Need help reading a schematic.

[stj]

Re: Need help reading a schematic.

they round it to the nearest available resistor value, it may have been just over 1/4w

[Spork Schivago]

Re: Need help reading a schematic.

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

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.

[stj]

Re: Need help reading a schematic.

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

[Spork Schivago]

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.

[Spork Schivago]

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.

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!

[redwire]

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.

[Spork Schivago]

Re: Need help reading a schematic.

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!

[Spork Schivago]

Re: Need help reading a schematic.

The Ohm's Law formula you use depends on what numbers you know. (I might have forgotten the E squared in my post, sorry if that confused).
Two ways to do it, look at voltage across the resistor or current through the resistor.

Applying Ohm's Law: P= I^2*R or P=E^2/R

For your LCD display:
White/Blue backlight likes 20mA and about 3.4V: (probably 1 LED)
R2= E/I = (5-3.4)/20mA = 80 ohms
P= I^2*R = 20mA^2*80 = 32mW
LED power P= E*I = 3.4V*20mA = 68mW

Yellow-Green backlight likes 100mA and about 4.2V: (probably 10 LED's)
R2= E/I = (5-4.2)/100mA = 8 ohms
P= I^2*R = 100mA^2*8 = 80mW
LED power P= E*I = 4.2V*100mA = 420mW

SMD 1206 is rated 250mW or 167mW derated by 2/3
SMD 0805 is rated 125mW or 83mW derated by 2/3
SMD 0603 is rated 100mW or 67mW derated by 2/3
I'd use an 0805. They aren't super small like pepper flakes.
A (more modern) low power backlight also means the voltage regulators will run much cooler.

Maybe voltage drops are confusing you. Start with power 5V and then subtract the two voltage drops (across R2 and then backlight LED) and then you hit zero 0V (GND).

Yeah, the missing squared was throwing me off! I was looking at my circle and just kept on seeing I = V / R. Now that I know you was definitely using the power formula (wattage), it all makes sense! I do struggle a little with the voltage drops some times but I'm getting better. I'm okay when it's just one device and I'm calculating the voltage drop across that device, but I seem to struggle when there's more than one device.

Also, the guy originally used a 16 ohm resistor but if I really understand this, with the white / blue background, I should be using an 80 ohm resistor? Because the Voltage / Current shows us the resistance is 80 ohm. Or am I misunderstanding that too?


Eventually, I want to replace the LCD screen with a touch screen QVGA LCD type screen. Thanks for all the help and showing me how to figure that out!

[stj]

Re: Need help reading a schematic.

The schematics say it's an HD44780 for the LCD display, but for some reason, what I got was a BC1602A

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.

[redwire]

Re: Need help reading a schematic.

The Ohm's Law formula you use depends on what numbers you know. (I might have forgotten the E squared in my post, sorry if that confused).
Two ways to do it, look at voltage across the resistor or current through the resistor.

Applying Ohm's Law: P= I^2*R or P=E^2/R

For your LCD display:
White/Blue backlight likes 20mA and about 3.4V: (probably 1 LED)
R2= E/I = (5-3.4)/20mA = 80 ohms
P= I^2*R = 20mA^2*80 = 32mW
LED power P= E*I = 3.4V*20mA = 68mW

Yellow-Green backlight likes 100mA and about 4.2V: (probably 10 LED's)
R2= E/I = (5-4.2)/100mA = 8 ohms
P= I^2*R = 100mA^2*8 = 80mW
LED power P= E*I = 4.2V*100mA = 420mW

SMD 1206 is rated 250mW or 167mW derated by 2/3
SMD 0805 is rated 125mW or 83mW derated by 2/3
SMD 0603 is rated 100mW or 67mW derated by 2/3
I'd use an 0805. They aren't super small like pepper flakes.
A (more modern) low power backlight also means the voltage regulators will run much cooler.

Maybe voltage drops are confusing you. Start with power 5V and then subtract the two voltage drops (across R2 and then backlight LED) and then you hit zero 0V (GND).

[Spork Schivago]

Re: Need help reading a schematic.

Redwire,

Google says this about finding a resistors power dissipation (I didn't know what I was looking for was called power dissipation, thank you!)

First, we use Ohm's law ( V = I x R ), to find the current through the resistor. The voltage across the resistor is V = 9 V. The resistance of the resistor is R = 100 Ω. Then, we can use the power rule ( P = I x V ), to find the power dissipated by the resistor.

The current the back light uses is 20mA though, so can I just use P = 20mA x 1.5V? Or P = 0.02A * 1.5V
P = 0.03 Watts
0.03 * (2 / 3) = 0.02 Watts

0.02 as a fraction would be 1 /50th. That just seems reeeally small! I guess I'm still confused here.

[Spork Schivago]

Re: Need help reading a schematic.

