power - Will placing a strong magnet near my electric skateboard Li-Ion Battery pack damage or discharge it?

The original clips securing my battery cover are broken and i thought of devising an alternate way to keep the cover shut involving a hard drive magnet and a hinge

the battery pack would sit in proximity (3-5 cm away) to the magnet while in use (1-2 hours/day), but I would remove it when the session ends.

Here's a rough plan. If you can make any sense of my drawing skills, the battery pack sits right under the cover. The hard drive magnet would be inside the battery case (dashed trait)

Before refining this idea I just wanna make sure this setup would not be harmful to my battery pack or other electronics (bluetooth remote receiver, ESC, ...). What if I decide to switch to a LiPo battery pack later?

Answer

As far as I know is there no direct harm to a lipo battery if you place a magnet next to it.

I have put magnets ofthen next to electronics to see what happens and never had some problems with that.

Also in the brushless motors you are going to use there are also fairly strong magnets inside of them, so don't worry.

I'm aware that this sin't a very scientific answer, but I'm not a lipo or ESC specialist. I'm just an experienced electronics guy. But I'm 99.99% sure nothing bad will hapen if you do this.

series charging three 18650 batteries with three chargers off the same 5V supply

being from a CS background I am a complete noob at this. I'll keep this short. I have a couple of 18650 batteries that i salvaged from an old laptop battery. I want to make a balanced 3S charger with over-discharge protection. I know that I could use a 3S BMS, but BMS aren't good at charging as they dont balance the cells properly.

I've seen many YouTube videos, regarding DIY balanced 18650 chargers and most of them uses this schematics this works perfectly for charging only and also does the cell-balancing.

But i wish to obtain the output from the charging modules as they also features a over-discharge protection IC in them. And embedded them into a project, without any hassle.

So, the question: will this work? those are TP4056 with over-discharge protection.

dac - Smoothing a High Current PWM

I am using a MOSFET to control the average voltage to a device by varying the duty cycle to the base as shown in the diagram below. My question is what is the best circuit to use in the "filter box" in order to "smooth" the PWM into its analogue equivalent. I have been told i can use a choke for this purpose, however is this the best approach.

Please note I want to smooth the PWM as the load is a peltier device and hence should not be driven by PWM. Further more there will be about 8A of current drawn by the peltier.

noise - How to remove mains hum from a BNC cable?

BNC cables are used as inputs to a data-acquisition hardware. If long BNC cables are picking up the 50/60Hz mains hum through the air like antenna, how can I filter this? Would connecting the BNC conenctors' GNDs to the earth help? Or using a capacitor?

Answer

Co-ax and twisted pairs are usually good at removing common mode noise.

Ground loops in coax installations can be a problem as they cause unbalanced noise. This becomes more of a problem if the co-ax shield carries any stray or intended 50/60Hz current.

Make sure signal co-ax screen is not used or involved in current paths that include conductors other than internal core. Running the Co-ax parallel with unbalanced high current 50/60Hz conductors could ealisy cause stray currents to flow in the screen if it is grounded at both ends. Any current on the screen that is not on the core will show up as noise.

Also not that with co-ax capacitive pickup should be minimal if the sensor is shielded. Electromagnetic pickup should be cancelled out except at the terminal points.

You should make sure the grounding at the aquisition end is as good as possible and the co-ax screen is isolated from ground at the sensor end. Grounding the sensor housing locally is usually still a good idea (unless it is small and can be left to float with at co-ax screen potential) but having the co-ax shield grounded to two unequal grounds is always a bad idea.

audio - How to make my own volume control for headphones?

I have headphones that don't have volume control built in on the cable, so I thought why wouldn't I try to make one? However, I don't know where to start.

Is it enough to just use a logarithmic potentiometer, or do I need something more?

If just a potentiometer is enough, what should be its ohmage? I also use FIIO E3 headphones amplifier. Is it better if I put volume control before or after the amplifier?

At what should I pay attention to avoid creating hum or other unwanted noises?

My headphones are: http://www.sennheiser.co.uk/uk/home_en.nsf/root/private_headphones_dj-headphones_500156

Answer

The writeup you linked to doesn't say much useful, but it appears these are bare speaker-type headphones (guessing from the size and shape). That means they are probably small speakers with 4 or 8 Ohms impedance.

You don't want to put a pot in line with speakers. That wastes power, doesn't present the right load to the amplifier, and probably messes up the frequency response due to the impedance change. The best place to put a volume control is in the signal path, not the power path. This means put it at the input of the amplifier that drives the headphones.

Whatever is driving the amplifier input is probably a "line" output. These usually have a few 100 Ohms impedance with a nominal 1V signal. A 1 kΩ logarithmic taper pot would be about right. Nowadays logarithmic tapers are harder to find because old fashioned analog pot volume controls aren't used that much. Nowadays the signal is handled digitally somewhere anyway, so the volume control is done by a digital multiply. If you can't find a logarithmic taper pot, it's not that big a deal. A linear taper will have a lot of change at the low volume end and not much at the high volume end, but for just setting a comfortable headphone volume it's probably good enough.

You can make a linear pot non-linear by putting another resistor accross the output. This doesn't make it logarithmic, but should spread the volume range out a little. Personally I wouldn't bother with this unless you've tried it directly and really didn't like the result.

As for avoiding hum and noise in audio, make sure everything is shielded. If necessary, mount the pot in a small metal box with the box grounded to the bottom lead of the pot, which should also be the ground for the input and output cables.

transistors - Why does the base-emitter voltage of a BJT decrease with temperature?

According to Sedra/Smith Microelectronic Circuits, \$v_{BE}\$ changes by \$-2\text{mV}/\text{°C}\$. I cannot understand how this could possibly be the case given the equations I am familiar with.

With all currents kept constant, we have:

\$\large{i_E = \frac{I_s}{\alpha}e^{v_{BE}/V_T}}\$

To keep \$i_E\$ constant, any change in \$V_T\$ would have to be accompanied by a change of the same factor in \$v_{BE}\$, otherwise \$\alpha\$ or \$I_s\$ would have to change, which as far as I understand is not possible.

So, how can \$v_{BE}\$ be inversely proportional to \$V_T\$?

Answer

\$I_S \$ is highly temperature dependent. As the temperature of the material increases, more electron-hole pairs are thermally generated, increasing \$I_S \$. Here's a link that gives the formula SPICE uses for \$I_S \$

Temperature appears explicitly in the exponential terms of the BJT and diode model equations. In addition, saturation currents have a built-in temperature dependence. The temperature dependence of the saturation current in the BJT models is determined by:

\$ I_S(T_1)=I_S(T_0)\left[\dfrac{T_1}{T_0}\right]^{XTI}exp\left[{\dfrac{-E_gq(T_1T_0)}{k(T_1-T_0)}}\right] \$

I believe that \$I_S \$ is roughly cubic in \$ T \$

How does ground mains work?

When I studied how AC mains power works, I learned that one of the wires is connected to the ground or a body of water so that it can get back to the power station. The concept baffles me. Every source of information I've come across fails to explain how it works or quickly glosses over it as if it is self explanatory.

If power can travel through the water or the earth back to the power station, then why aren't we getting vaporized when we walk on the ground near power lines? Also, how does an isolation transformer prevent you or your equipment from getting fried? If I touch both terminals of the secondary, am I going to get fried?