batteries - Supplying 220V to a cell phone battery?

I have to do this demonstration about batteries for a school project and I was thinking about the effects of supplying excess energy to a battery. I was wondering what would happen if I supplied 220V AC DIRECTLY to a Li-Ion battery rated at 3.7V? I would place the battery in a closed glass box, in case it explodes. But do I risk starting a fire? I want to know exactly what would happen if I do so..

Answer

It will explode, it will catch fire, you will be in trouble with your school administration.

This belongs to the class of questions best answered with:

If you have to ask this question then you don't know enough to do it safely.

What kind of DC motor is this and what does the circuit do?

I've had this motor at least for 5 years (salvaged from something) and I remember it working then. However it doesn't work now (5 VDC mobile charger) and so I opened it up expecting to see just the split rings. But I was surprised to find this circuit built in it. Why does it need that circuit and what does it do? The motor body says - (If the picture is not clear) 6JS E 06 R 2400N 6V

Link to a better quality picture http://postimg.org/image/l4unckkin/

Answer

That looks like a DC motor of the type used in tape cassette players.

Why does it need that circuit and what does it do?

For music reproduction with accurate pitch constant motor speed is required. Achieving this in battery powered equipment requires addition of a voltage regulator or speed controller as DC motors' speed varies (nearly linearly) with voltage. The potentiometer in the motor allows the speed to be factory set.

I modified one such motor on a good tape deck, adding an external pot and switch to allow some pitch adjustment so that I didn't have to keep retuning my guitar between tracks / albums.

The TDA1151 seems to be one simple device intended for such applications.

Does conductance in the transmission line model represent a physical quantity?

For context, someone asked a question on amateur radio SE which I'm having difficulty completely understanding myself:

Why is coax loss logarithmic?

I'm not looking for an answer to that, but rather it's made me realize there are some more fundamental concepts I don't fully understand.

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Say we take a typical transmission line model, with inductance, capacitance, resistance, and conductance distributed throughout the line:

^{simulate this circuit – Schematic created using CircuitLab}

The characteristic impedance is:

$$ Z_{0}=\sqrt {R+j\omega L \over G+j\omega C} $$

So wouldn't that mean if we expect to have a transmission line with a real characteristic impedance, it must be that \$R/G = L/C\$?

So then I wonder, do non-real characteristic impedance transmission lines exist? Are they typical? What does it even mean to have a non-real characteristic impedance?

If no, my understanding is G represents the conductivity of the dielectric, while R represents the resistivity of the conductor. Each being lossy non-ideal properties I'd guess the goal is to minimize them, but if hope to achieve a real Z₀ this means carefully matching the non-ideal properties of both the conductor and the dielectric, ideally in a way which holds over a wide range of frequencies. Sounds difficult.

Or is it that R and G don't directly correlate to physical quantities, and they are just values mathematically selected to fit the equations to the behavior of the circuit?

circuit design - Decoupling capacitor placements and ratings - how to choose?

I am building a circuit with 2 hybrid stepper motors each powered by a stepper motor driver A4988 and both drivers are connected to an Arduino UNO R3.

Before soldering the components together on a PCB, I would like to clear my doubts, confirm what I'm doing and prevent any disasters or improve it, if possible.

I have done some research about the importance of using capacitors, but I still have some unanswered questions.

Here are the specs for each hybrid stepper motor:

Rated current (Amps/Phase): 1.68

Recommended voltage: 12 - 24 V

Holding torque (kg.cm): 4.4

Resistance (Ohms/Phase): 1.65

Inductance (mH/Phase): 3.6

Here are two variants of the section of the circuit schematic relating to the stepper motors and drivers:

I have a few questions since during the circuit design, I've been puzzling over several aspects of the decoupling capacitor placements and their respective capacitance and voltage ratings.

1. 
   
   

   I have made two versions of the schematic in the picture above but I can't figure out which one is better. And why? In design B, I have placed a 100nF ceramic capacitor between the 5V and GND of each stepper driver - is it better from a design perspective? Although design B uses one extra 100nF capacitor compared to design A, I am trying to make the best practical circuit possible and explain it coherently.

   
   


2. 
   

   Why connect a 35 V electrolytic capacitor when only a 9V supply is used to power the motors? How is this 35 V calculated? I found this in a schematic during my search but I want to understand why that value was chosen before i build it.

   
   


3. 
   

   I have read that as a good rule of thumb to always use a small 100nF ceramic capacitor and a bigger 100uF electrolytic capacitor in parallel to the supply and ground. But in this schematic, there is only a 100 uF between the MV and GND, so should I add a 100nF ceramic capacitor in parallel to improve this design??

   
   


4. 
   

   Why is a 100nF ceramic capacitor used between the logic voltage 5V (supplied from the 5V output from the Arduino UNO R3) and GND? I read that it filters out high frequency signals. But shouldn't the voltage output from the 5 V pin on the Arduino already be stable??

