Problem with dual precision Monostable Multivibrator  CD4538BCN - Page 1
Forum section: electronics / projects

author: Longtimetech
author_rank: Newbie
timestamp: February 09, 2013, 04:54:03 pm
content: I have a general question on the timing trigger Rx/Cx circuit. I understand how the cap charges to the rail, but do not understand what discharges it for the next pulse. Is there something in the chip that discharges the timing cap and if so what is it called. I have a circuit where the Q and Not Q outputs would not toggle. I changed out the entire chip with another exact model--no change. The second side of the chip works perfectly! But I cannot get the first side to function at all. I changed both .1 Uf caps out and this side still won't function. I have verified all connections and resistive timing components and cannot find a thing wrong. I am really puzzled by this problem. If anyone out there can answer my question on the discharging of the timing caps, that might lead me to what is wrong.I have read over many of the chip datasheets from various manufactures and nowhere in those documents to they say how the trigger timing caps get discharged!!I am clueless on this. This is a timing circuit in my old brown Huntron Tracker 2000. I am stumped.....

author: Alana
author_rank: Frequent Contributor
timestamp: February 10, 2013, 12:21:17 am
content: I think you may be missing connection from T1 to ground in your circut. Check it out, or show us your schematic. Sometimes there may be an issue with bad, leaky cap, especially if your Rx is large.

author: CarlG
author_rank: Regular Contributor
timestamp: February 10, 2013, 08:02:01 am
content: In the logic diagram of the CD4358 datasheet, it's N1 that dicharges the cap. The diagram is missing a dot connecting N1-drain to pin 2(14). Compare with HEF4358 . More detailed (complex) but they've got the dot in placeAlana's guess is a good guess, but if you say you've checked all connections then I don't know...

author: Longtimetech
author_rank: Newbie
timestamp: February 10, 2013, 09:54:47 pm
content: I am attaching the circuit of half of this chip that does not function. I can watch the caps charge at pins 14 & 12 on power up, but only one time and they never seem to get discharged to repeat the cycle. The Q (10) stays at the +5 vdc rail, and the Not Q (9)stays at the - 5 vdc rail. +V= 5 vdc & -V= -5 vdc. The other half of the chip works perfectly. Same supplies. I AM stumped on this one.In this circuit there is really no ground, as you can see. Could the cap actually discharge back through the negative supply?I will look up the HEF4538 datasheet and compare. This circuit is unusual in that one end of each timing cap is connected to the - 5 vdc supply. It is possible that the datasheet is missing that dot as you mentioned. I have more chips and a good digital trainer that I plan on reconstructing the circuit on. I have travelled to my mother in laws (200 miles from home) so I won't get a chance to get back to this till next Saturday or Sunday.Thanks for both of you for looking and your suggestions.

author: Longtimetech
author_rank: Newbie
timestamp: February 10, 2013, 10:12:42 pm
content: Sorry first picture of the circuit was too big. This should be better.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 10, 2013, 10:22:36 pm
content: From what I can tell from a quick look at the schematic that circuit is wired-up as a power-on reset type of circuit. It will generate a pulse at the Q and Q# outputs at the time of power-on (assuming C27 is discharged at power-up.) It will generate this pulse just once during a power cycle and otherwise do nothing else. The length of the pulse is determined by R51 and C28.In other words, the circuit may be working properly if you are expecting some other behavior. C27 will only discharge when the power is removed.EDIT: I just noticed the signal names of POR and POR#. This is standard naming convention for power-on reset signals so that is definitely what this circuit is doing.

author: Longtimetech
author_rank: Newbie
timestamp: February 11, 2013, 12:47:24 am
content: Oh, I get it what you are saying. I was thinking that this would just keep on switching back and forth and I see now that the heck it is doing in the other parts of the circuit. It is resetting a bunch of other stuff to a proper power on sequence. I totally read a lot more into this little circuit than Huntron intended!!You guys were a fantastic help. I changed that chip and timing capacitors for nothing. I guess I am really out of practice. Thanks again for your help. I feel a little silly. I also should have caught the meaning of the POR.