Low power TRIAC dimmer circuit without inductor

Thread Starter

Gear_Clinkz

Joined May 25, 2013
16
Hello,

I want to make a dimmer circuit for a 5W AC 240V incandescent light bulb for a table-top night light.
I'd rather not use an inductor based circuit since I don't have an inductance meter.
I have a TIC216M TRIAC and a DB3 DIAC available so if possible I'd like to use them.
I've tried a few circuits but they don't seem to be working properly. One could adjust the light only in three different brightness levels and the other
had a strobe like effect on the light.

Can anyone recommend me a circuit that uses the components mentioned above?

Thank you in advance.
 

ebp

Joined Feb 8, 2018
2,332
The inductor in a phase angle control dimmer circuit is there for suppression of radio frequency interference (RFI). Dimmers generate RFI because of the fast switching of current when the triac turns on. The inductor slows the rate of change of current which reduces the RFI. With such a low power load you should be OK without an inductor.

You may be having problems with the "hold current" of the triac. Once a triac has been triggered it requires a certain minimum of current in the main terminals path to keep it turned on. The dimmer works by applying a brief trigger pulse when the diac conducts and then relies on hold current to keep the triac on for the rest of the half cycle. With just a 5 W load on 240 VAC, the current would be about 21 mA RMS. But you must consider the current at different points in the sine wave. If you try to turn the triac on early or late in a half-cycle, the instantaneous voltage will be much lower and the instantaneous current proportionately lower. The triac you are using as quite low hold current, so this probably is not a problem, but if you get bad behavior, try testing with a somewhat higher power lamp. If it turns out that hold current is a problem, I suggest a triac with lower current rating - hold current is usually lower for triacs with lower main terminal current rating.

Incandescent lamps typically have a resistance when cold that can be a tenth or less of what it is when hot. This complicates things, but generally is advantageous in terms of hold current. There is some chance this phenomenon is contributing to the odd behavior.
 

Dodgydave

Joined Jun 22, 2012
11,285
Plenty of circuits for triac dimmers, the inductirs are for rfi suppresion.
Here is one i use a lot, it uses steering diodes to prevent low light hysteresis. Triac is a Bta08-600, Mt1 to Neutral, Mt2 to load, diodes 1N4007.

Figure2(1).jpg
 
Last edited:

Thread Starter

Gear_Clinkz

Joined May 25, 2013
16
The inductor in a phase angle control dimmer circuit is there for suppression of radio frequency interference (RFI). Dimmers generate RFI because of the fast switching of current when the triac turns on. The inductor slows the rate of change of current which reduces the RFI. With such a low power load you should be OK without an inductor.

You may be having problems with the "hold current" of the triac. Once a triac has been triggered it requires a certain minimum of current in the main terminals path to keep it turned on. The dimmer works by applying a brief trigger pulse when the diac conducts and then relies on hold current to keep the triac on for the rest of the half cycle. With just a 5 W load on 240 VAC, the current would be about 21 mA RMS. But you must consider the current at different points in the sine wave. If you try to turn the triac on early or late in a half-cycle, the instantaneous voltage will be much lower and the instantaneous current proportionately lower. The triac you are using as quite low hold current, so this probably is not a problem, but if you get bad behavior, try testing with a somewhat higher power lamp. If it turns out that hold current is a problem, I suggest a triac with lower current rating - hold current is usually lower for triacs with lower main terminal current rating.

Incandescent lamps typically have a resistance when cold that can be a tenth or less of what it is when hot. This complicates things, but generally is advantageous in terms of hold current. There is some chance this phenomenon is contributing to the odd behavior.
Thank you so much for the information on the inductor. Solved some of my long lasting questions. I do remember someone telling me about the hold current but he didn't elaborate so I totally forgot about it.

I just tried it with a 25W light bulb and it flickers again. The hold current is 30mA for the TIC216M but the lamp draws about 100mA. So there must an other problem too.
 
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