   
   
   


5. 
   

   The recommended voltage for the motor is 12 V, so if I were to use a 12 V DC power supply instead of 9 V, what capacitor voltage ratings should i use? How do I calculate this?

   
   


6. 
   

   I have set the current limit to each stepper motor to about 500 mA (from my online search, I read that a limit of 500 mA is safe so as not to damage the wires,) so is that something to be considered when choosing the capacitor ratings?

   
   

At this point, I am just blindly using what I found in my search online going through several schematics without really understanding why. I would really appreciate some clarifications. Maybe someone with PCB design experience can enlighten me on these issues.

Arduino as AVR Programmer

Can you use the Arduino as an AVR programmer?

Answer

I use the mega-isp solution mentioned above - the Arduino avrisp sketch is here

http://code.google.com/p/mega-isp/downloads/list

I do not use the shield they have - I made up a simple cable using two 6 pin dual-in-line header sockets to fit the programming ISP connector on your Arduino. (I did not have two dil sockets so I used standard single in line and superglued two 3 pin sockets together.

You then need a piece of ribbon cable or some wire.

Connect each pin on one socket the the corresponding one on the other, with the exception of pin 5 = Reset. Connect one end of the cable to pin 5 - this is the end that will connect to the target AVR that you wish to program. Connect the other end of the pin 5 wire to a floating pin connector.

To use connect the cable to the arduino by placing over the ISP programming connector and connecting the floating lead to digital 10, connect the other end to the target ISP connector. You need to ensure that the pin 1's match up.

Program with avrdude using

avrdude -P com7 -p t2313 -c avrisp -b 19200 -U flash:w:fred.hex

where

com7 - the com port that the Arduino based programmer is on
t2313 - the type of AVR you wish to program (m328 for ATMega328).
fred.hex - the name of the hex file you wish to progam into the target.

I have used this to program Tiny2313 with no problems.

Note: The cable will carry 5v to power the target - if the target is already powered then do not connect pin 2 of the cable.

How to find the total resistance of this circuit

Without giving resistor values, what are the steps to breaking down this circuit to find total resistance?

 

Edit:
Like this?

pwm - Detect ring signal using a low pass filter

I created 2 boards communicating each other like an intercom system. One of them has a button to send a ring signal to the other. The ring signal consists of PWM signals created by an MCU. I want to detect the ringtone by the MCU of the receiver side.

My ringtone contains 4 outer pulses with a period of 300 ms:

200ms of this signal contains the inner pulses with a period of 1.25ms:

So, I want to add a filter circuit to the receiver side of the ringtone and convert the outer signal to a straight PWM signal. Below is an example output

:

So far, I have tried adding an RC low pass filter with a cut of frequency of the outer signal (1/300ms = 3.33Hz). But I couldn't even get close to the example output. Is there anything I misunderstand about the concept since I'm not very experienced in circuit designs?

Regarding my circuit: I just added a series resistor and a parallel capacitor to the node:

Edit: I didn't want to talk about the circuit that handles converting the audio line signal back to the PWM since this is not the subject on this question. But I suspect that my RC filter does not work properly because of that part. I tried the solutions on the answer but they didn't work as expected.

Below is the schematic of my optocoupler side, line input contains the audio signal of the PWM. The audio signal converted back to the PWM by the optocoupler then I get the PWM signal which I gave the screenshots above. But when I add the RC parts to the OPTO_OUTPUT node, PWM signal changes in terms of volts too. Do you think I have a mistake on adding the RC parts?

Answer

If you want the 300ms signal to be received, while filtering the 1.25ms one, then it's the high frequency signal you need to filter out, not the other. Which means your chosen time constant is too large and affects the 300ms one. The high frequency signal has a period of 1.25ms, so choose a time constant that is more than 10x larger, say 25ms, which is also more than 10x smaller than the 300ms. Here's a quick test in LTspice:

V(a) shows the modulated output, V(b) shows the filtered output with a time constant of 22ms, and V(c) shows the recovered signal with a minor hysteresis of 10mV to counter the non-ideal filtering of the RC. Notice that the filtered signal has a somewhat thicker trace, that's because of the residual. You could use a Bessel or Gaussian filter for better results, but that would only add to the complexity and, besides, you'll still need the recovery of the signal, so that means you can simplify things.

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If your Vcc's value is 5V, then the collector resistor is too much, remember the optoisolators have a strong Ic dependency. If so, then try a 2k2 resistor, which will only use Ic~2.2mA, and you could give up the extra R and place C directly across the collector (10uF, adapted value for time constant). This will get Ic higher when C discharges, though. Otherwise make R=2k7 and C=3u9 (for example). Don't forget that the signal is inverted now, so I've used an inverting hysteresis comparator (the Schmitt trigger). I don't have a TLP opto, so I've just used whatever you see, adapt to your needs. I'm not home right now, but here's an attempt at exemplifying (V(n004) is the 300ms signal):