control lamp using bt139 triac with moc3041 and MCU.

Thread Starter

Rawankhader

Joined Sep 22, 2017
28
Hi,

i need to control lamp on/off using triac bt139 and mcu .and moc3041

i found the following circuit

gg.png

should i modify the circuit ?

what is the rating of resistors ?

what is the type and the voltage value of the capacitors?

Regards
 

ericgibbs

Joined Jan 29, 2010
9,575
hi R,
Why do you have the extra 360R and 0.05uF components in the circuit.?
The 0.01uF has to be a mains rated capacitor and the 39R say 0.5W
Also note the 330R in the d/sheet
E
AA1 10-Aug-18 17.52.gif
 
From this App Note:

The 330 ohms gate resistor (RG) is only necessary when the internal gate impedance of the triac or SCR is very high which is the case with sensitive gate thyristors. These devices display very poor noise immunity and thermal stability without RG. The value of the gate resistor in this
case should be between 100 and 500. The circuit designer should be aware that use of a gate resistor increases the required trigger current (IGT) since RG drains off part of IGT.



 

Thread Starter

Rawankhader

Joined Sep 22, 2017
28
hi R,
Check thru this Article link for capacitor information. @ndavis17
https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/
I would always fit the 330R.

E
Thank you E,

As the x rated capacitor act like short when it fails .. using fuse with load is a must ?



From this App Note:

The 330 ohms gate resistor (RG) is only necessary when the internal gate impedance of the triac or SCR is very high which is the case with sensitive gate thyristors. These devices display very poor noise immunity and thermal stability without RG. The value of the gate resistor in this
case should be between 100 and 500. The circuit designer should be aware that use of a gate resistor increases the required trigger current (IGT) since RG drains off part of IGT.

thank you Xavier :)
 

ericgibbs

Joined Jan 29, 2010
9,575
hi,
All the mains lamp failures I have seen are the result of the filament burning out, ie: going open circuit.
I would not normally fit a an extra fuse in the lamp supply, the house lighting fuse or cut out will be rated at about 5Amps.

E
 

ericgibbs

Joined Jan 29, 2010
9,575
hi,
A point that you should consider is, if you are connecting your lamp project to the house power ring circuit wiring, which in my house is a 30 Amp fused circuit , you should either use a fused mains plug rated at 3 A or 5 Amp or a fused outlet box.

This project should be enclosed and in a secure location where unskilled persons cannot come into contact with the electrical circuitry.
It is the sole responsibility of project builder/installer that their local electrical safety codes are followed and equipment displays the appropriate warning/hazard labels.

E
 
Last edited:

ebp

Joined Feb 8, 2018
2,332
The primary purpose of the RC network is as a "snubber" to limit the rate of rise of voltage across the triac so it doesn't trigger simply due to that. It very rare to require the network for anything other than an inductive load.

When phase angle control for dimming is used, a certain amount of radio frequency interference (RFI) is generated due to the fast rise of current when the triac turns on, especially when turn-on is near the peak of the AC sine. A typical well-designed dimmer circuit will use an inductor in series with the load to limit the rate of rise of current to suppress RFI. When the inductor is used, a capacitor directly across the dimmer circuit terminals is commonly used. A snubber on the triac side of the inductor is rare.

For simple on-off control of an incandescent lamp there is no need for the snubber or RFI filter inductor. All you need is the optocoupler, the gate current limiting resistor and the triac.

If the RC snubber as shown at #1 is used, an X cap would be suitable. X caps are designed to greatly reduce the probability of short-circuit failure. It it were to short, the series resistor would most likely be burned up. You actually get better overall general safety by using a flameproof fusible resistor with an ordinary cap than you do by using an X cap and an ordinary resistor. If the resistor were zero ohms and the capacitor failed short-circuit, the consequence would be exactly the same as if the triac failed short-circuit - the load gets turned on and control is lost. Fusing does nothing for this since the current is no different than it is with a normally operating circuit.

Failure of an incandescent lamp can sometimes cause triac failure due to gross overcurrent. Occasionally when a filament burns out a short segment of it can find its way across the wires that support the filament and connect it to the lamp base. This can lead to extremely high current (and a brief bright flash) that can overstress the triac and cause it to fail, almost always short-circuit. Fusing to protect the triac is difficult because the normal cold versus hot resistance of the filament is around 1:10 for common lamp types - a 100 W lamp takes 1 kW when first turned on. If you use a fuse fast enough to protect the triac from the filament failure problem it will probably blow every time you try to turn the lamp on.
 
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