All About Circuits Forum Triac &/OR SCR to control voltage?
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#1
11-13-2009, 06:52 AM
 surfline Junior Member Join Date: Aug 2009 Posts: 30
Triac &/OR SCR to control voltage?

Hi,

I am curious about the capabilities of using a TRIAC or SCR in a circuit to convert AC voltage to a desired DC output voltage. For example, if our input is the 110VAC, 60HZ source from house, would it be possible to trigger / fire the pulse of the TRIAC or SCR at times where the 110VAC, 60HZ sin wave is just at a certain voltage level we want, for example 30 volts. So our TRIAC gate pulse would just fire (by a microcontroller ) when our 60hz sinusoid is at 30 volts in the cycle, for a very short pulse width. We would then send this possibly through a transformer and then bridge rectification and smoothing circuit. Could we get a clean 30V DC output from this method?

Also, couldn't we just eliminate the transformer? Do people ever use TRIAC's / SCR's as an alternative to transformers?

Thanks,

Surfline

Last edited by surfline; 11-13-2009 at 06:59 AM.
#2
11-13-2009, 11:23 AM
 Thav Member Join Date: Oct 2009 Location: Raleigh, NC Posts: 82

It sounds like you want to take a slice of an AC waveform, like when the utility voltage passes between 25 and 35V you want the SCR to be on, right? Unfortunately you can't do that with SCRs. Once you fire an SCR (or TRIAC), they can't be turned off until their current has reached zero and tries to reverse.

You could try that with a FET or IGBT which are fully controllable devices. At that point however, you're better off making a modulated rectifier so you get good power factor.

You can use semiconductor devices to transform voltages (like that rectifier I mentioned above). This is the principle behind all switch mode power supplies. You still require an inductor that will be smaller than a transformer needed to do the same job, and you would lose line isolation.
#3
11-13-2009, 12:07 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

You can't safely use a circuit that is directly connected to the AC line. A transformer is necessary to provide isolation and a safer level of energy limiting.

I have seen big power supplies that used SCR's in the rectifier bridge. The internal circuitry controlled their conduction to establish a voltage on the filter capacitors just enough above the required voltage output that the pass transistors could run in saturation. This eliminated losses in those transistors.
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#4
11-13-2009, 03:02 PM
 Papabravo Senior Member Join Date: Feb 2006 Location: Michigan, USA (GMT-5) Posts: 5,849

In my opinion, connecting circuitry to the mains without a transformer, is both foolish and unsafe. It is also against the policy of this board to support a discussion of such circuits. That said, the big problem with using an SCR in any application, is that it is easy to trigger and hard to turn off. In order to turn it off the current must be reduced to a low value near zero by some external menas. A triac is easier because the AC waveform reduces the current to zero each half cycle.
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 Tags and or or, control, scr, triac, voltage

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