Power supply peculiarity

[ashes1312]

Hi guys,

Go easy on me,newbie here.

I designed a device which has to run on main power and switch to battery when there is a power failure.


This works fine but I get a noise of around 19V on my 5V DC line when switching on the device.



This maybe causing my IC's to malfunction. Tried adding a 5.1 v Zener and a cap after the regulator to no avail.

What is causing this and how can I fix it?

Cheers!

 

[Pjdd]

Looks like a damped oscillation. Did you place C9 and C10 as close as possible to the regulator pins?

 

[ashes1312]

Looks like a damped oscillation. Did you place C9 and C10 as close as possible to the regulator pins?

Yep. They are physically right next to the regulator!

 

[FlapJack]

I can think of three things.

Relay coil oscillation.
Relay contact bounce.
Relay contact bounce confusing the Max regulator chip.

Use both channels of your scope, CH one on the 5 vdc line and move the other channel to the relay coil, contacts and output of Max chip. See what point correlates directly with channel one noise on the 5 vdc line.

 

[KlausST]

Hi,

With the capacitors 200nF and 1000uF at the 5V line it is almost impossible that those voltage spikes really exist.
Therefore i'd say most if it is an erroneous measurement.

You need a solid GND plane or at least a solid star point with short and wide tracks to the devices.

And you need to setup your scope GND connections to this point.

And after that do a test:
* connect scope GND to circuit GND star point
* connect scope signal to the star point also
* set scope for single shot triggering
* disconnect/reconnect mains
* most probably the scope triggers and shows some signal
This signal is the induced signal into the scope lines. Obviously this is not a true signal.
It may have different sources.(capacitive or inductive influence, but also EARTH loop, maybe scope included)

***
19V on a 5V line may immediately kill some 5V devices.
Because you say the devices fail...there will be problems.
I expect wiring problems (ground loops, long wires, no star conndction.) from your supply to your application.
Also the application PCB may cause the failure.

To better assist you, I'd like to see a photo of the complete circuit (power supply and application and wiring)
Also a photo of the PCB layout can help to find the problem.

Klaus

 

[sathiyanarayananL]

hi

can you just take a snap of your assembled circuit and post it here to see the placement of the power components

 

[c_mitra]

It is a 10MHz damped oscillation. Unlikely to be caused by relay bounce. There is a pole nearby. Is IC2A configured correctly?

 

[Pjdd]

Maybe it's the switched-mode power supply. Are you using a low-ESR type for C8? You could try paralleling it with a ceramic or plastic capacitor. By the way, by 50mH I assume you mean microhenry, not millihenry.

 

[udhay_cit]

I think you can replace the diode D10 with a schottky diode & check it again.

Udhay