# Tag Info

39

Safety capacitors are classified by X and Y ratings. Let's properly define everything, and then it should become clear how those capacitors can be rated for both X and Y at the same time. Class X Capacitors: These are capacitors are only for use in situations where their failure would not present an electric shock risk, but could result in a fire. That ...

12

It's pretty simple: The Zener diode voltage is at (or slightly above) the normal Vcc voltage. For example, a 5.6V Zener for a 5V Vcc. When Vcc is below the Zener voltage, no current flows through the diode, and the 1k resistor keeps the base of T1 low. No current flows through the collector to the emitter of the transistor. When Vcc is higher than the ...

11

Basically it's a power zener diode circuit. The zener begins turning on the BJT when sufficient voltage is across the line. The transistor takes current and the more that the line voltage tries to rise above the zener voltage (plus a Vbe drop of around 1 volt max), the more current is taken by the BJT. The BD139 can handle currents up to a few amps and that ...

10

It depends on how fast you want it... Zener diodes have pretty high capacitance, so you'll need a low value series resistor, which means it will draw a lot of current from the signal source. With your 100R value, a 15V source would have to provide (15-3.3)/100 = 117mA current, and the resistor would burn 1.3W. Both are inconvenient. If the signal is slow ...

8

The wire has inductance and, the longer the wire is the more inductance it has. An inductance likes to maintain the current flowing through itself so, when your switch opens, the small stored energy in the cables magnetic field tries to maintain current flow and produces a sizable voltage (aka back-emf) in doing so. This can easily exceed the maximum voltage ...

8

You can get rough calculations of the breakdown voltage from calculators. This paper has some measurements from an etched 0.55mm tooth gap: Wan, F., Pilla, V., Li, J., Pommerenke, D., Shumiya, H., & Araki, K. (2014). Time Lag of Secondary ESD in Millimeter-Size Spark Gaps. IEEE Transactions on Electromagnetic Compatibility, 56(1), 28–34. doi:10.1109/...

7

The current through the zener will be determined by the source impedance of your 12/24 volt supply and the associated wiring. Most supplies will have a low enough internal impedance that the zener diode will fail when it conducts. A slow blow fuse is not applicable in this situation since by the time the fuse blows, it is likely that the damage has been ...

7

Here is one method: simulate this circuit – Schematic created using CircuitLab If the errant user leaves the +12 on there the resistor will see almost half a watt and the transistor a few hundred mW. You could increase R1 to a few hundred ohms to reduce both those numbers. Note that the input current is diverted to ground through Q1 rather than into ...

7

Yes, that would work fine, but you can make this even more simple: simulate this circuit – Schematic created using CircuitLab Zener diodes behave more or less as a "normal" diode when in forward mode so we can use that to clamp negative voltages. When using a 12V, 400 mW zener diode the maximum current through the zener diode can be 400mW / 12 V = ...

6

No, X caps should fail open to prevent the risk of fire. However, there have been some parts that have caught fire after a number of years in the field. It's common wisdom that paper dielectric caps are more reliable because paper soaked with epoxy self-heals better than polypropylene. In some cases the epoxy has undergone some sort of fatal change. ...

6

Any accountable credits of this answer have to go to @bobflux . bobflux's answer points out all the aspects to be considered. I just would add a variation of bobblux's. simulate this circuit – Schematic created using CircuitLab

5

A linear regulator is inappropriate here. Use one of the many many buck regulator chips out there. Pick one that can handle a bit more than the max possible output after the full wave bridge, and you don't have to worry about that part at all. A buck switcher will be smaller and cheaper than a linear regulator after you include the cost and space of ...

5

Your circuit has issues and I would not try to "patch" these as there are simpler, more reliable solutions! I would use a poly fuse which is a self-recovering fuse. After it blows it will recover (become "good" again) after some time by itself. Of course you could also just use an ordinary glass fuse but make sure that the user can replace it without ...

5

Sometimes a stupid but convenient design just needs stupid protection.

5

The polarity protection works correctly as explained in Mosfet in reverse polarity protection. The rest is the Typical Applications given by Microchip in the MCP16301/H datasheet. So, I don't see any issues there. I don't know if you have considered the inrush current when applying 30V while C2 initially forms a short: it should not exceed max Pulsed Body-...

