# Tag Info

64

If you're here asking about the basics of a line-connected power supply for running some hobby project, then you shouldn't be trying to do this at all. This is NOT where you want to learn by experimenting. The cost of mistakes are too high. The results can burn down your house or electrocute you, or someone else. If you need a little current at 5 V to ...

28

1) When using a Zener diode as the regulation element like in this circuit: the disadvantage is that the circuit needs to be configured such that there's always some current flowing through the zener diode. The zener diode acts as a shunt regulator, it "burns off" the current that is "left over" instead of limiting the current that flows when little ...

27

Rather than going to 'no resistor' consider what happens if we just use resistors of different (lower) values and look at the pattern. As we reduce the resistor value the current through the zener will rise. Even if the voltage source is not perfect the power dissipated by the zener will cause it to fail due to overheating.

24

Because the limited current sourced by the meter is not enough to bias the Zener diode into the reverse avalanche region. Instead you are biased some where on the knee of the Zener reverse characteristic curve. It is typical for meters to source about 1 mA in the diode test mode. Most Zener diodes will not reach their steep breakdown region until 10 mA ...

23

There are many problems with that circuit, of which the most important is it is not isolated from mains. A transformer will provide you with isolation. If you mount the Atmega in a plastic box, then it might be safe to use, but you will not be able to work on it safely, unless you use isolated equipment and are a careful engineer. You wouldn't be able to ...

22

Consider that you effectively have this: simulate this circuit – Schematic created using CircuitLab where C is the junction capacitance, plus any external capacitance (leads, breadboard, etc). Some of the current from R1 leaks through D1, but the rest charges C. Once the voltage reaches a certain level, avalanche breakdown occurs and current flows ...

21

The reason for it is noted on the schematic: https://cdn.sparkfun.com/datasheets/Components/General/SparkFun_Beefcake_Relay_Control_Kit_v20a.pdf Flyback Arrestor -- Place D1 with a normal diode such as the 1N4148 for all cases. Use a zener (such as the 1N4739) at D2 to allow a certain flyback voltage. Keep Vcc + Vzener under 30 volts (Breakdown ...

19

All three circuits display examples of pull up, pull down, voltage dividers, and current dividers: R1 and R2 are pull up resistors. You have to have both because you have two switches which can be in a different state (one high, one low). The 40Ω resistor is the top half of the voltage divider. The zener is the bottom half of the voltage divider. The ...

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 ...

17

Zener diode voltages largely follow the E24 resistor value intervals, a range of a intervals specified with +/-5 % accuracy. Zener diodes have not particularly developed as a technology, with accurate voltage references using bandgap techniques instead. The intervals have stayed the same for many years. As an aside, Zener diodes are less commonplace in ...

16

The current through the resistor is, by KVL and Ohm's law: $\dfrac{V_{IN} - V_Z}{R_S}$ This is the maximum current through the zener diode and so, $R_S$ is chosen to limit the current through the diode to some value such that the maximum power dissipated by the diode is below the maximum power rating. For example, if the Zener is 12V, 1W device, we ...

16

Diodes of all sorts, including the ubiquitous 1N4148, packaged in transparent packages tend to have some sensitivity to light (both photoconductive and photovoltaic as you have observed). The 1N4148 can apparently produce 10nA in direct sunlight. I rather suspect your zener diode when used normally with several mA flowing would have negligible response to ...

15

The characteristics of TVS diodes are similar to those of zener diodes. However, the TVS diodes are specifically designed, characterized and tested for transient voltage suppression. Zener diodes are designed towards voltage regulation.

14

Just a bit of preliminary theory. As you probably know, without any flyback diode, be it a rectifier or a Zener, you'll have a (theoretically infinite) kickback voltage from the inductor (valve coil, relay winding or whatever) whenever you try to interrupt its current abruptly. In reality the kickback won't be infinite because the spike will trigger any ...

14

The reason the same symbol is sometimes used for TVS diodes (Transorbs) and Zeners, is a Transorb has a lot in common with a Zener. An ideal Zener and an Ideal TVS-diode would be indistinguishable in their characteristics. This leads to ... laziness in library management (or ignorance) and the same symbol is used. Regulator Zeners and TVS-Zener diodes ...

