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Binary 1011001101
Nobel Prize Winner
United Kingdom
569 Posts |
 Posted - Oct 13 2010 : 1:10:00 PM
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Holy shiznip its been a while since I was last here, just got back into electronics and realized ive forgotten a lot. Anyway, revisiting an old project.
So I need to trigger a bunch of SCRs or Triacs, I don't think theres any difference triggering wise. Ive seen people who use an optocoupler in two ways.
Take the mains and shove it through an optocoupler then through resistor into the gate.
Or
Taking the mains after the load (anode side), through a resistor, through an optocoupler then into the gate. I can't seem to find an explanation why. To me this way doesn't make sense because when a different load is connected to the scr it will change the gate voltage; a bad thing?
After all do I really need an optocoupler if it can be switched with something like transistors?
The application is just to switch lights on mains from a 5v source.
Cheers. |
Edited by - Binary 1011001101 on Oct 13 2010 1:11:30 PM |
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Aaron Cake
Administrator
Canada
6718 Posts |
Posted - Oct 16 2010 : 10:36:09 AM
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| Generally you'd want to switch it on with a known voltage, such as mains through a resistor. Optocouplers are used becuase SCRs/TRIACs don't isolate the triggering circuit. You need a common ground on the SCR/TRIAC side of the optocoupler. Actually, they make optocouplers with a DIAC inside specifically to trigger SCRs/TRIACs. |
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Binary 1011001101
Nobel Prize Winner
United Kingdom
569 Posts |
Posted - Oct 24 2010 : 2:29:58 PM
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Would something like this work?
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Edited by - Binary 1011001101 on Oct 24 2010 2:31:51 PM |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Oct 24 2010 : 6:22:50 PM
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An SCR turns on a light with DC pulses so with an AC supply the light is on only half the time and is very dim.
A Triac turns on the light all the time with full AC so the light is very bright. |
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Binary 1011001101
Nobel Prize Winner
United Kingdom
569 Posts |
Posted - Oct 25 2010 : 11:05:53 AM
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| What if I full wave rectify the mains to feed the gate with DC, it will still be on for half the time because the SCR will conduct for the half of the AC wave. But its a filament bulb so it shouldn't notice. |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Oct 25 2010 : 7:00:17 PM
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A light bulb that is turned on for only half the time is very dim.
Use a triac for full-wave control. |
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Binary 1011001101
Nobel Prize Winner
United Kingdom
569 Posts |
Posted - Oct 26 2010 : 10:32:39 AM
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Ill use the SCRs with 240VDC, cant afford to buy triacs haha.
But the triggering is all correct yes? I just got to calculate the needed resistance so I don't blow up the gates. |
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pebe
Nobel Prize Winner
United Kingdom
1078 Posts |
Posted - Oct 26 2010 : 1:51:27 PM
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| You will have to switch off the supply or short out the SCRs to turn the lamps off. |
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Binary 1011001101
Nobel Prize Winner
United Kingdom
569 Posts |
Posted - Nov 17 2010 : 5:42:55 PM
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Im back, i'm going to use rectified AC but not smoothed with and SCR and optocoupler. Im just stuck on what value resistor to use at 250v or so. The SCR i'm using is a TIC106D with a MOC3022 optocouple.
The mains goes through a ??? k resistor through the optocouple then to the gate, the load is connected to mains then goes through the SCR. Can someone verify if this is the correct way as many other people (the optocouple's datasheet) take the gate voltage after the load.
Normally id guess values until something works not knowing much (if any) electronics related maths, but I really don't want to blow up these components this time round. |
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SCR/ Triac triggering
