I have a floating 24VDC power supply. However, when I test the voltage from + to ground, I see some strange behavior. The voltage fluctuates from 0 to 24VDC non stop. Any idea what this means? It is like there is a capacitive connection to ground...it fills the cap and opens the circuit over and over...
Oscillating Floating 24VDC
- Thread starter fifty60
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gar
Senior Member
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- Ann Arbor, Michigan
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- EE
181018-0806 EDT
fifty60:
You have presented a question with far too little useful information. Do some thinking, measurement, and analysis, and you can possibly answer your question.
Why do you think it is floating? What was the frequency of oscillation? Is it periodic? What meter was used? What do different impedances across the meter do? What is the waveform? Is anything connected to the power supply? What powers the power supply? And so on.
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fifty60:
You have presented a question with far too little useful information. Do some thinking, measurement, and analysis, and you can possibly answer your question.
Why do you think it is floating? What was the frequency of oscillation? Is it periodic? What meter was used? What do different impedances across the meter do? What is the waveform? Is anything connected to the power supply? What powers the power supply? And so on.
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I am using a handlheld Fluke 75 multimeter. The power supply is a 240W 120VAC to 24VDC power supply. There is not ground wire ran to the - of the 24VDC power supply, therefore it should be a floating supply. When I test between + and -, i get 24VDC. When I go from + to the din rail the power supply is connected to, i get oscillations of OL to 24VDC. The meter will read OL for approximately 1 second, and then it will read 24VDC for approximately 1 second. It continues to do that over and over.
I should not be reading anything consistent to ground on a floating power supply, right? What could be causing the oscillating behavior?
I should not be reading anything consistent to ground on a floating power supply, right? What could be causing the oscillating behavior?
If the supply is isolated from ground you be getting pick up. Dou you have a low impedance meter you could check it with?
gar
Senior Member
- Location
- Ann Arbor, Michigan
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- EE
181018-1045 EDT
fifty60:
Using a square wave generator with lowest frequency of 0.2 Hz, 2.5 seconds on and 2.5 seconds off, and using a Fluke 27 I do not get discrete 0 and X readings per measurement sample. The integrating (averaging time) of the Fluke 27 is too long and not synchronized to the measured signal. Thus, I get varying readings between 0 and X.
With a Simpson 270 taut band meter I get 0 and X minimum and maximum readings at 0.2 Hz, same at 0.5 Hz. But at 1 Hz minimum is 0,2 of X and maximum is 0.8 of X.
I don't know how your meter would respond, but I suspect similar to my 27.
Use a scope and shunt with some different resistance values.
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fifty60:
Using a square wave generator with lowest frequency of 0.2 Hz, 2.5 seconds on and 2.5 seconds off, and using a Fluke 27 I do not get discrete 0 and X readings per measurement sample. The integrating (averaging time) of the Fluke 27 is too long and not synchronized to the measured signal. Thus, I get varying readings between 0 and X.
With a Simpson 270 taut band meter I get 0 and X minimum and maximum readings at 0.2 Hz, same at 0.5 Hz. But at 1 Hz minimum is 0,2 of X and maximum is 0.8 of X.
I don't know how your meter would respond, but I suspect similar to my 27.
Use a scope and shunt with some different resistance values.
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If its floating, why do you care what the voltage to ground is?
It sounds like the 24V + to - is fine.
With a high impedance volt meter, measuring from a floating power supply terminal to ground doesn't really complete any circuit. You are basically just picking up stray capacitance, and any voltage that is coming through that. And what you measure may depend as much on the meters internal circuits as anything.
Its kind of a drawback that digital meters with high input impedance's have. It improves accuracy for most measurements, but they don't always read 0 volts when an old analog meter would have.
It sounds like the 24V + to - is fine.
With a high impedance volt meter, measuring from a floating power supply terminal to ground doesn't really complete any circuit. You are basically just picking up stray capacitance, and any voltage that is coming through that. And what you measure may depend as much on the meters internal circuits as anything.
Its kind of a drawback that digital meters with high input impedance's have. It improves accuracy for most measurements, but they don't always read 0 volts when an old analog meter would have.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
181018-1216 EDT
steve66:
I think the question that fifty60 is asking is --- does this oscillating voltage between the DC supply +24 terminal and the DIN rail imply a power supply problem? Possibly.
We don't know if there is any load or 24 V output wiring connected to the power supply at the time the oscillating 0 to 24 V was observed. If no wiring or load, then the logical assumption is that the power supply is the origin of this variation, and I suggest that should not occur.
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steve66:
I think the question that fifty60 is asking is --- does this oscillating voltage between the DC supply +24 terminal and the DIN rail imply a power supply problem? Possibly.
We don't know if there is any load or 24 V output wiring connected to the power supply at the time the oscillating 0 to 24 V was observed. If no wiring or load, then the logical assumption is that the power supply is the origin of this variation, and I suggest that should not occur.
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rlundsrud
Senior Member
- Location
- chicago, il, USA
If the power supplies turned off, what do you get if you measure resistance between the negative and the ground? If it's not a low impedance measurement, the voltage measurement with respect to the frame ground really isn't relevant.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
181018-2011 EDT
rlundsrud:
I think your suggested resistance measurement is something useful to do, but it alone won't identify why oscillation occurs.
