Hot answers tagged

53

Advantages of relays over triacs: Very little voltage drop when on. This means they don't dissipate much power. For high power devices, the cost of dealing with the heat often outweighs the cost of the component that dissipates the heat. Good isolation. The relay coil is inherently electrically separated from the relay switch. Making that isolation ...


52

Mechanical relays won't last long if forced to switch at 5 Hz, but solid state devices (thyristors, triacs or transistors) are perfect for your task. Instead of coming up with your own thyristor/transistor circuit, I suggest that you use a solid state relay. It's easier and safer. They consist of an optocoupler (for electrical isolation of the logic level ...


31

Three major issues here. The first is that you don't seem to be aware that you're effectively building a dimmer circuit when you start using electronic switching. If you Google "theatre light dimmer schematic" you'll find a few useful links, including an old ePanorama article about different types. You may find useful links on the Everyday Practical ...


22

Your best bet is probably to use SSRs (solid state relays) which include drive and isolation circuitry in one package. They also include a thyristor and usually a heat spreader to interface with a heat sink, if necessary, that is electrically isolated. The mechanical relays are unsuitable, they will be noisy and wear out very quickly. Keep in mind that ...


20

Adding to the points of Olin's answer: If you don't need the fast switching times of semiconductor devices, relays are pretty robust and cheap, compared with the circuitry needed to implement a solid state switch capable of switching the same amount of power.


18

TRIACs switch off at (near) zero current. It is common for switches that switch passively at zero current to experience a voltage step that will cause circuit parasitic inductance and capacitance to ring. There are 2 problems: The peak voltage of the ringing can exceed the rating of the TRIAC. TRIACs also have a maximum rated \$\frac {\text {dV}} {\...


15

A triac is used to control an AC load, just like a transistor can be used to control a DC load. It is drawn like a couple of diodes in each direction which are then triggered to turn on. They will turn off when the current drops below a certain threshold. So if you just want it on or off you leave the signal on or off and at the next zero-crossing of the AC ...


15

When the current across the triac falls under \$I_H\$, which is the holding current, the triac stops conducting. With a pure resistive load this happens at the very end of the sine wave cycle, and voltage and current are in phase. When the load has an inductive component (e.g. a motor) then there is a lag between current and voltage. At the moment when the ...


15

You can't switch mains. First you shouldn't switch it because mains power is dangerous, and you should not be running it through a homebrew gadget (it violates the electrical code and will void your insurance if it causes a fire/injury). But the deal-killer is no mains lamps are fast enough. Incandescents have too slow a rise/fall time; at 5Hz you ...


14

In the answers to this question is explained how you can do that complete zero-crossing detection circuit with just U1, R12 and 2 series resistors on the 220 V side. One solution uses a common optocoupler, the other one a Darlington optocoupler, which needs less current to drive the optocoupler's LED, so that's less power in the series resistors (less than ...


13

While you may have undoubtedly found a load of principes of operation of SCR vs TRIAC it seems you miss out, at least (but likely not limited to): The Recovery Time of a Thyristor Type (the family of devices): If your signal might reverse quicker than the Thyristor can recover in the case of an SCR it will still return to blocking state ("eventually"), as ...


12

A triac is basically an electronic switch for alternate current. What it does is to close a contact between two terminals when a current is applied to its "Gate". It will continue allowing current to flow until it drops below a certain threshold. It can be seen as an alternative to the relay. See also this question


9

Zero-crossing is typically used for incandescent bulbs. You may have noticed that when incandescent bulbs fail they always fail when they're switched on. That's because the mains phase can be near its maximum when switching on. Combined with the low resistance of a cold bulb this results in a high current peak, which may burn the filament. When you switch on ...


8

The argument: If your heating element is rated to dissipate 2300 watts with 230 volts across it, then when it's dissipating 2300 watts the current through it will be: I = P/E = 2300W/230V = 10 amperes, and its resistance will be: R = E/I = 230V/10A = 23 ohms. In order for the element to dissipate 1400 watts, then, the voltage across it must be decreased ...


8

An SSR (solid state relay) doesn't need a bridge: - It handles AC and DC very nicely without using a bridge rectifier.


