Which diode to prevent "backflow"?

Thread Starter

kgstewar

Joined Apr 5, 2012
152
Hi all,

I have a couple of CD40192 counters that get reset to zero via two separate switches. S1 should only reset 40192A while S2 should reset both 40192A and 40192B. I tried using a 1N4001 diode in the circuit shown below but it got fried and now conducts in both directions. Is there an appropriate diode that would work in this circuit, or should I try a different approach?

Thanks!

Kevin
 

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kubeek

Joined Sep 20, 2005
5,650
If a 1N4001 got fried you have some serious trouble in there, as it can handle 1A without a sweat. Probably the actual circuit is not connected the way you show, my bet is that S1 is a SPDT between ground and +V.
 

Thread Starter

kgstewar

Joined Apr 5, 2012
152
If a 1N4001 got fried you have some serious trouble in there, as it can handle 1A without a sweat. Probably the actual circuit is not connected the way you show, my bet is that S1 is a SPDT between ground and +V.
Perhaps I've done something different than what I've shown, I'll certainly check. It actually worked for awhile so maybe I'll replace the 1n4001 with another and see what happens. Reverse voltage of 12V is not too much for this diode?

Kevin
 

Ron H

Joined Apr 14, 2005
7,014
Perhaps I've done something different than what I've shown, I'll certainly check. It actually worked for awhile so maybe I'll replace the 1n4001 with another and see what happens. Reverse voltage of 12V is not too much for this diode?

Kevin
The diode will not be hurt by 12V.
I would use 1N4148, but 1N4001 will be OK .
You need to add a 10kΩ pulldown resistor from each reset input to ground.
 

kubeek

Joined Sep 20, 2005
5,650
Perhaps I've done something different than what I've shown, I'll certainly check. It actually worked for awhile so maybe I'll replace the 1n4001 with another and see what happens. Reverse voltage of 12V is not too much for this diode?

Kevin
1N4001 is good for 50V peak, 1N4007 is for up to 1000V peak. These diodes don´t simply die unless severly abused. Don´t put another one in until you find the problem, you may damage other components in the process.
 

Thread Starter

kgstewar

Joined Apr 5, 2012
152
You all convinced me that the 1n4001 should be bomb-proof, more or less, so I pulled the diode and checked it out of the circuit. It's totally fine, so the problem lies elsewhere. Unfortunately, I have discovered that the problem is more mysterious.

The schematic I showed is a small, small part of a much more complicated scoreboard circuit. The reset features work fine when the circuit is first powered up and continues to work fine for about 15 minutes. After that, S1 resets both counters, instead of just 40192A. If I switch the circuit off and then back on, the problem remains. If I power it off for about 15 minutes or so, then power it back on, it again works well for about 15 minutes.

I checked all the ICs and transistors to see if anything is getting hot, but all seems cool, although it seems like something must be warming up to cause the problem to arise after 15 minutes....

Anyway, I realize this is hard to diagnose without a schematic, so perhaps this is my motivation to put together a complete schematic now.

Kevin
 
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Ron H

Joined Apr 14, 2005
7,014
The zero bias capacitance of 1N4001 is about 30pF. See attachment from Diodes Inc. datasheet. Depending on the stray capacitance at the S2 reset, S1 switching could cause a transient at S2 which is large enough to initiate a reset.
Try adding a 1nF cap from the S2 reset to ground.
My personal choice for signal diodes is 1N4148. 1N4001 is a rectifier. Sure, it's also a diode.
 

Thread Starter

kgstewar

Joined Apr 5, 2012
152
Thanks Ron, I'll try adding that cap.

I can pick up some 1n4148s at my local radio shack. Do these have lower capacitance than the 4001? Is that their advantage?

Many thanks!

Kevin
 

Ron H

Joined Apr 14, 2005
7,014
Thanks Ron, I'll try adding that cap.

I can pick up some 1n4148s at my local radio shack. Do these have lower capacitance than the 4001? Is that their advantage?

Many thanks!

Kevin
1N4148 is 4pF max @0v. Typical is 1pF or less.

EDIT: What is the value of your pulldown resistors?
 
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Thread Starter

kgstewar

Joined Apr 5, 2012
152
Thanks Ron, and forgive the basic question, but which side of S2 does the 1nF cap go on? The side closest to the IC?

Kevin
 

Ron H

Joined Apr 14, 2005
7,014
The cap goes from the reset pin to ground.
You may have addressed this already, but do you have power supply decoupling caps (0.1uF) from each IC's power supply pin to ground? You can get erratic performance if you don't.
100k pulldowns should be OK.
 

Thread Starter

kgstewar

Joined Apr 5, 2012
152
The cap goes from the reset pin to ground.
You may have addressed this already, but do you have power supply decoupling caps (0.1uF) from each IC's power supply pin to ground? You can get erratic performance if you don't.
100k pulldowns should be OK.
Yes, decoupling caps on all IC power pins (10uF and 0.1uF to ground)
Thanks!

Oh, and I just realized I have a 1 uF cap from the reset pin on 40192A to ground already so that it goes to zero at power on. Perhaps this is the problem?

I realize that I am making this difficult by not providing an accurate schematic from the get go. So sorry!

Kevin
 

Thread Starter

kgstewar

Joined Apr 5, 2012
152
Actually, let me put together a proper schematic. I hate that I'm taking your time without having given you a complete picture!

Many thanks, as always.

Kevin
 

Ron H

Joined Apr 14, 2005
7,014
Yes, decoupling caps on all IC power pins (10uF and 0.1uF to ground)
Thanks!

Oh, and I just realized I have a 1 uF cap from the reset pin on 40192A to ground already so that it goes to zero at power on. Perhaps this is the problem?

I realize that I am making this difficult by not providing an accurate schematic from the get go. So sorry!

Kevin
The 1uF caps should prevent crosstalk across the diode, but they might be hard on your switches. The current spike can be on the order of amps for a very short time.
I would think you would want to reset the ICs on power up. To do that, the caps need to connect to vcc, not ground.
 

Thread Starter

kgstewar

Joined Apr 5, 2012
152
The 1uF caps should prevent crosstalk across the diode, but they might be hard on your switches. The current spike can be on the order of amps for a very short time.
I would think you would want to reset the ICs on power up. To do that, the caps need to connect to vcc, not ground.
Of course you are right, the 1uF caps I have now go to Vcc, not ground. I'll add the .001uF cap as you suggested and report back.

Thanks!

Kevin
 

Ron H

Joined Apr 14, 2005
7,014
Of course you are right, the 1uF caps I have now go to Vcc, not ground. I'll add the .001uF cap as you suggested and report back.

Thanks!

Kevin
With the 1uF caps in place, the 1nF cap shouldn't do anything. I doubt that the problem is cap coupling across the diode.
 

timescope

Joined Dec 14, 2011
298
You can also make sure that all unused inputs of the 40192 are either connected to ground or the supply, Vdd. Floating inputs can cause weird problems.

Timescope
 

Thread Starter

kgstewar

Joined Apr 5, 2012
152
I swapped out the 1n4001 for a 1n4148 and the problem seems to have gone away! I did not add the 1nF cap as suggested but if the problem reappears I will give that a try as well. Many thanks to all and especially Ron.

Kevin
 
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