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Dual CMOS based timer problems...


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I have built a timer circuit based on CMOS 4538 which consists of 2 monovibrators. I am getting some sort of problems, such as bouncing and wrong timing. Do you see anything wrong with the schematic that i posted because I don't see it. Maybe i overlooked something... There is no noise in the device it is used for. If I test it out of the circuit it works fine, but once I connect it in and it freaks out. The other thing I noticed is that if I use 2 chips on separate boards for the purpose it stops doing it. I am lost...

post-2662-14279142389089_thumb.jpg

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Ok now. I do not know if you saw the 680nF capacitor which you should note is connected before the bridge rectifier. As hopefully we all know capacitors have reactive resistance to AC, which means that we limit the current to way lower value, and this is why the zener is still not blown up  ;D. This is very basic AC mains power supply which is used when you need only small ammount of power, as in this case since we power a CMOS and basically 2 LEDs that are limitet to 5mA each if I remember correctly. I will hook up 1 and 15 to ground just for the heck of it. The manufactor says that those are already internaly connected to ground and I actually measured them - they are indeed. The AC1 and AC2 notations in the schematic are only for the CAD to know where the other parts of the schematic connect; this is why you see them on more than one place. The AC mains is plugged in pins 1 and 2 of the connector.

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Hi there Kain

This will surely not work, take a close look at the PSU:s rectifier! The output to the Zenerdiode is taken from the input to the rectifier bridge and the input to the bridge is connected to its output. Also, small, lowpower PSU:s for things like this usally not have rectifier bridge. As you see, the catodes of Q1 and Q2 is related to AC1, then i belive the "ground" of D2 and IC1 also should be connected to AC1! If you then connect the output of the bridge to VCC and move the connection of the 680 nF condenser to both inputs of the rectifier i belive that it works. Usally you also have a little resistor in series with the 680 nF condenser, in the range 100 to 300 ohms, to reduce inrush current, usally 1 to 2 wattage. Hope this help.

//Staigen

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Ok now. Do not pay this much attention to the PSU part. I know it's not correctly drawn, but I know how to build it correctly and it does it's job well -  I am getting stable 13V out of it. As about safety I know that it is not safe to have mains all over, but you didn't know that this device is in fact enclosed in a box, not to mention that practically in this box there is as much unsafe AC as you want ;D. I am not the one who chosed this type of PSU anyways. Assuming that the bridge is connected correctly (not as in the schematic print), and knowing that I am having stable 13V to the chip, where do you think this thing fails? I mean the way the CMOS schematic is - is there anything wrong with this? This is what I am asking mainly.  :)

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Hi there Kain

Shame of you, sending up a shematic with such a serious error in it, and asking us to analyze it, and why it is not working as expected :( :)! Shame shame shame
Back to the CAD software!

Ok now. Do not pay this much attention to the PSU part. I know it's not correctly drawn, but I know how to build it correctly and it does it's job well -
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Ok. There is the schematic with the bridge rectifier connected correctly. By the way, if this sort of power supply is not using bridge rectifier I wonder why do we have this published in here:

http://www.electronics-lab.com/projects/power/022/index.html

This is exactly what I used without much change (maybe little change in the current limiting capacitor so I can get little more current).

post-2662-14279142396983_thumb.jpg

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Hi there Kain :)

So you dont see the error in the shematic? The thyristors Q1 and Q2 are connected to AC1, but the "ground"(the negative connection of C2) of IC1 is not! The gates of Q1 and Q2 shall have their steering pulses related to their catodes! The output of IC1 is related to the "ground", and not to the catodes of the tyristors, i belive that is the error! A simple solution would be to connect the "ground"(the negative connection of C2) to AC1( a short over the left diode in the rectifier bridge)

//Staigen

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Hi there again Kain :)

I'm not shure what happens, when you connect the thyristor the way you did, but it's not correct, thats for sure! BTW, isn't the resistors in the gates( R18 and R19) a little bit large(100K), maybe you should try smaller ones here(10K or so). I know the thyristors are highsensitive, but....?

//Staigen

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In this case there is not need of lower resistance actually. The thyristors that are on the schematic are sensitive gate ones and require very little current to triger. The funny thing is that it actually works even the way it's made right now.  The things go wrong when I wire up the timer in some machine. Sometimes one of the timers retrigers (bounces) or sometimes both of them time wrong. I understand that the gates are floating, but the reason why the thyristors fire up is because they are sensitive gate ones. I believe that if they were regular it's likely they won't fire up. Just a guess though... :) So let me guess - you said in these cases we do not use bridge rectifier because we leave components float with respect to ground. Correct?  ;D

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Bad news - this solution did not help at all. The timer just goes on on itself and I know that it is not false trigering of the gate because it's not pulled down with resistance. The way i know is because I see the indicating LEDs going on like crazy without any input. I am totally lost now ...

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I haven't bypassed it but I doubt that this is going to solve the problem. In fact I wonder if the timers are faulty at all. Any of those boards is perfectly working out of the machine that they are made for. Once put in the machine the first timer (the one that is on the lower part of the sheet) which works in "interval-on" configuration starts getting all crazy. Now, the only thing that I am suspecting is a problem with long wires. If you see on the schematic there are 2 nets with labels POT 1 and POT 2. One is in fact the positive terminal of the DC PSU while the other goes stright in the RC timing circuit (actually the potentiometer is part of it as you can see). You cannot see the potentiometer because it is not on the PCB but on face panel where we can adjust timing from 1 to 10s. Since the wires are sort of long (little less than a meter each terminal) I believe that this is very good noise source stright in the timing circuit... and thus the main problem. Now, my question is what are you thinking about this guess of mine, and if the guess is correct how can I fix the situation besides using short wires (since this is obvious way  ;D). Thanks for all replies so far  :)

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