# How to build a variable frequency controler?

Discussion in 'Electronic Basics' started by [email protected], Jul 4, 2005.

1. ### Guest

I am looking for advice. I need a varable control device that can take
standard 60 Hertz, 110 volts and give me a range from 15 Hertz to 200
Hertz at 110 volts. This will be used to adjust the frequency of
pulsating light given from a fluorescent light bulb. Some people find
reading difficulty under fluorescent light. My theory is that this may
be related to a sensitivity to the frequency. I have a basic knowledge
of what needs to be done, but I am not sure what components I need to
purchase and how to put them together. Any help will be appreciated!

2. ### arem_29Guest

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3. ### Roger JohanssonGuest

What you need is basically what a music amplifier does.
It takes 110V and makes DC voltage which powers an amplifier which can
drive a loudspeaker.

Add a variable frequency generator and connect it to the input of the
amp.

If the output stage of the amp gives high enough voltage you can use it
as your new, variable frequency power supply.

I described this using an ordinary household item, an amplifier, to
explain what needs to be done in simple terms.

You may prefer to build an amplifier yourself, or buy one, but in any
case you need a power stage in the amp which can deliver the voltage
and current you need.

The variable frequency generator isn't too difficult to build, there
are schematics on the web.

One alternative I would consider is to use an ordinary amp with an
extra high voltage stage after it. Common amps for music seldom give
enough voltage to make 110V AC.

A transformer at the output of the amp is another possibility. If the
amp delivers +-30Volt the transformer takes it up to 110.
The size of this transformer and the power demands on the
amplifier depends on how much current you need.

By using a readymade amp I would save myself the most of the work,
building the transformer stage, rectifier and amp circuits.

Anyway, you you what you need now. A variable sinus wave oscillator, a
power amp, and maybe a transformer if the power amp isn't
specially made to produce high enough voltage by itself.

An output voltage checking and regulating circuit can be added, if you
need better regulation of the resulting AC output voltage than just
setting it manually using a voltmeter.

5. ### Tom LeMenseGuest

[snip]
True enough - but typically if one sticks to frequenceis ABOVE the rating
(in this case, 60 Hz) it's OK. Go below that, however, and the transformer
will magnetically saturate and undoubtley fail due to overheating.

TJL

7. ### Tom LeMenseGuest

Hi John,

No, I've not operated a ballast transformer above 60 Hz, but I *have*
operated many 60Hz power transformers at higher frequencies, up to 400 Hz,
without incident. It's a strong function of the primary inductance, which
is sized for 60 Hz - once the frequency is higher, the transformer is less
prone to saturation, so runs cooler. On the other hand, I've seen the
disaterous results of what happens when a 60 Hz xformer is pressed into
long-term service at 50 Hz, too...

TJL

8. ### John FieldsGuest

---
Yes, in order to maintain the same permitted flux density in a
transformer operating at a frequency less than it was designed for,
its input voltage must be lowered by the ratio of the frequencies:

f1
Vin2 = Vin1 ----
f2

so, for a transformer with a 240V 60Hz primary, the permissible input
voltage at 50Hz would be:

50Hz
Vin2 = 240V ------ = 200V
60Hz

AIUI, a fluorescent ballast is more like an inductor than it is a
power tranformer in that it has to generate a high voltage kick in
order to ionize the gas in the tube and then, once the arc is
established, supply a more-or-less constant current to the tube for
the remainder of the half-cycle. Since the inductive reactance of the
ballast is what limits the current through the lamp, and since
inductive reactance varies with frequency, I would expect the lamp to
dim if the input input frequency to the ballast was increased.

http://home.howstuffworks.com/frame...l=http://www.misty.com/people/don/f-lamp.html

9. ### Roger JohanssonGuest

Yes, but that can be compensated by adjusting the voltage, using the
volume knob of the amplifier.

This would allow some experimenting with different frequencies, which
was the intent as I understood it.

11. ### Tom LeMenseGuest

Yes, certainly as the frequency is increased the voltage will 'droop'. I'd
attribute it to
hysteresis effects in the core, especially with the frequency >> 60 Hz.

If experiments didn't lead to more "pitfalls" they wouldn't be any fun,
right?

TJL

12. ### Guest

I think that inductive kick will blow up the audio amp. I had a friend
try this many years ago with MY amp. Never DID get around to fing the
old thing. What is wrong with using an electronic ballast? Or is this a
'I gotta know what frequency...'. I guess I'm just an old fart. The
solution is \$25 at Lowes made by Osram.
GG

13. ### Guest

I think that inductive kick will blow up the audio amp. I had a friend
try this many years ago with MY amp. Never DID get around to fing the
old thing. What is wrong with using an electronic ballast? Or is this a
'I gotta know what frequency...'. I guess I'm just an old fart. The
solution is \$25 at Lowes made by Osram.
GG

14. ### KubiackGuest

a écrit :

If the only thing you want to experiment is the light frequency, you
don't have to use a fluorescent light (witch is designed to operate at
50/60Hz).
You can build a very simple generator with a 0-5V 1Amp square output and
drive a series of white leds to illuminate what you want to illuminate.

On this other hand, if you really want to build a sine generator with a
110V output, you have a lot of work.
Such generators already exists and are used to drive asynchronous motors.