5

In effect it's an over voltage spark gap. If a line surge occurs, it will cause breakdown across the shark's teeth and (hopefully) offer some over-voltage protection to other parts of the circuit.

5

TL431 will work fine in this application, with a few caveats: When VREF<2.5V it is off but it will still draw a small cathode current to power the internal circuitry. When VREF=2.5V, in order to regulate, it needs to be able to draw a cathode current at least equal to "minimum cathode current for regulation" which is 1mA. So R3 should be around ...

4

You can probably clean this up by using a series resistor at the FPGA end. This will slow the rise and fall times slightly but should eliminate the overshoot and ringing. You will probably have to determine the value experimentally, 50-100 ohms is a good starting point.

4

Your interpretation is correct (note that the datasheet under Features (page 1) indicates a 12kV rating on the OUT pins vs 4kV on the IN pins which makes the in/out characteristic non-symmetric. The equivalent schematic looks suspect to me, though- check Texas Instruments TPD2S017 part, which is basically the same thing (with the in/out nomenclature ...

4

Your circuit with F1 overlooks how a crowbar circuit is designed to operate--the SCR is intended to conduct a very large current, forcibly blowing the supply fuse and isolating your circuit from the overvoltage. The idea of a crowbar circuit presumably takes its name from the act of throwing a metal crowbar onto a live circuit causing protection elements to ...

4

Most likely, no. When the SCR fires on an overvoltage, the crowbar current that is needed to blow the fuse also passes through the FET. Since the FET's source is then dragged down to near the negative rail, it'll drop out of conduction, leaving the intrinsic diode passing that current, and it'll almost certainly pop before the fuse. You could reverse the ...

4

The first circuit you show, with the resistor in-line with the load current, is what you require. That way, the Zener diode will limit the voltage, and the excess voltage will be dropped across the resistor. In your suggested circuit, the excess voltage will also be dropped across the resistor, but since the resistor is in series with the diode, and the ...

4

simulate this circuit – Schematic created using CircuitLab If you have need a resistive divider, it can limit current instead of an explicit current limiting resistor. The diode rail clamps can only clamp to the rails if there is voltage on the rails. They won't offer protection if the ADC is unpowered. But zeners or TVSs which breakdown in reverse ...

4

There is a parasitic diode in your MOSFET (aka the body diode) that will keep your output current flowing irrespective of how you believe you are controlling the device: - Maybe you meant the MOSFET source and drain to be swapped but screwed-up?

4

USB charging has become very complex but also very powerful. There are several official and de-facto standards used concurrently: USB 1.0 / 2.0 / 3.0 Apple chargers Qualcomm Quick Charge 1.0 / 2.0 / 3.0 / 4.0 USB Power Delivery All standards will initially deliver 5V (or sometimes up to 5.2V to compensate for losses in the cable). Plain old USB charger The ...

4

I suggest you limit the energy to an intrinsically safe level using a higher voltage zener diode before dealing with other requirements.

4

You can't really calculate a resistance, but using the current directly is a valid approach. Assume maximum current is flowing in or out of the pin (typically you'd be worried about the current flowing out of the pin when the input is low and into the pin when the input is high) and ensure there is adequate noise margin remaining under those conditions. The ...

4

I would like to ask if this circuit would work as expected, as I have never used such shunt regulators before and don't know their pitfalls. The TL431 is not particularly fast device so, the saviour here is the 10 uF capacitor in your circuit (C1). C1 really does need to be here to slow things down enough so that the TL431 can handle the slower surge. But, ...

3

Try this as an example of using an SCR and a zener: - Picture source. If the voltage exceeds Vz + Vgt then the SCR fires and shorts out the supply. Only when the power supply is removed or dropped in voltage to a low level does the short circuit reset. Added section If you need a tighter tolerance for crowbar activation than that offered by zener-sensing ...

3

To answer your questions, these are my suspicions based on your observations: The symptoms you observe are very likely an attempt to provide "12V" to 12V fans, from a 24V supply. Such a theoretical attempt to provide 12V with high efficiency from a 24V supply probably deserves full marks for trying, but the designer IMHO perhaps did not think his or her way ...

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