PC pwr supply noise

[Mosaic]

Hi:
I use a pc PSU for my hobby work and it produces some 50 and 100 nS spikes enough to cause probs on my logic analyzer, when debugging a circuit supplied by the psu.

A battery supply is clean.

Since i usually draw an amp or less for PCB circuits I am looking for advice on making a suitable filter to handle these short spikes.

I guess the very short 50 to 100nS transients are getting past the standard 1 uf & .1uf Decoupling I have on the pcb.

Perhaps an multistage LC may be in order?

 

[colin55]

I always use a battery.

You can eve use an old car battery if you want more than 1 amp. I have a 3amp-hr 12v battery for some testing.
In fact I always use weak cells so that I know the circuit will work, even when the battery gets weak.

 

[Mikebits]

Perhaps an LC Pi network would help. Something like in the link below.

https://www.radio-electronics.com/i...f-filters/simple-lc-lowpass-filter-design.php

 

[Mosaic]

I always use a battery.

You can eve use an old car battery if you want more than 1 amp. I have a 3amp-hr 12v battery for some testing.
In fact I always use weak cells so that I know the circuit will work, even when the battery gets weak.

LOL, I did just that....but I still want to learn to handle such quick transients.

 

[ronv]

Pretty high frequency. How about just a ferrite bead followed by say a 4.7 ufd ceramic cap.

Very short leads on the cap.

 

[dr pepper]

Your PC supply doesnt sound very well, are you using a load resistor to maintain a minimum load?
Does the noise come from the supply, or is the circuit causing the spikes, if your switching an inductive load that could be the cause.
A 100n silver mica cap accross the power supply terminals and as short as possible power supply leads might help.

 

[Mosaic]

No load resistor. No inductive switching.

 

[ChrisP58]

What is the magnitude of the spikes? I would expect that they would have to be a significant percentage (>10%?) of the DC voltage before they would show up on your logic analyzer. Also, what is the period?

Be sure to look at the impedance vs frequency curve of the decoupling caps. They may not be very effective at the problem frequency. Particularly when you consider that a transient pulse will have energy at a frequency much higher than the actual power supply switching frequency.

Check the board layout to make sure you the decoupling caps are well distributed, starting with some good bulk capacitance right next to where the power wires land on the board.

How much of the available current of the PC supply are you actually using? Some supplies are characterized assuming that a fair percentage of their power will be used. PC supplies are expecting to see a good deal of capacitance on the motherboard, where most of their power will be used by on-board regulators to drop their voltage to the 1 volt (sometimes less) that the CPU actully runs on. PC supplies are fairly cheep and good at their intended purpose, but can be pretty lousy when used elsewhere.

 

[Lund_Pa]

Is the power supply ground connected to digital ground? If so you need to break that connection and have an analog and a digital ground.

 

[Mosaic]

Well those are all useful measures but the circuit is intended to be batt. operated. I need a solution for the pwr supply generally. The transients occur around 300uS to 500 uS in period.

 

[ronv]

Filter

You didn't state the amplitude, but the ferrite will reduce a 1 volt spike to < 100 mv.

 

[Sceadwian]

Try loading the 5volt line on the power supply with a decent resistor, it may reduce the transients.