7

This is what you call triac abuse; a triac is not made for this kind of application. Like Steve says, triacs remain switched on on DC loads. You may have zero crossings where they normally would switch off, but I'm not sure how the triac reacts to very fast switching, like in your 56k EIA-232 signal. The zero crossing may be too fast to switch it off. I'd ...


7

Is a reverse polarity protection diode required across the INPUT of an optoisolator? Summary: A diode would often not be used in the location shown BUT it does protect against extreme conditions and costs very little to implement so is not a terrible idea. I have never seen a diode in this location in a "real world" circuit. The diode protects against ...


7

There are enough interesting things that haven't been said that another answer may be useful. As well as adding new material this will overlap various others for better overall completeness. TRIAC is an AC switch except in a special case. Turnon occurs when opto is driven above enable level. If opto is turned off and TRIAC load current is above holding ...


7

Edited: This is a good example of a question that superficially APPEARS to be Arduino related, but all the material dealt with is 100% EE related. If people are so utterly hateful of all things Arduino (as some truly are) then it behooves them to leave the question alone for others to deal with appropriately. The question has now made the EE -> Arduino -> EE ...


7

dV/dt is the derivative of the voltage with respect to time. In other words, it's the change in voltage (delta V, or ΔV) divided by the change in time (delta t, or Δt), or the rate at which the voltage changes over time.


7

If you had a curve of \$y = x^2\$ like below: - The slope at when x = 3 (y = 9) can be estimated by calculating how much y changes divided by how much x changes. The change is called "delta" hence the slope is \$\Delta y/\Delta x\$. Taken to infinitesimal changes, mathematically it gets "renamed" to dy/dx. It can even be algebraically proven by adding dy ...


7

There is no need for tricky software. simulate this circuit – Schematic created using CircuitLab Figure 1. Replacing D2 with a bridge rectifier will turn on the opto-isolator on both positive and negative mains half-cycles. Your 59k value for the current limiting resistor, R5, seems suspiciously low. You should check your calculations for this and ...


7

Relays are bad for that sort of thing because every opening of the circuit will arc which will burn the contacts and wear them out. OK, so you are switching A/C so sometimes you'll catch it at the null current so no arc, but mostly you'll have a lot of arcing going on. This will both subject the relay to high wear and create a lot of heat. Thyristors will ...


7

The opamp does not provide sufficient current to blow the fuse. The fuse is rated for 200mA (for the lowest current rated fuse in the family), the opamp can only supply 80mA (if running at 5V at 2.7V it's only 30mA) , or less then half the current to blow the fuse. Lets suppose that ground was attached to the other end of the fuse, instead of the crowbar ...


7

In the circuit as you show it you require no C1. As pointed out in the prior discussion, the capacitor may turn on the Triac on sudden rises in opamp output. The TL431 is not really suitable for what you are trying to do since it requires a minimum Ik to set the reference (0.4mA). The strange conduction you are seeing is in all probability due to the ...


6

The SSR is a low voltage DC device, but may also be used for low voltage AC, while the triac device is for AC usage only, typically mains voltage. A triac has a PNPN-structure, which means that there will always be a voltage drop across it, while FETs are resistive devices, and for those an \$R_{ON}\$ is specified, in this case 0.25\$\Omega\$ maximum. The ...


6

Zero Crossing refers to the load voltage at which the triac will switch. Wikipedia has a little information on the theory of Zero Crossing. Basically, the triac will hold off its switching until the alternating waveform of the load signal crosses the 'zero' or midpoint of the waveform. This helps alleviate sudden voltage spikes when the switched load ...


6

Several comments: If you don't need isolation, then (obviously) you don't need any device that includes isolation, such as a SSR. If you do need isolation, you probably don't need it for each channel. A single isolation gap, for all 16 channels, should be enough. All this could save you money. Snubbers don't protect the switches (the SSRs, in your case). ...


6

Playing with mains can lead to death of drive equipment, load, TRIAC, yourself or users if you get it wrong. Try hard not to. Use of an isolated driver IC greatly enhances the chances of you and your drive electronics living much longer. Anything driving mains equipment directly is potentially at mains potential at any time unless you have a ...


6

To answer this, consider the simpler to understand SCR instead of a triac. A triac is sortof two SCRs back to back and therefore can pass current in both directions. A SCR only works one way but has the same issue of holding current. Here is a equivalent circuit of a SCR: SCRs are actually built as one integrated device, but you can conceptualize them as ...


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