ac rectifier dc MOSFET dimmer circuit - Page 1
Forum section: electronics / projects

author: Lo.Fe.v
author_rank: Newbie
timestamp: May 28, 2023, 10:19:48 pm
content: Hello everyone, I am new to this, in the forum and in the world of electronics and I need help, I am doing my final electronics project, and this is the circuit that the teacher asked us to do, the light bulb should turn on by the arduino (the code will be given to us by the teacher), but through calculations I need to get the resistance R1 and R2, someone who can help me? I have no idea, ask on chegg but it asks me for the threshold voltage, I don't know how to get that

author: wasedadoc
author_rank: Super Contributor
timestamp: May 28, 2023, 10:23:50 pm
content: You get the MOSFET's threshold voltage from its data sheet.

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 28, 2023, 10:45:21 pm
content: Since this is a school project, I'm just going to ask you this:Why do you think you need R2 and D1?

author: Lo.Fe.v
author_rank: Newbie
timestamp: May 28, 2023, 10:51:18 pm
content: my teacher asked us to include the resistances with the calculations in the project report

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 28, 2023, 11:14:27 pm
content: Did the teacher draw the schematic and then tell you to fill in the values?Or is the schematic 100% your own design?What is the AC input voltage? What type of light bulb (voltage & wattage) is going to used?

author: Lo.Fe.v
author_rank: Newbie
timestamp: May 28, 2023, 11:22:33 pm
content: Yes, he drew the circuit and then told us to find the valuesThe AC voltage is 127 volts, and the bulb is 120 volts and 60w

author: Benta
author_rank: Super Contributor
timestamp: May 28, 2023, 11:43:49 pm
content: You either need a new teacher, or he's asking trick questions, both are possible.As Kim C said, D1 and R2 are unnecessary.If you turn off the Arduino, the lamp will always be on.And the only thing you need to care about is the R1 current vs. the CTR of the optocoupler.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 28, 2023, 11:56:59 pm
content: Quote from: Lo.Fe.v on May 28, 2023, 10:19:48 pmAs above, the circuit drawn actually has the Ardunio turn the light bulb OFF, not ON.Quote from: Lo.Fe.v on May 28, 2023, 10:19:48 pmYou need to worry more about the threshold of the 15V Zener, than the mosfet. The zener voltage determines the resistors.You then check the RDS of the MOSFET at that Zener voltage, and include power calculations for each circuit element.Quote from: Lo.Fe.v on May 28, 2023, 11:22:33 pmYou should also include a Spice run of the teacher's circuit, showing theoperating voltageon that light bulb

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 29, 2023, 12:24:57 am
content: Quote from: PCB.Wiz on May 28, 2023, 11:56:59 pmNot to mention the cold filament turn on current! (Guessing around 15cold filament resistance for a 120V 60W incandescant bulb)

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 29, 2023, 04:12:57 am
content: A perfect use case for a Photovoltaic MOSFET DriverAnd just need to calculate the resistance for the driver

author: Decapitator
author_rank: Contributor
timestamp: May 29, 2023, 07:17:57 am
content: I find myself wondering which part of the circuit will blow up first. Perhaps the bridge rectifier, unless the supply voltage is separately derived without an earth to neutral connection. Otherwise the earth ground connection on the negative side will kill it instantly. Might be the bulk capacitor. It would be smart to confirm it's ripple current rating. Running a 120 volt lamp off of a ~180 volt supply at a reduced duty cycle while avoiding visible flicker by keeping the pulse rate high can be a stressful combination. Switching mosfets like the IRF730 run coolest when rise and fall times are kept to a minimum. That implies low impedance gate currents. The 4n35 driven by a 5 volt Arduino port through through a 1k resistor into a LED with a 1.5 volt forward drop results in only 3.5 mA of current. With a guaranteed CTR of 100%, that only allows 3.5 mA of sink current which has to both remove the gate charge and overcome the current supplied by R1. A cool mosfet can better handle large pulse currents and there will be plenty of them when the filament is cold and a 180 volt supply. The lamp itself could fail much sooner than expected due to something known as "DC notching" which was first observed with small indicator lamps when run on DC at reduced brightness levels. The presence of nitrogen is a known factor in the migration of tungsten molecules from one part of the filament to the adjacent area during DC operation creating weak notch. General service (ordinary) lamps are often filled with nitrogen because it is cheap and reduces filament evaporation. That's good for lamps typically run on AC, not so good for lamps run on DC at reduced brightness due to dimming. Ever notice the extra bright flash when you flick the switch and the light burns out? That's caused by the arc formed across the break and the currents involved can be rather high. An AC arc will generally self extinguish at the next zero crossing. DC arcs can last considerably longer and will probably destroy a small mosfet of this type. Also note that if the Arduino doesn't turn the opto coupler on for the appropriate intervals at he appropriate rate the lamp will burn out almost immediately from over voltage.