from the 4017 are very low.
Thanks for the great help! WOW, there are a zillion n-channel mosfet's to chose from and my electronic knowledge is limited (I'm a mechanical engineer in an electronic manufacturing environment) Do I need something in the IRF 500 series TO-220 pkg.?
Also, are the mosfet pin hookups E = source, B = gate, C = drain? What about using individual resistors for each LED instead of just one to take care of the Vf? I notice in the schematic that the LED's are powered from 9V/2A source which is enough for 172 T1, 10ma LED's (1.72 Amps) but since I'll be using a wall adapter, I'll need to cut back on LED'S because the wall units only go up to 1.5 A., but are pretty big. I guesss I was drawn to this schematic because of the quantity of LED's, scrolling and few parts.
Also, are the mosfet pin hookups E = source, B = gate, C = drain? What about using individual resistors for each LED instead of just one to take care of the Vf? I notice in the schematic that the LED's are powered from 9V/2A source which is enough for 172 T1, 10ma LED's (1.72 Amps) but since I'll be using a wall adapter, I'll need to cut back on LED'S because the wall units only go up to 1.5 A., but are pretty big. I guesss I was drawn to this schematic because of the quantity of LED's, scrolling and few parts.
Part numbers beginning with "IR" were originally designed by International Rectifier.
very generally, if the 3rd character is an "F", it's a standard-level MOSFET, requiring Vgs=10 to be turned fully on; if an "L". it's a logic-level MOSFET, requiring Vgs=5 to be turned fully on.
So, instead of IRF parts, you'd want something with IRL in the beginning.
The IRLD014 and IRLD024 are nifty little N-ch MOSFETs you should take a look at. I thought I mentioned those to you before, but I guess I didn't. They can sink 1.7a/2.5a respectively, Vdss=60, and come in a convenient 4-pin DIP package, which is great for breadboarding and easy to use with pre-drilled perfboards.
Mouser, Digikey and other vendors stock them.
If you're going to be running LOTS of LEDs, look at something that will sink more current with Vdss in the 20v to 30v range.
Something like the IRLU2703 would work.
very generally, if the 3rd character is an "F", it's a standard-level MOSFET, requiring Vgs=10 to be turned fully on; if an "L". it's a logic-level MOSFET, requiring Vgs=5 to be turned fully on.
So, instead of IRF parts, you'd want something with IRL in the beginning.
The IRLD014 and IRLD024 are nifty little N-ch MOSFETs you should take a look at. I thought I mentioned those to you before, but I guess I didn't. They can sink 1.7a/2.5a respectively, Vdss=60, and come in a convenient 4-pin DIP package, which is great for breadboarding and easy to use with pre-drilled perfboards.
Mouser, Digikey and other vendors stock them.
If you're going to be running LOTS of LEDs, look at something that will sink more current with Vdss in the 20v to 30v range.
Something like the IRLU2703 would work.
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