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Re: Need help reading a schematic.
Originally posted by redwire View PostEdge-lit (one LED) blue/white 64-80 ohms; yellow-green 44-56 ohms to give 20-25mA. You have a bit of range there.
The back-lit displays use more LED's so the original 16 ohm R2 was for a 5 LED array.
I was hoping to find some sample projects that I could work on. You know, like word problems that say stuff like build a circuit that does blah. So I could work on trying to build a circuit and see how I'm doing. Thanks for all the help in explaining the power dissipation stuff with the resistors. That's really helpful and now I know!-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
I don't really understand what you mean by 5 LED array but I'll try to figure that out.
Just to make sure I finally understand this, if I had an LCD screen that used 40mA of current and I was supplying 12V of power but the backlight was using 6V, I'd use a 150 ohm resistor, right?
R = V / I
R = (12 - 6) / .04
R = 6 / .04
R = 150 ohm
And then to calculate the power dissipation for that 150 ohm resistor, I'd use the power formula like this:
P = V^2 / R
P = (12V - 6V)^2 / 150 ohm
P = 6^2 / 150 ohm
P = 36 / 150 ohm
P = 0.24 watts or 240 milliwatts, right? Roughly a 1/4 watt resistor, is that right? Even if I take 2/3's of 0.24 watts, I get 0.16 watts, but that's still too big for an 1/8th of a watt.-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
Originally posted by stj View Post-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
Originally posted by keeney123 View PostOne teacher I had told me they under rate resistor power by 40% for safety factor. This was on the carbon resistors.-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
Originally posted by stj View Postthey round it to the nearest available resistor value, it may have been just over 1/4w-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
So, I have a few questions here. I just successfully soldered my first out of twelve UV LED PCBs. It had 8 150 ohm resistors and 32 UV LEDs. The person who sold me the board with the UV LEDs say he cannot provide a datasheet or info on the UV LEDs because it'd hurt his business. 12 boards with 32 UV LEDs came to around 155$ USD. They're supposed to be 395nm 160 degree viewing angle UV LEDs but I can't seem to find any online that match those specs and for 395nm, they're awful blue. I would think a true 395nm wavelength wouldn't be very bright to the naked eye.
Anyway, on to the questions! So, the 12 boards get daisy chained together. I will have 6 on the top, 6 boards on the bottom. It'll be 2 columns by 6 rows so each six will look like this:
|--|--|
|..|..|
|..|..|
|..|..|
|..|..|
|..|..|
|..|..|
|--|--|
All 12 boards are powered by a 15 volt, 36 watt power supply. In the instructions though, he says when I finish one board, I can test it by applying 15 volts to it.
Here's my question. If all 12 boards take one 15 volt power supply, isn't 15 volts way to much for just one board? There's 32 UV LEDs on each board. There's 8 rows with 4 LEDs per row. So, the four rows are connected in parallel and each of the four rows should share the same voltage because of this. I guess where I'm getting confused is with the parallel circuit combined with a series circuit. I know the voltage drop for parallel circuits is equal to the supply. Ie, the voltage drop for each of the four rows should be 15 volts, correct?
If that's the case, then to calculate the current going through the resistors in the back there, the 150 ohm ones, using Ohm's law. I = V / R
I = 15v / 150ohm
I = .1 A
I = 100 milliamps.
Is this right so far? How do I calculate the voltage drop across each LED? I know the voltage, 15 V, but I don't know how much current they're getting or how much resistance they have without a datasheet.
Also, I was under the impression that no matter how much current I use I can go over the minimum amount required for the circuit. For example, if a device requires 12 VDC, 1 Amp, I could power that device with a 12VDC, 800 Amp battery. I want to make sure this is correct so I don't end up frying my board.
I've attached a schematic and board layout to make it a bit easier to visualize. Any help is appreciated.-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
The current draw is dictacted by the load not by the power supply, I.E. you can use 12V 10A power supply but if the load only draw 1A, the 10A power supply will not cause the problem, it is when you are trying to draw more current than the power supply can handle, then that will be the problem.
Now your calcualtion:
If that's the case, then to calculate the current going through the resistors in the back there, the 150 ohm ones, using Ohm's law. I = V / R
I = 15v / 150ohm
I = .1 A
I = 100 milliamps.