R2 would depend on the display, modern displays can use a single high-eficiency led with a good light-pipe,
older ones could use anything from 2 to 10 or more - specially if they are actually "back" lit..

No, no, not the resistance. I want to know how to calculate the smallest wattage resistor I can safely use. IE, is an 1/8th watt 16 ohm resistor the smallest wattage resistor, or is 1/16th watt 16 ohm or 1/32th watt 16 ohm. I want to use SMD resistors but I see a lot of SMD resistors that are small wattage, like 1/16th seems common, but there's also 1/32th of a watt. Most through holes I see are always 1/8th watt or usually they're a 1/4 of a watt. Never smaller.

[Spork Schivago]

Re: Need help reading a schematic.

Thank you RedWire! So my math formulas were straight up wrong then? I used the magic circle. Was I calculating how much wattage the device was using instead of how much power it was actually dissipating? I know my numbers were wrong and what you said make sense except for the formula. I thought V / R = I. I thought wattage was found using something like:
V * I = P
V^2 / R = P
I^2 * R = P

The schematics say it's an HD44780 for the LCD display, but for some reason, what I got was a BC1602A ( ) It's supposed to be blue in colour but I haven't had a chance to actually power it on yet to check. Still gotta finish soldering all the UV LEDs.

If I'm reading the datasheet correctly, current for the backlight would be typically 20mA, right? For the backlighting voltage, it's using 3.5V for the blue type screen, right?

So V / R = P (I thought V / R == current, not watts)
5 - 3.5 = 1.5V
1.5V / 16 ohm = 0.09375 Watts

Then I just want to take 2/3s of 0.09375 watts.
0.09375 watts * (2 / 3) = 0.0625 watts

0.0625 as a fraction would be 1/16th, so I could safely use a 1/16th watt 16 ohm resistor or I could safely use two 1/32th watt 8 ohm resistors, in series?

[stj]

Re: Need help reading a schematic.

R2 would depend on the display, modern displays can use a single high-eficiency led with a good light-pipe,
older ones could use anything from 2 to 10 or more - specially if they are actually "back" lit..

[redwire]

Re: Need help reading a schematic.

You can calculate a resistor's power dissipation two ways- knowing voltage across it, or knowing current flow.

For resistor R2, it does not have 5V across it because the LCD backlight uses some. Check the LCD datasheet, it might be 3.2V say depending on color/size. You can also read the backlight current rating.
So R2 sees (5-3.2V)/16=0.20W (as an example)
Good practice is to use a part at ~2/3 of its rating. Why? Resistors are usually rated at 70°C/170°F which is a bit hot for long life.

To dissipate heat, physical size is all that matters and SMD resistors still need to be fairly big. Get your backlight numbers and try the math. You can also use two (SMD) resistors together and double the power handling.

[Spork Schivago]

Re: Need help reading a schematic.

The original schematic I posted on page 1, the PDF. I'd like to replace the through hole resistors with SMD resistors but I'm not 100% sure how to calculate what the power rating for the SMD resistors need to be. Ie, do they need to be 1/16th of a watt, an 1/8th of a watt, a 1/4 of a watt, etc. The through-hole one, R2, is an 1/4 of a watt through hole. But how do I calculate what's the smallest wattage SMD resistor I can use in replace of the through hole 16 ohm through-hole resistor.

[stj]

Re: Need help reading a schematic.

what's this about?

[Spork Schivago]

Re: Need help reading a schematic.

Would I use something like P = (v * v) / R? Where V would be the 5V that goes to the resistor, R = 16 ohm, which is the resistance of the resistor.

Something like:
P = (V * V) / R
P = (5 * 5) / 16
P = 25 / 16
P = 1.5625

I read that I should double that value when using SMD resistors, so
P = 3.125. That can't be right at all! What am I doing wrong here?

The current going through that resistor would be 0.3125 amps, right? Using I = V / R, I = 5 / 16, I = 0.3125.

[Spork Schivago]

Re: Need help reading a schematic.

So, I now want to modify the circuit and replace some of the components with SMD versions. For example, R2 is a 16 ohm resistor. I know wattage = voltage * amperage. But how can I tell what's the smallest wattage surface mount resistor that can safely be used as a replacement for the R2 resistor there? I believe the through-hole one is a 1/4 watt resistor but do I need to use a 1/4 watt SMD resistor? Thanks.

[Spork Schivago]

Re: Need help reading a schematic.

Oh cool, thanks! We don't have a lot of room. Two bedroom, one was turned into a work room. But I'd like to own a house one of these days. A two floor house would be nice, with an arcade / video game room. I could own a bunch of the old consoles and even some of the new. Hook them all up to a TV using some sort of fancy setup. Mabe even have a pinball machine or two, a soda machine and maybe even one of those coin changer do-dads.