CMOS output protection

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msboude
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CMOS output protection

Post by msboude » Tue Feb 04, 2020 1:09 pm

Circuit is being driven by a CDd4017BE. Am I going to the extreme using these diodes for reverse voltage protection? I think the transistors should be protection enough?
cmos-output.PNG
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msboude
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Re: CMOS output protection

Post by msboude » Tue Feb 04, 2020 1:13 pm

Also, should I throw a resistor between the output of the 4017 and the transistor base?

jorg
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Re: CMOS output protection

Post by jorg » Tue Feb 04, 2020 4:40 pm

R7 is redundant.
If the CD4017 pin goes low, your output is floating. Is that the desired result? In most synths, it would not be. You might want a pair of diodes at the transistor's emitter, both "pointing up" - one will prevent the emitter from going much below ground; one to prevent it from going much above V+. Keep the series resistor in place between the emitter and the tip contact of the jack.
Also, the tip contact on your jack is labeled "IN." It's meant to be an output, correct?
LogicOut2.PNG
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msboude
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Re: CMOS output protection

Post by msboude » Tue Feb 04, 2020 6:00 pm

Thanks for your reply, Jorg!

Yes, the "in" on the jack is actually an out, it's labeled as "in" on my thonkjack library part.

I see what you are saying about R7 being redundant. Thank you!

I'm hoping I can do without the diodes.... ?

I take it this fixes the floating output?

Thank you, I really appreciate the advice.
cmos-output.PNG
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Re: CMOS output protection

Post by guest » Tue Feb 04, 2020 11:25 pm

out of curiosity, what is the failure mode that you are concerned about here?
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Duga-3
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Re: CMOS output protection

Post by Duga-3 » Wed Feb 05, 2020 5:00 am

msboude wrote:
Tue Feb 04, 2020 6:00 pm

I see what you are saying about R7 being redundant. Thank you!

I'm hoping I can do without the diodes.... ?

I take it this fixes the floating output?
Without R7 the output will still float below about +2V (depending on the LED color). I think, the design with R7 included is much cleaner.

msboude
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Re: CMOS output protection

Post by msboude » Wed Feb 05, 2020 8:40 am

Okay, I think I have this sorted..

I heard that you want to protect CMOS chips from the outside world, so I am going to use this on each CMOS output. I am also trying to avoid a floating output and any issues when another output gets plugged into this output. I'm designing the output for a flip-flop module. This is for eurorack.

Don't mind my MS Paint picture, I'm at work and don't have access to Eagle.

Let me know if you see any issues that may arise. Thanks everyone, I appreciate you help!
CMOS-OUTPUT2.png
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jorg
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Re: CMOS output protection

Post by jorg » Wed Feb 05, 2020 9:10 am

Duga-3 wrote:
Wed Feb 05, 2020 5:00 am
Without R7 the output will still float below about +2V (depending on the LED color). I think, the design with R7 included is much cleaner.
Yes, you're absolutely correct. I woke with a start last night when I realized I'd given bad advice.

Duga-3
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Re: CMOS output protection

Post by Duga-3 » Thu Feb 06, 2020 8:11 am

msboude wrote:
Wed Feb 05, 2020 8:40 am

Let me know if you see any issues that may arise. Thanks everyone, I appreciate you help!

CMOS-OUTPUT2.png
Now floating anymore - but in this design, the output's "high" level is at only +9.5V (assuming the transistor gets +12V at its base). Putting the 10k resistor between emitter and ground would help. Outputting a high should then give about +11.4V at the output.

BTW: how can I quote with graphics included? I always only get the file name of the pic :-(

jorg
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Re: CMOS output protection

Post by jorg » Thu Feb 06, 2020 9:46 am

Duga-3 wrote:
Thu Feb 06, 2020 8:11 am
Now floating anymore - but in this design, the output's "high" level is at only +9.5V (assuming the transistor gets +12V at its base). Putting the 10k resistor between emitter and ground would help. Outputting a high should then give about +11.4V at the output.
True.
Also, omitting the diodes will allow an errant input (or static discharge) to drive the emitter beyond the rails.

msboude
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Re: CMOS output protection

Post by msboude » Thu Feb 06, 2020 2:16 pm

Alright! I have updated this!

Thanks for you help, guys!

I'm looking at the doepfer version of a similar output, looks like they use the transistor outs, but no diodes are there.. I think I should be okay?
CMOS-OUTPUT2.png
doepfer_a161.jpg
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msboude
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Re: CMOS output protection

Post by msboude » Thu Feb 06, 2020 2:20 pm

And I understand the use of the diodes, I just think it's going to take up a bit too much room on the board.. but, maybe I can gravestone them.

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the bad producer
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Re: CMOS output protection

Post by the bad producer » Thu Feb 06, 2020 4:18 pm

I've always used Ken Stone's outputs:


schem_cgs36v14_pulse_divider.gif
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jorg
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Re: CMOS output protection

Post by jorg » Thu Feb 06, 2020 4:21 pm

Looks like Ken Stone and Dieter Doepfer both do about the same thing. I haven't built anything for the mass market so I will definitely defer to their more experienced opinions.

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Re: CMOS output protection

Post by guest » Thu Feb 06, 2020 4:50 pm

i was curious about the failure modes, so i got some 4000 series logic, put a 1k resistor at its output, and hit it with +/-12V. it survived just fine regardless of intended output state. where it didnt do so well, was when the power was disconnected from the chip, and the output was driven. it still worked but the body diode on the N channel output fried. what it really didnt like was getting an input signal with the powersupply disconnected, fried it entirely. the body diodes seem to do a decent job of protecting the chip when its powered, but something else goes when power isnt connected, and i havent figured out exactly what that is.
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Re: CMOS output protection

Post by hox3d » Fri Feb 07, 2020 3:58 am

Hm, weird: in the Music Thing Modular Turing Machine expanders (for instance Pulses), the 4081 output basically is a 1k resistor in serie. That's all.

Does it make it the module more prone to failures?
If I understand well, the problem could come from a hanging cable while the synth is unpowered?
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msboude
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Re: CMOS output protection

Post by msboude » Fri Feb 07, 2020 9:48 am

Awesome, some great information in this thread. I really appreciate all the help!

Thanks to guest for doing some further testing, that was a great experiment. Funny, I fried a few chips when I first started by not having power connected to them, now I double-check that power is connected every time. I also turn off power before making changes...

Thanks for the schematic, bad producer.. That helps confirm some things as well.

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Re: CMOS output protection

Post by guest » Fri Feb 07, 2020 11:39 am

ive found that the 4000 series CMOS is pretty sensitive, in particular when the power is off. the lower voltage HC logic is more robust.
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