Noise on the power supply line - Electronic Circuits Projects Diagrams Free

arrow · 17th March 2008, 10:27 AM

Hi

I have two circtuis on a common ground. The voltage on each circuit is regulated by its own V regulator (LP2951): One at 3.0V the other at 5.0V.
The 3.0V supplies a SD card, the 5.0V supplies a set of sensors.
What I notice is that every time the SD card is written to, there is a voltage (up or down) spike on the 5.0V supply!
I am using a 68uF cap on each of the V regulators.

Can someone please help me in eliminating/ reducing the V spikes on the sensor 5.0V regulated supply?

Thank you in advance.
a.

Pommie · 17th March 2008, 11:08 AM

Have you got 0.1uF capacitors in parallel with the 68uF ones?

Mike.
Edit corrected as per Nigel's post.

arrow · 17th March 2008, 11:16 AM

HI Mike

Thank you for your suggestion.

I have just added a 0.22uF cap in series with the sensor 68uF cap, and I still get the spikes on the sensor power line.
I have a 0.22uF cap. close to the SD chip.

(I have never understood why one would need a small capacitor in series with a larger one. Does it matter what kind of capacitor it is? - I think I am using a tantalum for the 0.22uF)

Thank you
a.

Pommie · 17th March 2008, 11:31 AM

Have a read of this page.

Mike.

**Nigel Goodwin** · 17th March 2008, 11:34 AM

Quote:

Originally Posted by Pommie

Have you got 0.1uF capacitors in series with the 68uF ones?

Surely you mean in parallel?.

Pommie · 17th March 2008, 12:04 PM

Quote:

Originally Posted by Nigel Goodwin

Surely you mean in parallel?.

I do indeed.

Mike.

Boncuk · 17th March 2008, 12:21 PM

Hi arrow,

try this:

The 47n capacitors already take care of reduced spikes from mains via the transformer. Additionally connect a small inductance (47 - 100µH) + a 100n capacitor close to the power supply pin of the card reader/writer. Doing the same with the connected sensors spikes and resulting problems should be minimized.

Finally use separate GND traces from the power supply to the sensors and to the card reader/writer.

arrow · 18th March 2008, 08:27 AM

Hi

Thank you all for replying. I tried most of the above, and it did not seem to work. I ended up putting in an RC low pass filter with R=1ohm and C=0.022uF, and that worked pretty well.

All the best
a.

Hero999 · 19th March 2008, 07:55 PM

What current are you drawing, are you sure you can stand the voltage drop across the resistor?

You could have also tried an inductor or just a simple ferrite bead.

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17th March 2008, 10:27 AM	(permalink)
arrow	Noise on the power supply line Hi I have two circtuis on a common ground. The voltage on each circuit is regulated by its own V regulator (LP2951): One at 3.0V the other at 5.0V. The 3.0V supplies a SD card, the 5.0V supplies a set of sensors. What I notice is that every time the SD card is written to, there is a voltage (up or down) spike on the 5.0V supply! I am using a 68uF cap on each of the V regulators. Can someone please help me in eliminating/ reducing the V spikes on the sensor 5.0V regulated supply? Thank you in advance. a.

17th March 2008, 11:08 AM	(permalink)
Pommie	Have you got 0.1uF capacitors in parallel with the 68uF ones? Mike. Edit corrected as per Nigel's post. Last edited by Pommie; 17th March 2008 at 12:05 PM.

17th March 2008, 11:16 AM	(permalink)
arrow	HI Mike Thank you for your suggestion. I have just added a 0.22uF cap in series with the sensor 68uF cap, and I still get the spikes on the sensor power line. I have a 0.22uF cap. close to the SD chip. (I have never understood why one would need a small capacitor in series with a larger one. Does it matter what kind of capacitor it is? - I think I am using a tantalum for the 0.22uF) Thank you a.

17th March 2008, 11:31 AM	(permalink)
Pommie	Have a read of this page. Mike.

17th March 2008, 12:21 PM	(permalink)
Boncuk	Hi arrow, try this: The 47n capacitors already take care of reduced spikes from mains via the transformer. Additionally connect a small inductance (47 - 100µH) + a 100n capacitor close to the power supply pin of the card reader/writer. Doing the same with the connected sensors spikes and resulting problems should be minimized. Finally use separate GND traces from the power supply to the sensors and to the card reader/writer. Last edited by Boncuk; 8th July 2008 at 12:49 AM.

18th March 2008, 08:27 AM	(permalink)
arrow	Hi Thank you all for replying. I tried most of the above, and it did not seem to work. I ended up putting in an RC low pass filter with R=1ohm and C=0.022uF, and that worked pretty well. All the best a.

19th March 2008, 07:55 PM	(permalink)
Hero999	What current are you drawing, are you sure you can stand the voltage drop across the resistor? You could have also tried an inductor or just a simple ferrite bead. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez