152

The main reason is because you can't safely connect diodes in parallel. So when we use one resistor, we have a current limit for the whole diode section. After that it's up to each diode to control the current that goes through it. The problem is that real world diodes don't have same characteristics and therefore there's a danger that one diode will ...


54

OK, let's do the calculation. A simplified model for a LED is a fixed voltage source in series with a small resistor. Let's pick this LED from Kingbright. The slope is 20mA/100mV, so the internal resistance is 5\$\Omega\$. The the intrinsic LED voltage is 1.9V. Let's assume that the LEDs need 20mA and that our power supply is 5V. Then the LED voltage ...


48

It's the voltage rating on the resistors that is important here. They are powered from rectified 230 V AC and they need to have the correct voltage rating to suit their application. Two resistors in series having an individual rating of 200 V gives a total voltage rating of 400 volts (near enough if you ignore tolerances on values). Take a look at the good ...


38

Because UL 1741 does all the heavy lifting for you. That's what makes it "so easy". UL 1741 is a complicated spec for "grid-tie solar inverters". Aside from doing their usual inverter thing (itself no small matter), a 1741 inverter also senses the presence of the grid, and obviously syncs its output to the grid. A UL 1741 inverter is intentionally ...


34

First some theory: In general, a PC power supply isn't expected to operate in a redundant mode (i.e. with outputs tied together). In industry parlance, this function is called OR-ing (not O-ring). If a power supply is designed with OR-ing in mind, there will be several additions to the circuitry: Some means of isolation (diodes or MOSFETs) Some means of ...


30

Both configurations will conduct power to the loads. When trying to figure out what's 'illegal', and why, you need to understand what fault conditions the authorities are trying to prevent. There may be a commentary in the relevant standards if you're lucky. In the UK, such an arrangement of a circular conductor is called a 'Ring Main', and was actively ...


28

MOSFETs are a bit unusual, in that if you connect several of them in parallel, they share the load quite well. Essentially, when you turn on the transistor, each one will have a slightly different on-resistance and a slightly different current. The ones carrying more current will heat up more, and increase their on-resistance. That then redistributes the ...


27

To add to what Majenko said, in the series configuration it doesn't work the way you expected either. That is because both supplies have a common ground. Series would only work to make 17V is both were independently floating, which they are not. It is not possible to connect two supplies with a common ground in series. Whether series or parallel, either ...


21

Take a look at this graph: - http://electronicdesign.com/files/29/1478/figure_01.gif Capacitors have a resonant frequency due to the inherent small series inductance they have. The "generalized" capacitor showing the various parasitic components is shown below: - It's the L\$_{ESL}\$ that causes this series resonance. For a typical 10 uF tantalum ...


20

The problem is keeping the signals on a parallel bus clean and in sync at the target. With serial "all you have to do", is be able to is extract the clock and as a result the data. You can help by creating lots of transitions, 8b/10b, bi-phase or manchester encoding, there are lots of schemes (yes this means you are adding even more bits). Yes, ...


20

Yes, this is exactly the intended purpose. It's called a wired-OR or wired-AND, depending on your logic. (Or, as Tony points out, even a wired-NOR!) However - you should not do this if you just want twice the amount of current sinking capacity. Only if you want the feature of being able to switch using separate inputs. Current will likely not be shared ...


19

Instead of thinking of these things as "resistors", try thinking of them as "conductors". After all, that's what they do: conduct. A resistor with resistance \$R\$ is a conductor with conductance \$S=\dfrac{1}{R}\$. When you provide multiple conductors connecting one point to another, the conductances simply add. What could be more intuitive? When you ...


18

This is a bit of a controversial subject. Some people seem to feel that a capacitor beyond resonance has no benefit for bypassing. Others point out that even past resonance the part is basically just a very small inductor short-circuited to ground and it still has fairly low impedance. This plot from Murata shows the (magnitude of the) impedance vs. ...


18

Yes. Fixing production errors by using jumper wire is a time honored tradition by those blessed with hindsight. For Power rails or low speed GPIO (Think Push Buttons inputs or LED outputs), a simple wire would work. As a precaution, you could cut the trace, and carry the full load on the jumper you are adding. If the jumper fails, the load would try to go ...


18

Capacitors eventually stop behaving like capacitors at high frequencies and exhibit resistive and inductive effects. The 100pF capacitor filters high frequencies that the 100nF capacitor could not. This is shown clearly on the diagram below which shows the impedance (Red) of a 100nF capacitor and a 100pF capacitor (Light Grey top curve) versus Frequency. ...


