| Electronics > Beginners |
| How to wire this setup? |
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| KL27x:
--- Quote ---But for the Arduino and the Servo it seems that Vdd/Power and Vss/Ground are the same thing. Any reason not to label them similarly? --- End quote --- I'm not even sure what the second D in Vdd means. I use it because that's what it says on the datasheet, lol. But I understand at least one of the D's is for "drain," and that Vdd is used to denote the supply pin of device that is composed of mostly CMOS/FETs or to denote the logic bus/rail. In my domain this is usually a microcontroller or logic chip. When BJT logic chips were the thing, it was Vcc, not Vdd - presumably one of those C's was for "collector." Perhaps it stands for Voltage Drain-to-Drain, for CMOS logic push and pull output transistors. So as far as I know, Vdd is the same thing as power/supply, just a little more specific. The only reason I can come up with for not using Vdd/Vss for anything is that someone may come along and correct you, lol. Is there any reason to use Vdd/Vss, at all? Personally, I find it useful because it is very short and unambiguous to write and read, and it sorta designates what voltage is your logic rail. CMOS stuff typically operates at 1.8 to 5V, but depending on your circuit, it could be anything between. --- Quote ---Also is the labels from the power rails and to the ground even necessary; or are they just on there for clarity? --- End quote --- I dunno why I put those on there. Certainly not necessary until it is necessary. And here, it is not necessary. :) Yeah, your calculation of base resistor is fine. It's more of a stupidity check. It's not that critical. In this case, I'd use whatever resistor I had between 1K to 2k. Anything from 300R to 4K would probably be fine, depending on your duty cycle, perhaps. The higher you go, you might get less draw through your LEDs. I think maybe the Hfe will show how much the transistor would draw from an ideal power source, e.g., voltage source with zero impedance.* So you would add the impedance of the transistor to that of the LED strip, and you might not get "full power" when you go much higher than 1.1K, and it will dissipate some extra heat in the transistor, which is probably the main failure concern when you use too little base current. But as long as the transistor doesn't get hot at 99% duty cycle and the LED does what you want it to, then you have enough base current. *But this would depend on the absolute voltage, so I'm missing at least one piece of the jigsaw puzzle as to what Hfe is exactly a measure of. For most of the reasons one would have to know this stuff, the work is already done for us. We have opamps, voltage regulators, and buffer IC's to choose from, galore. The new schematic looks fine, to me. :-+ |
| Youkai:
I started doing a test wire up to see if it would work and I ran into a weird issue. I set my 3 PWM pins in arduino to have a 100% duty cycle so the LED would just be on and I could test the circuit. Then I started wiring it up. My green LED activated before the circuit was complete. If you look at the wiring diagram (left side of the graph paper) you can see how it was wired. When I attached the green wire to the collector (I think) of the transistor; the LED lit up even though the emitter was not connected to ground. On the right side of the graph paper I tried (and probably failed) to show you how the breadboard was wired; in case I have some very fundamental misunderstanding of how this should be hooked up. In that drawing the + is the 12V from the wall outlet. The arduino is being powered by USB from the computer. Additionally attached is a picture of the breadboard though I doubt that will be terribly helpful. Finally when I was doing some testing I plugged in the wall outlet and tested it with a volt meter. It read 17V even though the thing says it's 12VDC. Is that normal? |
| KL27x:
It's been a lot of years since I used a TO 92 transistor. The first datasheet I pulled up for a 3904 NPN transistor shows C and E being reversed from what you have, there. That said, I tend to get it backwards on his particular package despite having the thing in front of me. If you have Arduino connected to base and the green LED strop cathode to the emitter, it's possible you have put enough reverse voltage on there to kill your transistor. There's a maximum Veb on a transistor. 3904, for example, is guaranteed for 6V minimum. You might also damage the Arduino. You also have no current limiting resistor on either of those transistors. This could damage the transistor, too. And it's not very nice to the Arduino, but probably not a killer by itself (if you had it wired the right way, at least). You also don't have a ground connection between the arduino and the breadboard/PSU. Even if they are both earth referenced, it's probably a good idea to add a direct connection so's your voltages are more stable. |
| Youkai:
Yeah I didn't put the resistor from my wiring diagram in because I was just doing a quick and dirty test to see if I could get it to work and if my soldering as good. I only planned on running it for a minute so I figured if a little more current went through than was idea I would probably be ok. This post from earlier has the back of the package for the transistors: https://www.eevblog.com/forum/beginners/how-to-wire-this-setup/msg1531484/#msg1531484 If I'm looking at the diagram correctly then; if you are facing the flat side the collector is the left-most pin. I believe I used the BC557B transistor. |
| Youkai:
Oh! Hmm. I looked at the data sheet/that picure again. I guess the 557b is a PNP. I should use the 547b (an NPN) I guess. Does that provide a reason for this activity? EDIT: after a quick google of PNP transistors it seems that this is likely the cause. My arduino pin was acting as ground and current was flowing through without the need of the third terminal. If someone could confirm that would be appreciated. |
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