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Ground Bounce/Transients and my digital circuit, help!

djdeposit

TRIBE Member
Hey this one's for the real geeks,

I have a circuit I'm working on that has a 5v power supply (which powers digital IC's such as gates etc.). Also, I have a 24v power supply which supplies power to solinoids (Such as relays and motors). The two sepertate supplies share the same Ground (RTN).

I have been noticing that the digital circuitry is susceptible to line transients, such as unpugging loads on the same AC supply line (the supply line before rectification).

I have thought about using a 5V Zener Diode, but am not able to locate one that can supply enough current to my digital Circuitry. I need one that can regulate @ 1 Amp (output current, not breakdown current).

I have also considered geting a Transient supressor on the AC side to avoid my digital circuitry from "Changing State" when transients are present.

What do you think is a good solution for this? Do you think bypass capacitors across the IC's Will prevent this nonsense from occuring?

I think it might be Ground Bouncing because it occures when I switch off the 24v DC supply

Ground Bounce: defines a condition when a device's output {really a number of outputs} switches from High to Low and causes a voltage change on other pins. The problem is cause by the large current flow through the ground pin which develops a voltage drop over the lead inductance. This voltage drop on the ground line creates two main problems; first it rises the chip off ground [0 volts] potential which increases the devices input threshold level, and increases the voltage level on an output pin which is not switching. Because a quiet output is effected by the other switching outputs, this is also called Simultaneous Switching Noise. It's really a question of loss of noise margin which is listed below. The faster the slew rate of the logic family, the worse the problem becomes. With glue Logic, the ground pins may have been moved around to reduce the inductance. Using a surface mount device instead of a Through Hole will reduce the lead inductance. For FPGA's with hundreds of possible output pins the situation may change, and it's more up to the designer. Start a noise budget to determine if the ground bounce [rise in ground potential] effects the design. The voltage developed over ground lead is proportional to the rate of change in current, so the faster the logic family the worse the problem becomes: V = L * [di/dt]. The more outputs switch at the same time, the larger the current value, and the greater voltage bounce. Also occur when the outputs switch from a 0 to a 1 but to a much smaller degree. Series termination of the line is one method of reducing ground bounce. Series termination resistors slow the rate of change of the output, and so reduce the instantaneous current on the ground line. Also Resistor Pull-Ups on the line cause the ground bounce voltage to increase. The pull-up resistor allows the load capacitor to charge to it's flow value, so as the line switches maximum current is delivered back to the driver. Eliminate pull-up resistors on devices with an issue, use pull-down resistors or series resistors if possible. Reducing the loading on the driver also reduces ground bounce.
Thanks in advance!
 

zoo

TRIBE Member
this is a problem that you would only be able to offer a solution on if you're working with digital circuits on a daily basis :)

i know what you're talking about, but it's hard to say if a bypass cap will help; that assumes that the problem is high freq transients which CAN be effectively removed without altering the time constants of the system ..

do you plan on using that diode to eliminate ground noise? what if the noise is coming from the supply?

i dunno, i'm in 4th year elec eng @ mac and last term i did courses on both power electronics AND digital circuits / ICs / fgpas, so i feel a bit dumb that i can't offer more :)
 

djdeposit

TRIBE Member
any advice is appriciated. I know the noise isn't coming from the supply because I notice that the state changes take place when i remove loads from the line supplying the AC power.

For example. If I unplug my coffe machine from the same outlet supplying ac to my Digital circuit's 5 V powersupply, I notice changes in output state that shouldn't happen.
 

Shug

TRIBE Member
We're talking years since I was in 2nd Year Elec. Eng. before quitting and getting into computers....

... but I distinctly remember making a small circuit to prevent bounce noise... it was a switch of some sort...

Sorry, I'm no help... but if it's something elementary enough to teach in the first few sememsters of Elec., there's gotta be a standard solution to it.
 

zoo

TRIBE Member
that's kind of messed up ..

it does 'sound' like ground bounce as described in that passage, perhaps you need to indeed put a diode before ground

have you looked on digikey for one? i searched a bit and there are lots of results for 1A ..
 
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djdeposit

TRIBE Member
Originally posted by zoo
that's kind of messed up ..

it does 'sound' like ground bounce as described in that passage, perhaps you need to indeed put a diode before ground

have you looked on digikey for one? i searched a bit and there are lots of results for 1A ..
I'm going to check out Sayal for one.
The advice you game me was very similar to advica a digital designer I work with offerd. I also am going to try adding a large cap across the power supply. It could possibly be voltage sag.
 

djdeposit

TRIBE Member
Originally posted by Shug
We're talking years since I was in 2nd Year Elec. Eng. before quitting and getting into computers....

... but I distinctly remember making a small circuit to prevent bounce noise... it was a switch of some sort...

Sorry, I'm no help... but if it's something elementary enough to teach in the first few sememsters of Elec., there's gotta be a standard solution to it.
You are probably reffeing to a schmitt trigger switch bounce elimination circuit.
 

labRat

TRIBE Member
have you 'scoped out your power supplies and grounds to see if there are transients on those lines? if so, i'd suggest decoupling caps close to the power supply pins o fthe digital ICs ... the larger the better (1 -> 20+uF).
 

Shug

TRIBE Member
Originally posted by djdeposit
You are probably reffeing to a schmitt trigger switch bounce elimination circuit.
Ding ding, yup. That was it. Sorry, not applicable in this case.

Upon further googling "ground bounce" this afternoon, the general consensus was a pass cap... so I'd concur with the direction the thread is going so far.
 

djdeposit

TRIBE Member
Originally posted by labRat
have you 'scoped out your power supplies and grounds to see if there are transients on those lines? if so, i'd suggest decoupling caps close to the power supply pins o fthe digital ICs ... the larger the better (1 -> 20+uF).
I've tried this,
I also tried upping my bus capacitors to over 10,000uf. It didn't work :rolleyes:
I'm going to have to try getting a 1N5339 5.6v zener diode at 5W.
If that doesn't work then it's just a really shitty design and I need to try trouble shooting.. LOL

Thanks for the help dudes.
I'll let you know how this turns out.
 
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