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This is to expand on Leon's suggestion to use a hub. The USB hubs are not all created equal. Unofficially, there are several "grades": Cheap hubs. These are cost optimized to the point where they don't adhere to the USB spec any more. Often, the +5V lines of the downstream ports are wired directly to the computer. No protection switches. Maybe a ...


34

Use a hub. They are quite inexpensive, and your USB ports will be perfectly safe no matter what your device does.


33

It's for two reasons. Well, actually just for one, but with two factors. A MOSFET can conduct in both directions when turned on, as it truly is just a resistive channel that is opened or closed. (Just like a tap, it's open with a tiny resistance, closed with huge resistance or a small gradation in between.) But, a MOSFET also has what is called a body ...


26

This is a very handy reverse polarity protection scheme. A P channel MOSFET turns on when \$V_{gs}\$ is negative, probably starting around -3V to -5V. When power is switched on, \$V_{gs}=0V\$. This is where the parasitic diode drawn across the MOSFET in the datasheet comes into play (drawn for explanation purposes only, do not put a diode between the drain ...


23

Consider the operation of the circuit. When the transistor is on current is flowing in the coil from top to bottom as the circuit is drawn we now switch the transistor off. The current in the coil still wants to flow. For the circuit on the left this current can now flow back to Vcc via the diode the voltage across the coil has reversed direction and is ...


23

You are pretty close, except your transistor is upside down. You should also add a diode across the relay coil contacts to avoid back EMF and a resistor on the base of the transistor. simulate this circuit – Schematic created using CircuitLab Something else I noticed... It looks like you are trying to power your ESP through a resistor divider? That ...


23

As pointed out, you can't directly connect two hosts to a device - so if you just wired them together and somebody plugged it in on both ends, you'd have a problem. However you also have a problem if only one end is plugged in. USB, especially high-speed (480Mbps) mode is controlled impedance. If you wire both connectors data lines together you end up with ...


19

As someone who does this for a living, any cheap hub in-line should give you 100% protection if your motherboard provides reasonable short-circuit protection. We use them all the time, even when doing ESD testing on our parts (15KV zaps are pretty entertaining), and have never blown one up or taken out a host port. The Data lines from a cheap hub simply ...


19

Overview: A good wrist strap system has a large resistance between the skin contact and ground. Typically this is 1 MΩ, which is relatively large. This large resistance is there for your safety, in case an error somewhere connects you to a hazardous voltage. Even this high resistance is sufficient for protecting your devices from ESD. A typical multimeter ...


18

You didn't specify a chip, the following is mostly geared to the 8 bit atmega devices, but it's general information. Read the 'Memory Programming' section for you specific chip's datasheet for more specific information! That being said, and as you said, all AVR devices contain two Lock bits named LB1 and LB2. Programming these (to 0, low) will add ...


18

You could consider a Transient Voltage Suppressor (TVS). They are basically diode arrangements that are intended to protect circuits like yours from over/under voltages. Since they are designed to work with microcontrollers and other devices for this purpose, you can choose the voltage range you're using when you buy them (e.g. designed for 3.3V and goes ...


16

The function you describe is the use of "Clamping Diodes" to protect the ADC input from voltage swings too far above the positive rail, or too far below the negative (typically ground) rail. See the the Voltage Clamp section in this nice Diodes and Transistors guide, and specifically the schematic provided in it: The diode parameters of interest: Forward ...


16

It is used as discharger for overvoltage conditions - in case of overvoltage a discharge starts through the lamp and that protects the main circuit from overcurrent. A neon lamp is used because it is relatively cheap, very reliable and there's zero current through the lamp until the discharge actually starts.


16

Absolutely not. First: HDMI uses TMDS signaling. This is not compatible with TTL logic gates; it's a current-mode differential protocol, very different from the single-ended outputs used in TTL logic. Second: The TMDS signals used by HDMI run at over 1 GHz. This is far faster than the 74LS parts you're looking at can handle; indeed, I don't think it's ...


15

The diodes in this application are not there to block current, but to allow a low-impedance path for the coils to discharge themselves through. If such a path is not provided, then when the coil's supply is stopped at each cycle, the stored magnetic energy must find a path for discharge. This results in the coil expressing an arbitrarily high reverse voltage ...