You don't get the specific signal that fifty60 is describing without something generating it. If there is no load on the supply, then it is very strange that this type of signal would be generated by the power supply.
If there is some sort of load on the power supply, then first I would be drawn to the load as the possible source. So I would disconnect any load or wires connected to the power supply from the supply, and see if this oscillation still existed.
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rlundsrud:
I think your suggested resistance measurement is something useful to do, but it alone won't identify why oscillation occurs.
You don't get the specific signal that fifty60 is describing without something generating it. If there is no load on the supply, then it is very strange that this type of signal would be generated by the power supply.
If there is some sort of load on the power supply, then first I would be drawn to the load as the possible source. So I would disconnect any load or wires connected to the power supply from the supply, and see if this oscillation still existed.
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gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
181018-2431 EDT
I took an old power supply I designed about 30 years ago. It uses a standard iron core transformer, no switching circuits, rectifier filter, about 250 W capability, and has an LC Corcom AC filter at the input referenced to the EGC.
Using a 3 conductor cable so chassis and the LC filter connect to the AC source I measured 0.025 V DC from a +8 V output to the power source EGC No oscillation. Very low DC current flows, as expected.
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I took an old power supply I designed about 30 years ago. It uses a standard iron core transformer, no switching circuits, rectifier filter, about 250 W capability, and has an LC Corcom AC filter at the input referenced to the EGC.
Using a 3 conductor cable so chassis and the LC filter connect to the AC source I measured 0.025 V DC from a +8 V output to the power source EGC No oscillation. Very low DC current flows, as expected.
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winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
As others have described, the high impedance measurement of the voltage between output and ground is subject to tremendous artifacts.
There is no such thing as a truly isolated circuit. You always have parasitic coupling, capacitance, inductance, insulation leakage, etc. These parasitics are very likely immaterial to the actual function of the device, though if you are actually having problems with function they might be key to the cause of the problem.
When you use a high impedance meter to measure the voltage between the so called 'isolated' circuit and ground, you are measuring these unintentional connections.
The changing resistance measurement suggests a capacitor charging up in the leakage path. As the capacitor charges the current flow decreases and the apparent resistance goes up.
The slow oscillation you are measuring might be an artifact caused by pwm in the supply, or it might be a slow oscillation in the supply caused by having no load. In any case something is coupling to your meter and you are curious what it is.
The best we can do is suggest various tests. Below are my random thoughts:
Make the same measurements with a well defined load on the power supply, such as a well isolated resistor.
Make the same measurements with the meter shunted with a resistor.
Make the same measurements with an oscilloscope rather than a meter, again with different shunt resistances.
I think you will find that when you make the measurement with a moderate shunt impedance (say 100K) that the voltage will go away.
-Jon
There is no such thing as a truly isolated circuit. You always have parasitic coupling, capacitance, inductance, insulation leakage, etc. These parasitics are very likely immaterial to the actual function of the device, though if you are actually having problems with function they might be key to the cause of the problem.
When you use a high impedance meter to measure the voltage between the so called 'isolated' circuit and ground, you are measuring these unintentional connections.
The changing resistance measurement suggests a capacitor charging up in the leakage path. As the capacitor charges the current flow decreases and the apparent resistance goes up.
The slow oscillation you are measuring might be an artifact caused by pwm in the supply, or it might be a slow oscillation in the supply caused by having no load. In any case something is coupling to your meter and you are curious what it is.
The best we can do is suggest various tests. Below are my random thoughts:
Make the same measurements with a well defined load on the power supply, such as a well isolated resistor.
Make the same measurements with the meter shunted with a resistor.
Make the same measurements with an oscilloscope rather than a meter, again with different shunt resistances.
I think you will find that when you make the measurement with a moderate shunt impedance (say 100K) that the voltage will go away.
-Jon
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
2181819-1114 EDT
fifty60:
You still have not told us whether or not you have any load or wires on the power supply 24 V output.
When an ohmmeter shows a slowly changing resistance value it usually implies a fairly large capacitor somewhere in the measured circuit. The rate of change is a function of the ohmmeter source current and the capacitor value.
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fifty60:
You still have not told us whether or not you have any load or wires on the power supply 24 V output.
When an ohmmeter shows a slowly changing resistance value it usually implies a fairly large capacitor somewhere in the measured circuit. The rate of change is a function of the ohmmeter source current and the capacitor value.
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gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
181019-1751 EDT
fifty60:
Try a Simpson 260 on the 50 V DC range, 1 megohm input impedance, and see if the oscillation is still present. On DC the Simpson is 20,000 ohms/volt.
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fifty60:
Try a Simpson 260 on the 50 V DC range, 1 megohm input impedance, and see if the oscillation is still present. On DC the Simpson is 20,000 ohms/volt.
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There are a few loads on the supply, maybe 60W worth of electronics. I found the issue. A ground monitoring device is one of the loads, and I believe it is connecting to ground somehow periodically on its power input side. Going to power that device by a seperate power supply and see if the issue goes away.
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