18

Reasons someone might put two resistors in series in a volume design: A bit higher power was needed than what the commonly stocked parts can handle. Let's say a company standardizes on using 0805 resistors unless there is a good reason not to. They therefore end up with many 0805 values in stock, with only a few values of other packages. Now you need a ...


18

You have two problems Paralleling MOSEFTs in this mode is unstable. However, you could parallel the outputs of several totally independent current sources, each with its own power device, Rs and opamp. Most MOSFETs will not like operating in linear mode like this, they are designed and specified for switching operation. Internally, MOSFETs are hundreds, if ...


17

I tried to replicate, and I saw the same behavior. @SteveG did, too. After a few questions regarding the sanity of each of us, I think I found why, and it is pretty simple: Are you absolutely sure you are actually plotting the right voltage node? In both of your plots, it shows "V(n003)/I(V1)". But I actually realized that if I mess up just a bit with the ...


17

It is indeed possible to have parallelism on a superscalar processor. A superscalar processor can execute multiple instructions at the same time by using multiple execution units. There are certain limitations depending on the architecture. It is not true parallelism. If you have to calculate $$A = B + C,$$ $$D = A + 3,$$ you cannot execute both ...


17

Two parallel wires with no resistance - why it's wrong? This is wrong because there are no wires with no resistance. When trying to resolve Kirchhoff equations for this loop (or whatever their algorithm is), the simulator would run into division by zero exception. To avoid program crash, it likely analyzes these conditions and declares the circuit as error. ...


17

Yes. It does. Note that in general you can't blindly parallel transistors. You can parallel MOSFETs without special measures since as they get hotter they conduct less well which distributes the load more or less evenly in spite of individual component differences. Positive temperature coefficient. BJTs conduct BETTER as they get hotter so the BJT that ...


17

As DKNguyen pointed out it is important to "account for" or "manage" failures. We put capacitor banks on the 28 VDC power bus of spacecraft all the time. EVERY cap had its own fuse, so if the cap shorted out - which was the predominant failure mode - the fuse would blow and take the cap out of the circuit. We were also concerned about partial shorts of a cap,...


16

This is actually a very common technique to do, both with BJTs (traditional transistors like drawn above) and MOSFETs. With BJTs, you don't need to bother with separate trimmed base resistors, all you need to do is add current sharing resistors or sometimes called ballast resistors. Look at this page for instance, the first one I found with google that ...


16

The shift is from "parallel on a single clock" to "multiple serial links". Such as PCIe, where a card may have 1x to 16x "lanes". There are two factors involved, skew and size. Adding more connectors makes both the cable, its connectors and the receptacles on each device larger and more expensive. Look at how large things like Centronics printer cables and ...


15

Why is the serial connection faster than the parallel connection? You're making wrong assumptions. Take any serial connection. Now place 10 in parallel and call that the parallel version. Which one is faster ? So how come the serial connection is considered the future while the parallel one as a thing of the past? Says who ? Parallel connections are ...


14

I'll assume that the total length of each of your buses is under 1 meter, which is typical for plain I2C and SPI. Also, the bus capacitance is within the spec. There should be only one (1) set of pull-up resistors. As a result, the best place for them is on the main controller board. Don't put the pull-ups on the modules. If you want to have pull-up in ...


14

If your budget is less then 10-20 thousand dollars (or more realistically 100K+), you have no hope of getting an ASIC made. The common device used instead of an ASIC, in situations where you cannot afford the NRE (non-returnable expenses - basically the cost for producing the masks for etching your asic, as well as the design costs), is to use a FPGA. ...


14

It is really not that easy. It's easy for you because standards and regulations have been put in place that affect the power company and anyone selling you grid-tie inverters. The system is designed and regulated by law so that you can just buy a shiny box, pay someone to hook it up, and not worry. The grid-tie inverter is pretty complicated. It has to ...


13

In parallel, your overall voltage would be +12V. BUT Your +5V feed will be being over-powered by +7V. This is bad You will probably be doing damage to the internals of your power supply. Especially if the +5V section is using capacitors that are rated at less than 12V... pop


13

Yes, it is, but the easiest and most compact representation is the $${1\over R_t} = \sum_{i=1}^n {1\over R_i} = {1\over R_1} + \dots + {1\over R_n} $$ you already mentioned. To see the pattern you can do the math yourself for 3 resistors: $${1\over R_t} = {1\over R_1} + {1\over R_2} + {1\over R_3} $$ Do the first addition using the usual method: $${1\...


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