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Old 11-24-2015, 08:40 PM   #81
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Default Re: Need help reading a schematic.

Quote:
Originally Posted by stj View Post
HD44780 is the interface, originally it was the part number of the chip but they are cloned to hell now.

BC1602A is the display that is using the chip - part number suggests 16characters by 2 lines, A is probably related to led colour or viewing angle.
Thanks!
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Old 11-24-2015, 09:19 PM   #82
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Default Re: Need help reading a schematic.

Edge-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.
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Old 11-24-2015, 09:45 PM   #83
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Default Re: Need help reading a schematic.

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Edge-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.
Okay.

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!
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Old 11-25-2015, 12:46 PM   #84
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Default 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.
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Old 11-25-2015, 12:54 PM   #85
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Default Re: Need help reading a schematic.

http://led.linear1.org/1led.wiz



Last edited by stj; 11-25-2015 at 12:56 PM..
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Old 11-25-2015, 01:41 PM   #86
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Default Re: Need help reading a schematic.

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So I calculated the resistors value correctly, just not the wattage. How the heck did I get a 1/4 of a watt when they got 1/2 watt. Thought I really was getting this.
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Old 11-25-2015, 02:20 PM   #87
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Default Re: Need help reading a schematic.

they round it to the nearest available resistor value, it may have been just over 1/4w
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Old 11-25-2015, 02:25 PM   #88
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Default Re: Need help reading a schematic.

One teacher I had told me they under rate resistor power by 40% for safety factor. This was on the carbon resistors.
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Old 11-25-2015, 04:39 PM   #89
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Default Re: Need help reading a schematic.

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One teacher I had told me they under rate resistor power by 40% for safety factor. This was on the carbon resistors.
Thank you Keeney123. Does that mean a 1/4 watt resistor can actually handle by up to 40% more wattage than 1/4 watt? I was told I could safely use 2/3 of the wattage required for SMD components. So in my example, the math is correct?
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Old 11-25-2015, 04:40 PM   #90
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Default Re: Need help reading a schematic.

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they round it to the nearest available resistor value, it may have been just over 1/4w
Gotcha. Thanks Stj. So you think my math is good then? I'd like to avoid tools that automatically calculate the stuff for me right now, to make sure I truly understand it, but I'll definitely try to find one that doesn't round up, to give me a way to check my answers.
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Old 11-25-2015, 05:16 PM   #91
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Default 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.
Attached Files
File Type: pdf UV LED PCB Schematic.pdf (47.6 KB, 3 views)
File Type: pdf UV LED Board Layout.pdf (43.7 KB, 4 views)
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Old 11-25-2015, 05:32 PM   #92
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Default 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.
http://www.mouser.com/catalog/catalogusd/648/144.pdf
15V - (3.4Vf * 4) = 1.4V across 150 Ohms resistor = 1.4V/150 Ohms = 9.3mA.
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Last edited by budm; 11-25-2015 at 06:04 PM..
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Old 11-25-2015, 05:39 PM   #93
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Default Re: Need help reading a schematic.

if 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.
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Old 11-25-2015, 05:46 PM   #94
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Default Re: Need help reading a schematic.

Quote:
Originally Posted by budm View Post
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, then * by 8 to get total current the circuit needs.

What does the LED look like?
Ahhh! I wondered if that was true and I almost thought my logic there was incorrect but I went with it anyway. So, I need to use my multimeter to measure the voltage drop across the resistor and then when I multiply it by 8, that'll give me the total current that each series of LEDs + resistor uses, right? And that'd be the same for each row, because they're all the same and coming off a parallel circuit, right?
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Old 11-25-2015, 05:47 PM   #95
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Default Re: Need help reading a schematic.

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if 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.
Yeah, I thought maybe it was because I could buy the UV LEDs really cheap on aliexpress or whatever it was. I think I can figure most of this out on my own though to narrow down what exactly is being used.
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Old 11-25-2015, 05:55 PM   #96
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Default 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.
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Old 11-25-2015, 06:00 PM   #97
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Default 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 at 06:23 PM..
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Old 11-25-2015, 06:12 PM   #98
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Default Re: Need help reading a schematic.

maybe you should try a new battery in your meter.
just in case!
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Old 11-25-2015, 06:35 PM   #99
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Default 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?
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Old 11-25-2015, 06:42 PM   #100
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Default 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.
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