Use a phase -locked loop. Square up the 50 Hz to a square wave. The PLL [LM565], has a voltage controlled oscillator which will multiply, with proper components,the 50Hz to 400 Hz, use divider to divide by 5 giving 80 Hz. Dividing by 8 gives Almost 50 HZ which is comared to origional 50Hz & generating a correction V to VCO to make divided down 50 Hz exactly same as origional, the 400 HZ then is then also exact, giving an exact 80 HZ square wave. Use class D amp to give desired operating voltage & filter. LED current limiting resistors can be part of filter. Use a high V out [48V], to shorten the number of LED strings. At least I think that is how it works.

Another wonderful Rube Goldberg solution!
I'm not making fun of you and studiot, I'm making fun of the OP's supervisor.

If he was doing PWM, this frequency conversion would make sense, but he said it was a sine wave.

If it turns out there is a good reason for doing this, I'll have to develop an appetite for crow.

 

80Hz flicker is generally not detectable by the human eye. 50Hz is noticeable and objectionable to some people.

This will depend very much of duty cycle also. So I was somewhat unprecise. A motion picture do have a frame rate equal to 25 pictures pr second. As an example. So the magnitude of change is also important. A 50 Hz mains powered light bulb will change from on to off in a gradual manner. But we see the light as constant, at least then we do not stare directly into it.

 

This will depend very much of duty cycle also. So I was somewhat unprecise. A motion picture do have a frame rate equal to 25 pictures pr second. As an example. So the magnitude of change is also important. A 50 Hz mains powered light bulb will change from on to off in a gradual manner. But we see the light as constant, at least then we do not stare directly into it.

A 50Hz mains-powered bulb has a thermal time constant which reduces the amplitude of the flicker, and also doubles the frequency, so the flicker is basically invisible.
PAL TV, with its 50Hz field rate, has flicker which bothers some people. NTSC, while generally inferior to PAL, has a 60Hz field rate, and is generally not perceived as flickering.

 

Flicker has come up before here on AAC, last I remember[ imperfect] we decided 39Hz @ 50% duty cycle was acceptable for LEDs. This isn't the first strange or impractical request we've received, but if we can just answer POs question as stated ,I feel we have done our duty.

 

It would help if the OP told us what, he/she want to achieve by doing this. Someone must have given this some chain of thought.

 

Yep, it's a good example of those vague questions that force everyone to be guessing while they are trying to help, and in the end it's all for nothing because the real solution would have been easy if the question had just included the basic info of WHAT they were wanting it to do, not "how do I do this weird thing?"

 

Yep, it's a good example of those vague questions that force everyone to be guessing while they are trying to help, and in the end it's all for nothing because the real solution would have been easy if the question had just included the basic info of WHAT they were wanting it to do, not "how do I do this weird thing?"

I suspect our OP was afraid to confront his supervisor for whatever reason. It might be a cultural thing. I know that when I was working, I occasionally had to confront mine, and I didn't hesitate, but I was diplomatic if I thought he was touchy. I lived (still do) and worked in the US. Things might be different in Hong Kong.

 

thanks for all your help..
but, may i use rectifier to convert AC to DC then use inverter to convert DC to AC?
If use inverter is it possible to get 80Hz AC?

 

The definition of a AC voltage is. A voltage in which the polarity alternates. Then you are saying AC do you mean a pulsating DC voltage? Can you please post a schematic drawing of what you are planning to do, or some schematic of your current circuit

 

AC-DC converter
then, DC-AC inverter
220VAC input , 50 hZ from AC-DC converter, to get 5-10v dc output (by using rectifier)
the voltage I want to get from The final DC-AC inverter is around 5VAC , 80Hz with 1A current
for inverter, I am still studying the cirucit, may I get the 5VAC , 80Hz with 1A current output?
Also is it using the thyristor? (as i see many book is talkiing about this and I never use it before)

 

Oh I see. You need to build a sine wave oscillator. Then connect this sine wave to a amplifier circuit. I would recommend you to use an audio amplifier IC as amplifier as they are cheap, and easy to get your hands on.

 

The only reason I can think of why you might want such a low inverter frequency is backlight/backplane driving.

http://www.tstonramp.com/~pddwebacc/lcd_backlights.htm

If this is what you are doing you need a square or pulse wave not a sine wave.

The more you explain the more help members will be able to supply..

 

Yes, the final pulse wave is square wave and actually I am doing the LED backlight.
Why I want to get the sinewave before is that I have a circuit to change the sinewave to square wave.
At the beginning , i just want to have frequency conversion form AC voltage to AC voltage and 50Hz to 80 Hz.

However, I cannot do that....
So now , AC to DC by rectifier then DC to AC by inverter

1. But when I read reference about inverter, I don't understand what thyristor is ,
and is it I must use thyristor to control the output freq ??
2. it's seems inverter can have square wave output , if it is true, then i wanna to
get this pulse wave with 80Hz directly, but how to design it?

To t06afre
amplifier circuit??
what amplifier circuit?

 

Perhaps a 555 timer, and a transistor/FET as the switching element could be the solution you want. I guess you also have to include a voltage regulator so you do not damage the LED backlight circuitry

 

4 pages of comments and we come back to a 555. LOL

No reason to use a thyrister, just hook up one of the hundreds of LED drivers as per the application note and use a 555 to turn the led on and off 80 times a second.

 

Perhaps a 555 timer, and a transistor/FET as the switching element could be the solution you want. I guess you also have to include a voltage regulator so you do not damage the LED backlight circuitry

As I found the information of inverter, to build up a single-phase full Bridge inverter , it needs I need to build up four chopper (,aybe 4 transister),
and build up a oscillator to provide square wave to control the on-off time of these four chopper to control the voltage freq on the load?

for the chopper, any cirucit ref?

 

You want to rectify 5 V AC 50 HZ, filter and generate 80 Hz or greator square wave [ 50% duty ] 5 V PP, using some electronics becides diodes?? Use comparator to select 50% point on 100PPS signal, driveng P ch, logic level, MOSFET to chop 5 V DC @ 100 PPS rate, to drive 100 75 Ω current limiting resistors & LEDs.

 

You want to rectify 5 V AC 50 HZ, filter and generate 80 Hz or greator square wave [ 50% duty ] 5 V PP, using some electronics becides diodes?? Use comparator to select 50% point on 100PPS signal, driveng P ch, logic level, MOSFET to chop 5 V DC @ 100 PPS rate, to drive 100 75 Ω current limiting resistors & LEDs.

Sor, what comparator or mosfet you suggest? and if i use the mosfet, does it have around 1A output?
And I wanna to get the sinwave or square wwave 5VAC

As I found the information of inverter, to build up a single-phase full Bridge inverter , I need to build up four thyristor or chopper .

there is a Gate of the thyristor, I don't kn how to develop the circuit to control the gate,
To conrol the gate, it is controlled by a signal or current?
If control signal, is it a need to develop a oscillaor as a input?

 

Comparator.... LM393; P ch MOSFET....NTD25P03L,30V, 72mohm, enough As. Output , about 5V PP, pulsating DC @ 100 pps- put it thru capacitor and call it AC; i'm finished here.