13

All voltage references have a tolerance and a drift with aging and temperature. This list includes zener diodes, series voltage references (like voltage regulators) and shunt references (like the normal zener). Here's what a BZX84C zener looks like: - Take the 5V1 device - its zener voltage range is somewhere between 4.8V and 5.4V for a zener current of 5mA ...

13

In the case of the Zener regulator, the supply is 10 volts, and the Zener will do its best to limit the voltage across itself to 6 volts. If the 40 ohm resistor wasn't there, a very large current would flow as the Zener tried to do its thing, and the magic smoke would be released from many parts. If I've done the math right, the 40 ohm resistor will carry ...

13

Well, like resistors and capacitors, zener diode voltage values tend to follow a range of preferred numbers such as: - So, quite often you'll find 4.3 volt, 4.7 volt, 5.1 volt, 5.6 volt zener diodes etc.. As an aside, the 5.1 volt zener diode in several supplier ranges has the lowest temperature coefficient of voltage change with temperature: - So, if you ...

13

The value you read is not wrong, it’s correct for the test current your meter uses. Zeners less than about 5-6V have very rounded knee characteristics- to the point of being fairly useless for many purposes. For example, the 1N4728 is specified at a zener test current of 76mA! Your meter probably uses a test current in the 1mA range, limited to 5 or 7V. ...

12

The one that has the smaller voltage. It will hog the current until it's voltage compliance brings the voltage into the range of the higher zener, which then will start to flow some current. But even if the higher zener is flowing some current the majority will still flow through the smaller voltage unit. Heating of the diodes also plays a role, ...

12

I believe the question you're asking is: "How can the 1.2k resistor have 8.3mA going through it if there is only 6mA going through the 1k resistor." The answer is that there isn't 8.3mA going through the 1.2k resistor. The zener diode will break down and conduct for any voltage above 10V. The misleading thing about this circuit is that the voltage at the ...

12

"Or is this too specific to be a real-world problem?" Not at all. It's a problem for me as I use them for cryptographic random number generation. I've recently been using BZX85C24 Zener diodes. Running it at 30uA can create a noise level of 1V peak to peak (if you measure it enough times). But that's in total darkness. Get some sunlight on it and the ...

12

It's an LED driver, with regulation and constant current circuitry. U1 is a single IC LED driver and, yes, ZD1 is a Zener diode. U1 is an AMC7136 low drop-out constant-current LED driver, or a clone of the AMC part.

11

A zener is a shunt regulator, which means that it is a load parallel to your actual load. The zener voltage is 6 V, so you'll need a bit more to allow a voltage drop across the series resistor R. If the input voltage is constant, say 8 V, R will see a constant current if also the 6 V is constant. That's Ohm's Law: V = R $\times$ I. Your load may vary ...

11

I'm wondering how one would choose between one or a combination of these options? It's easy, if you understand how inductors work. I think the problem most people have is that they hear words like "inductive voltage spike" or "back-EMF" and reasonably conclude something like So, when an inductor is switched, it's for an instant like a 1000V battery. ...

11

The left circuit (A) is what he means. The Zener diode limits the voltage across the inductor to its nominal value. The other diode is there just to block the Zener diode in the forward direction. Concerning the decay time, this is better than just having the normal diode, because a normal diode just yields a voltage of ca. 0.7 V across the diode. For ...

11

My grain of salt as a professional power supply designer engineer. The issue is not on power dissipation or losses, but on safety. This system will most likely not burn (you limit power trough capacitor, smaller = less power), but it is GUARANTEED to electrocute anyone touching our Atmega or any of its output. If you really want to do it, you must make ...

11

Varistors are "baked" semiconductors, their properties aren't tightly controlable. Think of them as a mesh of billions of Schottky junctions. Their big pro is they can both short a spike and turn it into heat smoothly because of the high mass of the actual mesh. In addition, as the varistor heats up, it gets more conducting and this can be used to blow a ...

10

You can place the RC either at the B side or the A side. When components are placed in series the order of them doesn't matter for the working. About the diodes. When you switch off the relay it will cause a (possibly large) negative voltage on the FET's drain, and a flyback diode is used to limit that voltage to a 0.7 V diode drop. So the diode(s) don't ...

10

The first thing to do is to redraw the schematic with a more logical layout. Put positive power at top and negative power at bottom so that current mostly flows down the page. Then arrange signals so their logical flow (more like cause to effect, not current flow) is left to right as much as reasonably possible. Feedback signals are exceptions in that ...

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