Newbie questions, please help !!!!! - Page 1
Forum section: electronics / beginners

author: RivaultUser
author_rank: Contributor
timestamp: December 31, 2012, 02:43:24 pm
content: Hi guys, I'm new to electronics and after I watched EEVblog video I found words that I couldn't understand.....What does it mean by :1. Cut off voltage2. Discharge current ( like in the graphic, it was on the x axis with mA )3. Ambient temperaturePlease give me childish explanation & an example if you can ^^There are also some questions that I want to ask :1. I know that transistor can be used to amplify current. By amplifying the current, is the voltage also increasing as well?2. I'm working with 4017 chip right now and I'm trying the knight rider circuit, I don't have diodes so I did this without diodes. And I FAILED!!! You know that I have to use 2 outputs for 1 LED and all of them don't turn on, only LEDs with 1 output turns on. What happened ?3. Are AC & DC at the same voltage can power up same devices ( like bulbs, LED, etc.)4. What's the different between adapter & switching ?Please helppp!!!!! I love electronics, but sadly I am not smart as my other friends, so I recently end up with asking stupid questions. Please bear with me!

author: c4757p
author_rank: Super Contributor
timestamp: December 31, 2012, 03:33:32 pm
content: Nothing special about those words - they aren't technical terms. Cutoff voltage is the voltage at which something cuts off, discharge current is the current that flows while discharging something, andpleasejust look up the word "ambient"1. No, but by giving it a load resistor, you can make this happen (see Ohm's Law: as the current through a resistor increases, the voltage across it also increases). This is only subtractive, though, you'll never get more voltage than you started with.2. Give me some time to look things up and I'll come back with an answer.3. Yes, for resistive devices like light bulbs. It all depends on how the device itself functions. LEDs don't like having voltage applied "backwards" so you'll have to bypass it with a diode in the opposite direction that lets current flow across it rather than through it, or put it in series with a diode that only lets current flow in one direction.4. A traditional adapter uses a line transformer, directly converting the AC voltage to another AC voltage.Thenit might convert it to DC, and regulate it if you're lucky. A switching adapter converts the AC directly to DC, then back to AC at a controlled frequency, through a transformer, then to DC again. Regulation is achieved by changing the characteristics of the secondary AC. (Abitof an over-simplification, but SMPS's are a bit complicated).

author: c4757p
author_rank: Super Contributor
timestamp: December 31, 2012, 03:41:42 pm
content: I can't find the circuit you are talking about. I foundone pagethat looks like it might be the one, but the diagrams won't load for me.Let me take a poke at it with a guess:Are the diodes in series with the LEDs? If the voltage is above about 5V, they're probably to protect the LEDs. When the LED shuts off it would have the whole voltage of the circuit applied across it in reverse, which as I said before they don't like. The extra diodes may be to prevent that. If you take them out you probably won't see a difference, but you're slowly damaging the LEDs.Look atthis LED datasheet. In the "Absolute Maximum Ratings" table on the second page, it says that the maximum reverse voltage is 5V. Above that, the LED may begin to conduct, even though as a "light emittingdiode" you'd think it wouldn't. All diodes have a reverse breakdown voltage, and LEDs have a particularly low one.

author: alanb
author_rank: Frequent Contributor
timestamp: December 31, 2012, 03:49:49 pm
content: On the Knight Rider circuit I assume you are refering to this circuithttp://www.talkingelectronics.com/FreeProjects/555/555-P3.html. The purpose of the diodes is to make each led (except from the ones at the ends) activated by two alternative outputs of the 4017.I suggest that until you get some diodes that you only connect one output to each led. This should light each led in turn but they will only appear to move in one direction.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 31, 2012, 04:08:29 pm
content: Quote from: RivaultUser on December 31, 2012, 02:43:24 pmsurrounding temperature, ie air around you. but when you measure it, be sure its done under the shade as measuring under the sun will also register "sun radiation" temperature, which will be higher than "ambient" temperature.Quoteadapter is something that can adapt from one condition to different condition. in technical term it means a device that connects one different device to another different device that you cant simply connect directly to the former device. switching is when something switch (change condition). in technical term it means something that can switch from ON to OFF condition vice versa.ps: i believe kindergarten teacher can be alot harder and messier than higher grade teacher. but if one knows and can "adapt" to the situation, then i believe things can get alot easier.

author: RivaultUser
author_rank: Contributor
timestamp: December 31, 2012, 04:15:11 pm
content: Quote from: c4757p on December 31, 2012, 03:33:32 pmThx for your reply man, I wanna ask again about the transistor.Why in this schematic :http://www.instructables.com/files/deriv/F6U/2LOH/H68GSWNV/F6U2LOHH68GSWNV.LARGE.jpguse a transistor for each LED??? The input Voltage is 12 V, so why use transistor again ? 12 V can definitely lights up LED !?

