Frequency Control for Single Phase Circuit

[hisham.i]

Hello...
I was trying to make a circuit to control the intensity of light of a lamp...i thought about using a reohstat in series with the lamp, but i want to try a circuit which changes the frequency of the current entering to the lamp.
How do we make such circuit, taking into consideration we are working in a single phase circuit.

 

[mgb_phys]

Why would changing the frequency change the intensity?
Are you just relying on the stray inductance of the circuit? It would be better to use the normal 50/60Hz frequency and introduce a variable inductance

 

[hisham.i]

I just want to know what is the effect of changing the frequency on the load...
would the effective value of the voltage around the lamp change if we change the frequency of the source?

 

[Adjuster]

Most commercial dimmers for AC lighting use triacs to cut out part of the AC wave. Why re-invent the wheel?
Are you perhaps worried about harmonic generation?

 

[Adjuster]

Changing the frequency (within reasonable limits) won't make much odds to a filament lamp. Old-time radio hams used lamps as a crude estimate of transmitter output at MHz frequencies before more refined measuring equipment was easily available.

Don't even think of doing this to fluorescent lamps though, whether coil ballast or electronic, this is just asking for nasty malfunctions.

And whatever you do, remember that mains electricity is deadly dangerous.

 

[hisham.i]

If i use a Triac, then i should have a variable dc voltage being applied on the gate of the triac, and while varying the input gate voltage then the output applied on the lamp will be changed and so the intensity of the light..right?

 

[dlgoff]

Just for reference.

Four types of pulse-width modulation (PWM) are possible:

1. The pulse center may be fixed in the center of the time window and both edges of the pulse moved to compress or expand the width.
2. The lead edge can be held at the lead edge of the window and the tail edge modulated.
3. The tail edge can be fixed and the lead edge modulated.
4. The pulse repetition frequency can be varied by the signal, and the pulse width can be constant. However, this method has a more-restricted range of average output than the other three.

http://en.wikipedia.org/wiki/Pulse-width_modulation#Types"

 

[Adjuster]

No, triacs are pulse driven, often by a diac circuit which produces pulses synchronised to the AC line, with an adjustable phase to control the dimming. Basically the triac blocks the AC between each zero-crossing until the next gate pulse.

Unfortunately, ordinary triacs can't be turned off at will, you need something more advanced to do that. However, if you are just trying to dim a lamp, what do you really need?

 

[vk6kro]

http://www.electronics-lab.com/projects/motor_light/029/dimmer.gif

This is a simple light dimmer circuit taken from this article:
http://www.electronics-lab.com/projects/motor_light/029/index.html
which includes a description of the components.
It is for 110 volts only. L1 is the lamp being controlled.

L2 is a neon lamp which is a bit unusual as a diac is normally used here.

You may know that Triacs are turned on by a voltage on their gate but they cannot be turned off by removing this voltage. The only way to turn them off is to remove the anode voltage. Fortunately an AC waveform removes this voltage every time it changes polarity.

 

[hisham.i]

However, if you are just trying to dim a lamp, what do you really need?

yes am just trying to dim a lamp...
But i think that the circuit which vk6kro gave me is what i need.
Thanks for all..

 

[hisham.i]

There is also another question on the same topic,
what is the difference between using just a rheostat is series with the lamp, and using such a circuit?

 

[vk6kro]

You could use a rheostat to control a lamp's brightness and for very small lamps, this might be an option.

For larger lamps, the problems get serious and are related to each other. They are heat dissipation, size, cost and efficiency.

If you just switch something on or off, the switch doesn't dissipate much power. Maybe a little if there is sparking.

If you put in a rheostat, it will get hot. If you controlled a 200 watt lamp and dropped the voltage on the lamp from 110 volts to 55 volts, then the power dissipated in the rheostat would be about 50 watts.

Current in lamp = 200 / 110 = 1.818 amps
Resistance of lamp = 60.5 ohms
Current in lamp at 55 volts = 55 / 60.5 = 0.909 amps
Power in Rheostat = 110-55 * 0.909 = 50 watts.

So, the rheostat has to be quite big or it will get very hot. It also gets very expensive.

There is another problem, though. To reduce the voltage on the lamp to even 10% of the 110 volts, the series resistance has to be about 544 ohms. So, the rheostat has to be able to have 544 ohms. But only the low resistance end of the rheostat will dissipate the 50 watts. So, the whole rheostat needs to be a lot bigger than 50 watts. Maybe 500 watts.

A commercial lamp dimmer costs about $10. So, the choice is fairly obvious for mains AC circuits. They are small, cheap, very safe and they can reduce the voltage on the lamp to zero, which a rheostat can't do.

Incidentally, if you make one yourself, you should not use it as a fixture in your house as it would not be an approved type and your insurance may not be honoured if it is found later, even if it didn't cause a fire.