| Electronics > Projects, Designs, and Technical Stuff |
| AC Rectified - DC Mosfet dimmer - Hints please |
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| bingo600:
--- Quote from: Hero999 on March 03, 2017, 08:46:56 am --- --- Quote from: bingo600 on March 02, 2017, 07:34:56 pm --- I'll be dimming 2 x 40w - lightbulbs (The big ballon ones with golden glow) --- End quote --- What sort of light bulbs? LED, compact fluorescent, incandescent? --- End quote --- Standard light bulbs w. clear glass , and those wires that glows golden inside. /Bingo |
| bingo600:
--- Quote from: BradC on March 03, 2017, 06:08:22 am --- --- Quote from: james_s on March 02, 2017, 10:21:03 pm ---Is there a reason for using such an unconventional circuit rather than the classic triac phase control dimmer? If you use a zero crossing signal to trigger an interrupt in your microcontroller it's fairly easy to adjust the trigger point. --- End quote --- The above circuit with the bridge rectifier in *series* with the load is a really neat way to allow you to use a DC device to control AC, so you can use a MOSFET rather than an IGBT. Additionally, it can be operated outside the parameters of being locked to a zero cross, so you can use either faster PWM *or* (as I've actually seen done) use the MOSFET as a resistor to give a clean but reduced voltage output (with the consequential power dissapation in the MOSFET). It'd even be a neat way of effectively implementing a trailing egde dimmer. I have a real issue with supplying high voltage DC to anything a finger can get into (such as light socket). --- End quote --- Would you care to point out with one you mean ? The top post (it think you mean that one) , or the one wo. the red cross And i'd be using an ESP8266 for control , no fingers involved ;) /Bingo |
| BradC:
I meant the one with the Red Cross. Not that circuit specifically, but the same theory. Bridge I series with the the load. |
| Seekonk:
I use opto isolators to switch FET often. FET gate capacitance will make a very slow turn off with a 22K resistor. I wouldn't use any more than 1K to reduce transition heating. Electronic wall warts are a convenient way to power these circuits. Surprised no one mentioned capacitive dropping to eliminate resistor heat. |
| Kees van Gelder:
Actually there is more sense in using a 400 Volt capacitor in the position that it is in than you give credit. When using Mosfets or IGBT you want them to work as a switch that switches fast between OFF and ON. You dont want the Gate voltage to linger somewhere halfway, making the MOSFET or IGBT to dissipate a lot of heat. Tou want it to be either OFF or ON on its RDSon. With the capacitor as is in the given circuit, the only RCtime you have to worry about is that of the 100k resistor and the capacity of the tarnsistor. For the average MOSFET that is in the range of 10pF. Hence your RC time being practically zero. Which means that the FET or IGBT will switch very fast. Now lets see how that is with a 25uF capacitor where you say it should be. Then suddenly the picture is different. The RC time will be 2.5 seconds. Which means that it takes 2.5 seconds to get to 63% of its gate voltage. All that time the Transistor is not at RDSon but at a higher resistance, which translates in dissipating power -> heating up. So I presume a bit more thought is gone in to that than you say. Having said that, the 100uF probably could go down a bit, say 50, but I'd certainly keep it in the position it is, The zener, as the author says is there just in case. Sure, you could leave it out, but it isnt a 'flaw'. I even see a circuit there with just the zener. Coming to the 'resistor between the connection of R4/R5 and the optocoupler', I presume also here fast switching was the goal, you want that Voltage to go down fast. Now if you put a capacitor there as you propose, yes chances are you will kill not only the Transistor but also the optocoupler so yes then you need a resistor, but then not only you are slow in charging but also in discharging -> more time spent outside RDSon -> heating up your MOSFET 5k6 instead of 6k8 ? Cosmetic Stays on when Arduino is off? yeah I guess that is the design, but I guess you could put the optocoupler in series with the 100k. regarding the discussion on using a TRIAC and do zerocross detection. Sure that works as well, but you will not be able to use PWM which seems to be the point of this circuit |
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