Question about TL494 op amps - Page 1
Forum section: electronics / projects

author: hydrogen18
author_rank: Contributor
timestamp: February 24, 2018, 09:29:17 pm
content: I have made a few circuits with TL494 for experimentation and hobby purposes. In all cases I have tied the op amp inputs to the power rails. I tie the non inverting input to ground via 10k ohm resistor and the inverting input to +VDC using a 10k ohm resistor. My understanding is this basically disables them because they produce no output. So they don't inhibit the duty cycle of the output from the TL494.I recently build a TL494 circuit with a current sense resistor. One side of the resistor was tied to ground, the other connects to the small transformer I made. From this junction I connected it to the non inverting input of one of the op amps. The inverting input of the op amp was tied to 2.5 VDC via 10k ohm resistor, obtained by dividing the TL494s reference voltage. So basically this resistor was causing no voltage drop that I could measure. The idea was eventually I'd get around to setting up a feedback resistor on the op amp as well as some potentiometers to adjust everything.This configuration totally inhibited the output of the TL494.Am I wrong for thinking it should still produce output?My thought was that with the op-amp's inverting input tied to +2.5 VDC it'd basically be disabled. I have a loose knowledge of what an op amp is and why it is useful (small signal amplifier in this case) but I have never actually built anything using just op amps. Once I modified that circuit with 10k ohms resistors to pull to ground and +VDC the chip produced the square wave output as normal.

author: Audioguru
author_rank: Super Contributor
timestamp: February 24, 2018, 10:28:05 pm
content: A TL494 is not an opamp. It is used to make a pulse-width-modulated power supply as shown in its datasheet that I guess you have never seen.

author: Benta
author_rank: Super Contributor
timestamp: February 24, 2018, 10:45:51 pm
content: Quote from: Audioguru on February 24, 2018, 10:28:05 pmAudioguru, try reading the OP before firing all guns. Not nice.

author: Phoenix
author_rank: Frequent Contributor
timestamp: February 24, 2018, 10:49:37 pm
content: I expect the op is talking about the internal error amplifiers and the pwm output.Got a schematic of what you're doing?Sent from my G8441 using Tapatalk

author: danadak
author_rank: Super Contributor
timestamp: February 24, 2018, 11:15:22 pm
content: The error OpAmp has limits on its CM input range, tying an input,w/ or w/o an R, to the positive rail violates its CM range.Regards, Dana.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2018, 12:47:38 am
content: I'm guessing the effect of that violation is to force the output low.OP: use VREF, not +VDC, for the positive rail tie. You should find the 'disable' polarity is opposite what you had. Then the feedback will work in an obvious manner.Tim

author: danadak
author_rank: Super Contributor
timestamp: February 25, 2018, 02:04:00 pm
content: Ref material on CM violation -http://www.analog.com/media/en/training-seminars/tutorials/MT-036.pdfhttp://www.analog.com/media/en/training-seminars/tutorials/MT-036.pdfRegards, Dana.

author: hydrogen18
author_rank: Contributor
timestamp: March 10, 2018, 08:20:09 pm
content: Thank you for taking the time to respond and linking the other reference material.Quote from: T3sl4co1l on February 25, 2018, 12:47:38 amIn reference to this, why is my disable polarity wrong? Is the following the correct "disable" action?1. Connect the inverting input to VREF/2 via 10k resistor2. Connect the non inverting input to ground via 10k resistor

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2018, 11:52:45 pm
content: VREF and 0V will do, or that.Tim