15

You are forgetting that these voltage sources are "ideal". So if your input is 20V directly from a supply, it will always be 20V. Throw a series resistor in there and you can see how it works. I used LTspice to model the circuit. R1 is the input resistance for some IC pin. I did a DC sweep from -10V to 10V with 1V increments. As you see, as I start to ...


14

Fuses are used for over-current protection, first please diagnose the mistake what causing the fuse to blow (like any shortages, overloading or some faults) and solve that before you replace new fuse. As long the voltage of fuse is more than the system voltage. you can use it. Since, what you are saying is right.. fuses protects the system/equipment from ...


13

Styrofoam is ESD death, alas. Some people wrap it in Al foil but blowing up a few photos of ICs with this done will show you that its very very easy to get a pin through a hole in the AL that does not touch metal but does touch foam. Murphy will have no problem t all blowing up your better ICs this way. Wrapping loosely in Al foil is safer. Break some ...


13

PTC fuses are not for all applications. However, in many cases the damage caused by high current is by heat, so high current for a short time can be OK. Look up the reaction time of normal fuses with a melting link, and you will see they aren't that fast either. Consider what exactly you are trying to protect. Would a short circuit really damage ...


13

Sometimes, relay coils are used together with switches, and no freewheeling (flyback) diodes are used. This will work, but every time you open the switch contacts, an arc will burn for a short time, which shortens the lifetime of your switches. With a transistor output driving a relay, a freewheeling diode is absolutely necessary, because the voltage spike ...


13

Analog devices makes a two chip solution for your problem, it provides full power and data isolation for the USB bus up to 12mbps, which should be fine for your needs: http://www.ubasics.com/usb_isolator Power - ADuM5000 Data - ADuM4160 There are a number of evaluation kits and breakout boards which make these easier to use for those not handy with a ...


13

The input impedance is high only when the circuit is operating correctly. If a fault occurs inside the equipment, you can no longer say for sure that that assumption is still true. By explicitly placing resistors in series with the leads, you can enforce a lower bound on the input impedance, and you can easily analyze the limited set of faults that might ...


13

The idea of the transistors is that: If the Left is low and the right is high R2 (and the left transistor a little) will negative-bias the base of the right transistor's base, allowing it to push the gate to the right voltage; closing the FET's channel and the body diode will block as well. If the right is low and the left is high, the left transistor's b-e ...


13

You have three components there that are all there for protecting the AVR, but all are doing a different job. The resistor is there to stop steady state high voltages. The capacitor is to remove ripple/RF/slow transients. The TVS is to suppress fast transients. In order to get the best out of your protection, you need to have the shortest (lowest ...


13

In practice, a power MOSFET has a body diode in parallel with the channel. These parasitic diodes are an intrinsic part of a power MOSFET. As a result, a power MOSFET can block current only in one direction. The switch in the battery protection circuit has to block current in both directions: charging and discharging. That's why there are two opposing ...


12

Relying on a capacitor to fail in an overvoltage condition in order to protect some other equipment is a bad design practice. Capacitors may fail open, short, or somewhere in between; unpredictability makes for poor protection. And I've personally seen overvoltaged caps emit flames. You should spec all your components such that its voltage rating is at ...


12

Use a wireless hub. I'd like to see someone create a USB device that can fry a motherboard through the wireless USB hub...


12

You are close. Your transistor is upside down and you need a base resistor. You should also add a fly-back diode to protect the transistor. simulate this circuit – Schematic created using CircuitLab As Ron spotted, you can not divide down the voltage like that to power your module, You should use an appropriate 3.3V LDO Linear voltage regulator.


12

Your typical double transistor current limiter may be your best bet. Shown below is the top-side and bottom side versions. simulate this circuit – Schematic created using CircuitLab Note there is a penalty of about a volt drop with this circuit. Buy dual transistors in a single 6 pin package. The small resistor will cause the current to fold back ...


11

There are ICs available to do the whole thing, including the switching MOSFETs, full isolation between inputs, etc. For example, the PS2115A from TI can autoswitch between two inputs, handle 2A, and is currently available for US$2.15 from DigiKey. Take a look at figure 14 for what you want.


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