What could I build with a TL494?

[giantsteps]

One of my students gave me a dead circuit board from a refrigerator so I could salvage parts from it. Got some PNP transistors and a LM358 dual op-amp as well as the usual caps and resistors. It has a LM358 chip. Any ideas for something I could build with it? (Besides a refrigerator control unit.)

Thanks,

Chris

[petemoore]

  LM358, IIRC is a dual opamp.
  Look for the data sheet.
  There are plenty of ways to apply it, just look for opamp triangles in schematics.
  Hafta suggest the relatively inexpensive new ones though.

[giantsteps]

Whoops I meant there was a TL494 chip as well. The dual op-amp I may use for a distortion just to try the flavor (it ain't a 4558.)

[giantsteps]

Hmm...reapproaching the TL494 dilemma. Any ideas?

Chris

[Pablo1234]

you could make a mono synth or do some small motor control stuff.

[PRR]

> the TL494 dilemma

Dilemma? It's a 13 cent chip. It or something like it is found in almost every power supply. It is very dedicated to power-supply control. It would take a lot more than 13 cents of head-scratch to figure anything "audio". The most obvious is a square-wave oscillator (it can easily push to 10Hz), but there are so many pins to enable/disable/load to make that happen that I'd rather just chuck it and use a 558.

LM358 is the dual of the quad LM324. Not as-is a good audio op-amp (horrid crossover distortion) but a generally useful glue chip.

[JKowalski]

Class-D amplifier? But that's a complicated thing to tackle. Class-D means that you compare (hi-speed comparator) your audio signal to a fixed high frequency trangle/sawtooth wave, creating a PWM output that is used to drive MOSFETs or other high speed switching devices that switch power into the speaker. The high frequency is way ultrasonic (100kHz-1Mhz) so it takes only a little bit of filtering to get the hash removed from the output signal with an inductor and capacitor, and you end up with the output signal at the speaker (averaged PWM). These are very high efficiency amplifiers because there is not any power devices intentionally operated in the linear region. The mosfets are either ON or OFF. Their on state resistance is very low, so they dissipate very little power while switching large amounts of it into the speaker. Think of a speaker.... The mosfets are shorting the speaker across the supply rails one way, then the other way, then the other way.... If they were ideal switches then the load would be the only power dissipation. They are close to ideal, so they load is practically the only source of power dissipation!

This chip could very well do a class D amplifier - the audio signal is put into an error amplifier, and is compared internally to the high frequency oscillator, so you end up with complementary PWM outputs. But as I said, this is a very difficult thing to tackle - not only do you have to take great pains designing it so it works well or at all, but the circuit board can easily make or break the device just because of parasitic capacitance/inductance screwing with your high frequency signals.




The chip is really only useful for one thing:

Creating a fixed frequency square wave output and adjusting it's duty cycle based on the voltage at certain inputs.

The error amplifiers are just op amps that output a voltage that the sawtooth oscillator voltage is compared to. Changing the voltage output of the error amplifier changes the duty cycle. This is used for SMPS applications. They are called "error" amplifiers because they detect either the output voltage of the SMPS OR the current through the switching device, and change the way the MOSFET is driven to compensate for the error in output. This is how SMPS stay regulated.  

The square wave has two outputs: One noninverted, one inverted. The Dead Time Control controls a period of time inbetween the on/off switching of these outputs where both are OFF. What this is used for is in driving things like MOSFETS handling decent power... When you turn a mosfet on/off at high speeds, it takes a certain amount of time to make the transistion. Often these MOSFETs are connected in series across the power rails and turned on/off alternately (see half bridge or full bridge) to drive power into the load. If both are trying to make the transistion, there might be a period of time where both MOSFETs are not fully on/off and operating in the linear region, effectively SHORTING the supply voltage with potentially disastrous consequences. The dead time makes sure that there is a dead time inbetween the switching so that both MOSFETS can never be conducting simulataneously. I don't see much use for anything else.

I am not entirely sure why I wrote all that but maybe you might find it interesting