Attached is the output of my power supply with about 1A load (ac coupled). It looks like the frequency is 120hz/60hz to the input ripple looks to be relevant.
This power supply is unstable with no cap out the input (oscillates at about 50khz), but adding at least 20uf output cap seems to stabilize it, just have the ripple attached.
Is this still unstable? I get this type of ripple no matter how much output cap I have above a certain threshold (about 20uF).
This power supply is unstable with no cap out the input (oscillates at about 50khz), but adding at least 20uf output cap seems to stabilize it, just have the ripple attached.
Is this still unstable? I get this type of ripple no matter how much output cap I have above a certain threshold (about 20uF).
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It's not unstable; as you noted yourself, it has some residual mains breakthrough.
Based on the pulse shape it looks like it comes from the narrow, high current cap charging pulses. These occur when the secondary voltage in its cycle climbs above the cap voltage and the diodes start to conduct to top up the caps.
These current pulses can be quite high, especially if you use a lot of capacitance after the rectifier. Even a fractional ohm resistance in the ground or return line can cause some mV across them. A simple test could be to remove all load from the psu, that should also make those pulses disappear or greatly diminish.
Then again, you may be just having your scope probe ground lead at a bad point - try to measure directly at the supply output or directly across the capacitor.
jan
Based on the pulse shape it looks like it comes from the narrow, high current cap charging pulses. These occur when the secondary voltage in its cycle climbs above the cap voltage and the diodes start to conduct to top up the caps.
These current pulses can be quite high, especially if you use a lot of capacitance after the rectifier. Even a fractional ohm resistance in the ground or return line can cause some mV across them. A simple test could be to remove all load from the psu, that should also make those pulses disappear or greatly diminish.
Then again, you may be just having your scope probe ground lead at a bad point - try to measure directly at the supply output or directly across the capacitor.
jan
Just adding to what Jan mentioned... try connecting your scope probe tip and its ground lead together. You should get a noise free trace of course. Now keeping them shorted connect the two to the same ground point you used before. If the trace now shows a problem, then that suggests a grounding issue via the scope.
Maybe the ripple minima at the regulator input are simply getting too close to the reg's dropout region. Did you try adding capacitance before the regulator? Can we see the input voltage waveform?
Also, are your scope traces always that thick? You might still be seeing some HF oscillation. But it's difficult to tell if it's "real" or not.
Maybe you should also zoom in on the diode turn-off time, while probing across the bridge's input.
Also, are your scope traces always that thick? You might still be seeing some HF oscillation. But it's difficult to tell if it's "real" or not.
Maybe you should also zoom in on the diode turn-off time, while probing across the bridge's input.
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Yes there are no pulses with no load, and increase in amplitude with load.
Is there anyway to diminish or eliminate these charge pulses?
You simply have not provided enough information. How much capacitance is between the rectifier and the regulator? How much load current? Ripple voltage max and min values, before regulator? Schematic?
As said before, these come from charging current through the wiring.
Did you try to change the scope ground lead connection? You may be picking it up that way, fasely thinking it is on the supply.
Connect the scope directly across the last supply cap: tip to the cap voltage, gnd lead directly at the cap neg terminal. Close as possible. Don't include any nominal ground wiring in your measurement.
If it is clean, good job.
If not, let us know.
jan
The charging pulses charge the cap and then return through the ground wiring. That's what we're seeing here.
jan
If it was the case, their frequency would be 120Hz, not 60.
The fact they are symetrical gives another clue: it cannot originate from one diode, because the waveform would be asymetrical.
This leaves one possibility: induction from the AC input.
Of course it isn't one diode! One up, one down, per chance?
Do you realise that this is going on in ALL power supplies, and that it is a matter of where you chose your grounding, also for measurement, that determines whether you 'see' it or not?
jan
Do you realize that there is only one single loop in the supply circuit where this type of signal can be observed?
I let you guess which one
Hmm, ok forget the regulated power supply for a second, I am getting these waveforms at the output of the rectifier diodes with just loading the output rectifiers and caps alone.
I'm starting to think perhaps the diodes I am using are the suspect?
I have 15000ufx8 at the output of the rectifier diodes and I am using 4 of these Digi-Key Part Search for full wave rectification.
With just adding a load at the rectifier output to draw about 400mA I am getting the waveforms pictured at the OP.
So lets forget stability issues or whatever. Diode, cap, or layout issue?
I'm starting to think perhaps the diodes I am using are the suspect?
I have 15000ufx8 at the output of the rectifier diodes and I am using 4 of these Digi-Key Part Search for full wave rectification.
With just adding a load at the rectifier output to draw about 400mA I am getting the waveforms pictured at the OP.
So lets forget stability issues or whatever. Diode, cap, or layout issue?
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