MEMM

[MeggerMan]

Hi Paul,

Just working out the frequencies now.
You can roughly calculate them but there is no pattern that I can work out to calculate the exact result.

TL494 push pull breadboard testing pics:




Fan light I was telling you about.



Regards

Rob

[MeggerMan]

Hi Paul,
According to a write up for the TL494 the frequency for push-pull setup should be:
http://focus.ti.com/lit/an/slva001d/slva001d.pdf

f = 1/(2 * R * C)

But as usual it does not quite work out correctly.

It is possible that there is some capacitance on the breadboard itself but I cannot see this being more than 100 pf.
Its out by about 35% on the larger capacitance (88nf) and gets better, 27% for 10nf, on the lower the capacitance so that rules out the stray capacitance idea.

I will add a supply capacitor across the chips supply rails just in case its that.
Last thing I need is an unstable push-pull pulse circuit.
Its lower limit looks like to be about 365Hz where the spec. tells you that its 1kHz.
Pity, I was hoping to get down to about 10Hz.

Regards

Rob

[gyulasun]

Hi Rob,

I suggest using a 1uF timing capacitor at  Pin 5 of the TL 494 to get to the lower 10Hz frequencies.
From the writings of your link, Page 8, in Fig. 6 you can see the 1uF value for the lower 10Hz operation and this goes with a 200-300 kOhm timing RT resistor, preferably a potentiometer of 220 kOhm. With these two values you can go up to maybe 1kHz from the few tens of Hz, so you have to switch in a lower value timing capacitor to reach higher frequencies. A good solution at the potmeters to connect a 22kOhm one in SERIES with a 220kOhm to get fine tuning possibility and to avoid switching the potmeters when higher frequencies are needed.

If you cannot get near to the lower 10Hzs with the 1uF/220kOhm, then I suggest bending out Pin 5 and Pin 6 pf the TL494 horizontally to disconnect them from the board and solder directly the cap and potmeter wires to these pins.

rgds
Gyula

[PaulLowrance]

Hi Rob,

That's odd.  Are you sure your cap isn't 88 pF? What's your timing resistor value?  My datasheet shows the TL494 having the following range -->

minimum 0.47 nF and maximum 10000 nF for timing cap
minimum of 1.8 Kohm and maximum of 500 Kohm for timing resistor.

That should give a range of 0.1 Hz to nearly 600 KHz, but that's probably not the valid frequency range.  I don't see how your circuit could have anywhere near a few nF stray capacitance.

What's your Vcc?  Valid ranges is 7 to 41 V. 

http://focus.ti.com/lit/ds/symlink/tl494.pdf


Regards,
Paul Lowrance

[MeggerMan]

It looks like the value of the discharge resistor causes some issues if it is too high for the larger caps. 20k or above with 0.47uf and it jumps all over the place.
At the higher frequencies like 1kHz and above it is not so much an issue.
With testing I can get down to about 10Hz but I have to use a resistor of about 1k + 10k pot.(big frequency swing though).
Also I have managed to get the duty cycle working, 47% down to 0% (off), although the meter flakes out at about 5% and then shows it as 99.8%???
I can see on the scope that it is about 2% but the meter tells me different.

24khz on the scope with storage so I could get a photo, sorry for the poor quality.
Notice the alternating pulses, it does work so far!


734kHz scope trace, maxed out with no caps.


Regards

Rob