strange ripple noise in DC power supplies ?

[skibum]

Hello,

I have 2 independent DC power supplies sharing the same ground.

I am using a full wave rectifier using center tapped transformers for both power supplies.

Like this:

File:Fullwave.rectifier.en.svg - Wikipedia, the free encyclopedia

One of the power supplies outputs 1 amp, the other about 20mA.

Whenever they are both running at the same time I get a strange 100Hz ripple noise on the ground. It is about negative 40mV. I have attached a picture of what it looks like.

It only happens when they are both running at the same time. If they run independently this noise is not present.

There is some type of interaction going on that I cannot figure out.

Any ideas?

[AndrewT]

Independent.
Does that mean two separate mains transformers?

[jan.didden]

Originally Posted by skibum

Hello,

I have 2 independent DC power supplies sharing the same ground.

I am using a full wave rectifier using center tapped transformers for both power supplies.

Like this:

File:Fullwave.rectifier.en.svg - Wikipedia, the free encyclopedia

One of the power supplies outputs 1 amp, the other about 20mA.

Whenever they are both running at the same time I get a strange 100Hz ripple noise on the ground. It is about negative 40mV. I have attached a picture of what it looks like.

It only happens when they are both running at the same time. If they run independently this noise is not present.

There is some type of interaction going on that I cannot figure out.

Any ideas?

This looks like the charge pulses for the reservoir capacitors causing a voltage across the ground wire.
Depends on the length and size of the ground connetion, and depending where you measure.
What is the level when you run either only the 1A or the 20mA separately?

jan didden

[Pafi]

I get a strange 100Hz ripple noise on the ground.

Measured relative to what? Between ground and ground? If you think it through, you may find the answer.

[Elvee]

Originally Posted by Pafi

Measured relative to what? Between ground and ground? If you think it through, you may find the answer.

My point exactly.
The reference ground first needs to be defined, and the rest follows.

A drawing of the actual wiring/physical configuration would be helpful, and would probably tell the whole story

[skibum]

Originally Posted by AndrewT

Independent.
Does that mean two separate mains transformers?

Yes, I have two separate mains transformers.

[skibum]

Originally Posted by janneman

This looks like the charge pulses for the reservoir capacitors causing a voltage across the ground wire.
Depends on the length and size of the ground connetion, and depending where you measure.
What is the level when you run either only the 1A or the 20mA separately?

jan didden

Hi Jan,

I just checked it again. I was mistaken. When I run the 1 amp seperately I am seeing this signal on the ground. What I mean by that is that I have my scope ground hooked up to my output ground and the scope probe on the center tap of the output side of the transformer (which is ground in my set up). It is the 1 amp PS causing the issue. Very weird......

[skibum]

Originally Posted by janneman

This looks like the charge pulses for the reservoir capacitors causing a voltage across the ground wire.
Depends on the length and size of the ground connetion, and depending where you measure.
What is the level when you run either only the 1A or the 20mA separately?

jan didden

Yes, I connected a wire from the center tap transformer input to the output ground and I still see the signal! But if I check ground at different places around the circuit the pulses are less or even non existent.

I do not understand "charge pulses for the resovoir caps causing a voltage across the ground wire". I do have 30,000 uF for the 1 amp circuit and 15,000 uF for the 20 mA circuit. Could the large amount of capacitance be causing the issue?

Sounds like that may be my problem. I do NOT have a ground plane for this -

?

[DF96]

You are seeing charging pulses in a 'ground'. True ground only exists at one and only one point in a circuit. All other points, even when connected to ground with a nice thick conductor, are not ground unless no currents flow along the conductor. You decide which point you call ground.

The bigger the caps, the worse will be the charging pulses so the worse will be the voltage drop along a 'ground' conductor. Treating the transformer secondary centre-tap as ground is a common mistake.

[AndrewT]

The charging circuit, from transformer through the rectifier to the smoothing caps, must not share any common wire or wiring with the load circuit that receives the smoothed DC supply.
These two separate circuits must be grounded at ONE POINT.