# LMC555 Problem (solved)

Discussion in 'The Projects Forum' started by SmokeMcPot, Aug 7, 2015.

1. ### SmokeMcPot Thread Starter New Member

Aug 7, 2015
4
0
Hello Guys

I build a circuit with a LMC555 Timer, i used this circuit very often and had no problems with it.
The circuit is a PFM Generator for a flyback, the frequency can be selected by applying a voltage on REG. The turn on time is determined with R19 and C14. To limit the current in the transformer a shunt resistor is used which will reset the timer at a certain current level.

But in a recent project i have problems with the reset entry of the Timer. The following figure shows the normal behavior of the circuit. In yellow we can see the voltage over the shunt resistor, which represents the current. After a certain limit, an over current is detected and the reset signal drops. The reset function is working well.

But when i apply a load on my flyback circuit under certain conditions, some serious problems with the reset entry appear. As you can see in the following figure, the timer ignores the reset signal and my transformer passes in to saturation.

I know the threshold of the reset entry is quiet low, min. 0.4V. But the reset signal applied is lower than this threshold.
Does someone have an idea, why this problem appears and whats the solution?
Thank you

2. ### ScottWang Moderator

Aug 23, 2012
5,477
864
How is the transformer?
You only shown a PT10, does the PT10 is the transformer?

3. ### SmokeMcPot Thread Starter New Member

Aug 7, 2015
4
0
Here you go. Flyback circuit:

And the current limit detection circuit:

4. ### SmokeMcPot Thread Starter New Member

Aug 7, 2015
4
0
I added the rest of the circuit below

5. ### ScottWang Moderator

Aug 23, 2012
5,477
864
Could you separate the reset pin of CD4093 from 555 and measure its waveform and voltage when the pin is low?
I just checked the voltage of Vol(voltage output low) of CD4093 is 0.05V, so it is quite low than bjt.

6. ### ScottWang Moderator

Aug 23, 2012
5,477
864
This is interesting, I never saw a 555 circuit controlled so many pins, it is still have some connections of pins are unclear, but since you concentrated in the reset pin of 555, the other is not that important for now.

7. ### ronv AAC Fanatic!

Nov 12, 2008
3,657
2,800
Since the reset really should reset the 555 maybe the 555 ground is not where you think it is. Can you put the probe ground right at the 555 and measure the reset pulse?

8. ### SmokeMcPot Thread Starter New Member

Aug 7, 2015
4
0
Yes the regulator circuit is missing. But its just a PI-Regulator.

I found the solution in the mean time. Due the commutation of the Power-Electronics, powered by the flyback, some perturbations occur. You can see this perturbations in the second scope image.

When this perturbations appear in the flank of the reset signal, the flipflop of the LMC555 will have a Latch-up problem. The solution is to add capacitor around 500pF between the reset line and the ground. Now the problem disappears.

9. ### ronv AAC Fanatic!

Nov 12, 2008
3,657
2,800
I see the noise on the current, but not on the reset. Glad you got it working.