View Full Version : What switch can I use for passing positive or negative cycles of Alternating Current?

08-15-2011, 06:42 AM
I know this is a very silly question but, i am somewhat knew to power electronics.

I am building an electronic circuit, and now i need to control the direction of the current flow, for which diode cant be used.
So which switch can I use to pass either positive or either negative cycles of AC - triac, scr, mosfet, any other ??

Like in one path i need to conduct with a switch which will be closed during the positive half cycle of AC, and open during the negative half of ac.
In another path i need the switch to close during negative and open during positive.

Also it should allow the entire positive or negative half cycles to flow when closed.
Also the control signal is from the AC signal itself.

I know for dc , i could have used npn and pnp bjt transistors for this actions.
But for ac, what should i use.

Thank You

Ben tenyson
08-15-2011, 07:42 AM
I am new to electronics to. I think you can do that by using a diode, connect the diode in series with you load. If want to filt out the negative voltage you just need to reverse the diode. You should be careful with the current rating of the diode while you choosing it.

08-15-2011, 10:35 AM
I'd suggest Triacs or anti-parallel SCR's / Thyristors would be suitable. Correctly fired device gate terminals allow you to control the particular AC half cycle over which a device conducts. Said gate control pulses can be derived from the same AC voltage sources.

08-15-2011, 11:35 AM
Thanks for the quick reply guys,

No Ben, a diode cant be used in my circuit, the line should be closed only during certain times.

TNK what type of Triacs or scrs should I use.

Like for DC, in bjt there is npn which i can use to on it using positive voltage, and pnp which I can used to on during negative voltage.
Is there somethin similar in triacs scrs. where the gate will switch on during either + or -, as per the type.

08-15-2011, 12:47 PM
You need to indicate what the voltage and current flow through the device will be.

08-15-2011, 01:57 PM
Right now i am using the voltage stepped down from the mains from 240V to 6V using transformer.
But later i want to try with much higher voltages.

Can you please tell me the type of ac switches for both low and high ac voltages.
And the two types pnp and npn like in transistor

08-15-2011, 07:10 PM
Thanks for the quick reply guys,

No Ben, a diode cant be used in my circuit, the line should be closed only during certain times.
A diode in series with a switch should do what you want, I think, I'm not certain though because I find your grammar a little hard to understand in some of your posts :confused:

What exactly are you doing?

08-15-2011, 08:24 PM
One thing nobody has mentioned clearly is that you can put a diode in series with an SCR or a triac. If that meets your other requirements it will force that one active switch to only work with one polarity.

08-16-2011, 03:38 AM
Firstly Sorry guys if i have not been clear enough.

For example in dc, with a npn bjt i can close the switch by applying positive voltage to base of the transistor.
And the reverse for pnp.

In scr,triac are there types that can be switched on using either + or -ve voltage to the gate, (just like in bjt's.)
What kind of the v/g should be applied to the gates of these switches to close them.
I am not much familiar with scr and triacs, am reading up from the net right now.
I hope ive been clearer this time, Any help will be appreciated,

08-16-2011, 04:47 AM
Attachment shows...using SCR's (Thyristors)

A simulated setup with continuous gating of the negative half cycles and a single gated positive half cycle at a specified time.

08-16-2011, 05:08 AM
Can any ac switches be used that do not require a opamp/comparator for driving the gate.
like how in bjt the dc voltage can be given directly to the base of the bjt through a resistor.
I want to avoid use of lot of components.

08-16-2011, 05:43 AM
Diacs are commonly used to control switching device [e.g. triac] gate input terminals. Google thyristor firing control - e.g. as used in dimmer circuits. Switching control for switched AC applications isn't as simple as with DC BJT switching control. If you want to do things like integral cycle control you will be stuck with more sophisticated control logic. Control isolation / electrical safety considerations may also come into the picture.

The problem for forum members in providing practical advice to you is that you actually don't seem to have a clear design goal. Or rather, you haven't given much background information thus far.

If you would care to elaborate your ideas in some detail with the overall requirements, voltage & power or current levels and so forth [a picture or schematic always helps] you might receive more helpful feedback.

08-16-2011, 05:56 AM
Thanks tnk for the nice explanation, and i am sorry for being so confusing, ill try to draw a circuit diagram and upload it.

So ac switching requires a lot more control, i guess thats why i coludnt find any simple ckt on the net for ac switching. all used some form of control mechanism, but since being self taught i just thought there might a way, since no one clearly put it to me as you have.

I was just now thinking that may be I could rectify the ac to dc using the charge pump mechanism and use this for my ckt, then i can use the npn and pnp bjts itself.
Can you advice me any npn and pnp type dc switcing elements that can be used for much higher voltages.

08-16-2011, 01:34 PM
PNP and NPN transistors won't work well for AC, as you will exceed the reverse breakdown voltage for the BE junction. Besides causing undesired current flow, this will permanently reduce the gain of the transistor(s).