author: c4757p
author_rank: Super Contributor
timestamp: December 31, 2012, 05:44:12 pm
content: That way the IC doesn't have to drive the LEDs itself, it just signals them. Many ICs aren't meant to output high currents. (Most can handle an LED, but might still not like driving multiple at once.) Look up the maximum current, total and per pin, for the chip.

author: mariush
author_rank: Super Contributor
timestamp: December 31, 2012, 06:12:04 pm
content: LEDs consume power when they light up. How much current they use varies from led to led, but generally it's around 10-30 mAIf the LED was connected directly to the 4017 chip, the 4017 would have to be able to send 10-30mA of power through each pin that has a led connected to it. I count 9 leds connected to a 4017, so if all leds would be lit, then the 4017 chip may have to be powerful enough to give 9 x 10-30 mA of power to the leds. The chip may not be able to provide that much power.For example, pick any of the 4017 chips here and check the datasheets:http://ro.farnell.com/jsp/search/browse.jsp?N=2025+203062&Ntk=gensearch&Ntt=4017&Ntx=mode+matchallpartialI've picked two and both were saying that the typical current output on the pins is about 1.5-3 mA - this may be too little to get the LEDs bright.So a transistor is used instead in front of each LED, and this transistor acts like a switch. When a little bit of current (for example as low as 0.5-1mA of power ) goes into the transistor base, there's a connection made between the other two pins of the transistor so current flows from the 12v wire through the led and down to ground through that 240 ohm resistor.Now, LEDs need a minimum voltage to light up, but then they'll work in a large range of voltages, as long as the current flowing through them is somehow limited.A red led would start to light up at about 2-2.5v and as you increase the voltage it will become brighter and brigher, up to a point where it just burns up. But if you limit the current going through it, the LED can work for a long time at a high voltage like 12v with no problems.That's what that 240 ohm is doing, it restricts the flow of current to a small value, like 25-50 mA of current. If you use a smaller value, the current would be higher and led would be brighter or may burn up. If you use a bigger value, led would be dimmer.As designed, the circuit is a little bit bad, because it relies on the knowledge that the chip 4017 will only light one LED at a time. As it is right now, no matter how many leds light up by the 4017, they're all restricted to a certain maximum current going through them, by that 240 ohm resistor.If one led lights up, a certain amount of current goes through it - let's say 50mA. If two leds would be lit, the same 50mA goes through both, so each LED would get about 25 mA. And so on.If the circuit were to light several leds at the same time, and you want all leds to light up equally bright, you'd have to put a 240 ohm resistor after EACH led.That resistor value is chosen using a formula called ohm's law:V = I x R where v is voltage, i is current , r is resistanceI = V / R or R = V / Iso if you want to limit the current going through the led, you pick a resistance value which does so.Your voltage is 12v, but the LED themselves have a certain voltage drop, how much voltage has to go through them to actually light up. That's about 2-4v, depending on the color of the LED. So the actual voltage that would go through the resistor is 12v - (up to 3-4v).. let's say for our example 3v.So now your formula becomes( 12v - 3v ) / 240 ohm = 9 / 240 = 0.037 ASo in your circuit, all LEDs on one 4017 get 37 mA if they're 3v leds. If all would lit up, this 37mA gets split between all 9 leds.But I think circuit is intended only to light one or two leds at a time, so this works OK in practice and you save a few resistors by using a single resistor to limit all leds.

author: RivaultUser
author_rank: Contributor
timestamp: January 01, 2013, 12:00:32 pm
content: Next questionsss :Is impedance is the same as resistance? ( like internal resistance??? I have a speaker with 8 Ohm written on it & people say that 8 Ohm is that speaker impedance. Does it mean like 8 ohm is the speaker's internal resistance?)

author: FenderBender
author_rank: Super Contributor
timestamp: January 01, 2013, 04:18:56 pm
content: You can look it up. Impedance is kind of the AC equivalent of resistance. Because AC oscillates (from positive to 0 to negative, and so on), there are more factors that will affect the impedance (such as capacitance and inductance).

author: RivaultUser
author_rank: Contributor
timestamp: January 01, 2013, 04:25:17 pm
content: Quote from: FenderBender on January 01, 2013, 04:18:56 pmSo for short, in DC circuits, impedance is the same as resistance ? Impedance is just resistance in AC circuit

author: JuiceKing
author_rank: Regular Contributor
timestamp: January 01, 2013, 04:55:13 pm
content: Quote from: RivaultUser on January 01, 2013, 04:25:17 pmImpedance is a combination of resistance and reactance, both of which oppose an AC current. Resistance is like friction in that it generates heat as it opposes the flow, whereas reactance is more like the action of a spring in that energy opposed is temporarily stored and not radiated as heat. Inductive reactance goes up with frequency, capacitive goes does down. All components have some of both as well as resistance, and their amounts and proportions will affect how much overall flow is impeded for a given frequency current.