Is this right so far? How do I calculate the voltage drop across each LED? I know the voltage, 15 V, but I don't know how much current they're getting or how much resistance they have without a datasheet."
That will be ture if you have ONLY the 150 Ohms as the only load, but you don't because you have 4 LEDs connected in SERIES with the resistor so the Voltage drops on the resistor will not be 15V.
You can find how much current of each row of LED is drawing by measuring the Voltage drops across the 150 Ohms resistor uisng Ohms law, then * by 8 to get total current the circuit needs. You can also measure the Voltage drops on one of the LED to find out what the Vf of the LED is.
What does the LED look like? The foot print of the LEDs are just T-1 or T-1 3/4
As you can see for the low current UV LED, the Vf is about 3.4V. we know that the circuit you have will be way low for sure due to the 15V power supply and 150 Ohms in series with the 4 LEDs.
https://cdn.badcaps-static.com/pdfs/...2aba06648d.pdf
15V - (3.4Vf * 4) = 1.4V across 150 Ohms resistor = 1.4V/150 Ohms = 9.3mA.Last edited by budm; 11-25-2015, 06:04 PM.Never stop learning
Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956
Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999
Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl
Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/
TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
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Re: Need help reading a schematic.
Originally posted by budm View PostThe current draw is dictacted by the load not by the power supply, I.E. you can use 12V 10A power supply but if the load only draw 1A, the 10A power supply will not cause the problem, it is when you are trying to draw more current than the power supply can handle, then that will be the problem.
Now your calcualtion:
If that's the case, then to calculate the current going through the resistors in the back there, the 150 ohm ones, using Ohm's law. I = V / R
I = 15v / 150ohm
I = .1 A
I = 100 milliamps.
Is this right so far? How do I calculate the voltage drop across each LED? I know the voltage, 15 V, but I don't know how much current they're getting or how much resistance they have without a datasheet."
That will be ture if you have ONLY the 150 Ohms as the only load, but you don't because you have 4 LEDs connected in SERIES with the resistor so the Voltage drops on the resistor will not be 15V.
You can find how much current of each row of LED is drawing by measuring the Voltage drops across the 150 Ohms resistor, then * by 8 to get total current the circuit needs.
What does the LED look like?-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
Originally posted by stj View Postif the seller actually has a datasheet, i would insist on the voltage, current and disipation numbers.
he obviously wants to hide the mfr/supplier name.-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
I got 2.116VDC, which can't really be right though. Maybe my multimeter needs calibrating or my Rigol DP832 needs calibrating. 2.116VDC * 8 = 16.928VDC, which is almost 2 volts over what I had the PSU set for (15.000VDC, 2 Amp). Something else is wrong too. When I laid my hand on the board to touch probes to the resistor, the board started flickering. Gonna try touching a soldering iron to each component to see if I can find a bad solder joint. Was almost certain I had them all good though.-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
So You are getting 2.116V between the two legs of the 150 Ohms resistor using 15V power supply, then 2.116V/150 Ohms = 14.1mA on each string of the LED, so 8 string = 14.1 x 8 =112.8mA for TOTAL current draw, so your power supply must be able to supply at least that much of current.
'2.116VDC * 8 = 16.928VDC, ' No, You are still confused about the circuit (sorry I was not clear about calculating the current through the resistor), you forget to calculate the current value after you know the Voltage drops across the 150 Ohms resistor.Last edited by budm; 11-25-2015, 06:23 PM.Never stop learning
Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956
Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999
Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl
Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/
TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
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Re: Need help reading a schematic.
Okay, I fixed the blinking. It was my alligator clips not making a good contact. Switched them with needle probes and resoldered the connections to be sure. The resistors have a voltage drop of around 2.2VDC. The UV LEDs have a voltage drop of around 3.2VDC. 3.2VDC * 4 = 12.8VDC + 2.2VDC = 15VDC. That makes sense.
The current going to the resistor would then be
I = V / R
I = 2.2 / 150
I = 0.0147 or 14.7 milliamps, right?
So for current for the whole circuit, I'd just do:
14.7 milliamps * 8 = 117.6 milliamps? The UV LED board only uses 117.6 milliamps?-- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full
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Re: Need help reading a schematic.
Hey, you got it now. So the power supply he sold you is on the over rated side as far as the current capacity is concerned. 15V 1/4A power supply will be good enough.Never stop learning
Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956
Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999
Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl
Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/
TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
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