Depends on the crest factor
https://en.wikipedia.org/wiki/Crest_factor
Non-TRMS assume a crest factor of 1.414.
Jitter by many times i meant that when i connect my scope to other/many audio systems this causes a hum.
The HOT side of a SMPS is a far cry from a isolated low voltage section.
The HOT side of a SMPS is a far cry from a isolated low voltage section.
Jitter, here is something that should help you.
Tektronix A6901 Ground Isolation Monitor | eBay
It is a “ground isolator” made by Tektronix for exactly the use I have been telling you for exactly the problem I mentioned.
“Excellent Tektronix ground isolator helps isolate anything electrical from the problems associated with ground loops or noisy circuits. Allows separation of sensitive communications or test equipment from noisy industrial machinery by allowing ground to float at up to 40V before cutting the circuit.”
Tektronix A6901 Ground Isolation Monitor | eBay
It is a “ground isolator” made by Tektronix for exactly the use I have been telling you for exactly the problem I mentioned.
“Excellent Tektronix ground isolator helps isolate anything electrical from the problems associated with ground loops or noisy circuits. Allows separation of sensitive communications or test equipment from noisy industrial machinery by allowing ground to float at up to 40V before cutting the circuit.”
a multimeter set to Vdc or mVdc samples the voltage and averages the samples to give a read out of the average DC voltage.
When set to Vac or mVac it again samples the voltage and averages the samples to give an average of the AC waveform.
It then applies a correction to convert that average of the AC waveform to an equivalent sinewave voltage. The correction from average sinewave to true sinewave is ~ 1.11times
Average Voltage of a Sinusoidal AC Waveform
A true rms voltage measurement is quite different.
It looks at the equivalent heating effect of the voltage waveform. Some actually measure the heat and use that to determine the voltage.
When set to Vac or mVac it again samples the voltage and averages the samples to give an average of the AC waveform.
It then applies a correction to convert that average of the AC waveform to an equivalent sinewave voltage. The correction from average sinewave to true sinewave is ~ 1.11times
Average Voltage of a Sinusoidal AC Waveform
A true rms voltage measurement is quite different.
It looks at the equivalent heating effect of the voltage waveform. Some actually measure the heat and use that to determine the voltage.
I remember a project for a meter like that in Practical Electronics back in the eighties. Nowadays its mostly done within 1 chip, a true RMS to DC converter and now that these are a lot more affordable I can't imagine there'd be much call to use the heating method anymore.
At most, for a near-squarewave, it will read ~10% higher, π/√8 to be perfectly accurate: the rms value of a squarewave of amplitude A is also A, as is its average value. But the non rms voltmeter will apply the correcting factor of rms/average for a sinewave, which amounts to the above value
the chip converts the samples to digital bits and then multiplies to give a result.
This leads to limited bandwidth and maybe other inaccuracies, whereas the heating method remains fairly accurate to high bandwidth.
The heater type is not obsolete.
The 1.414 is for DC. Did you change you DVM from AC to DC making these measurements. If not, it makes perfectly sense that you got a lower reading after rectification; your meter just showed the AC-component of the rectified signal.
Powerbob, can you explain the cause of the non-sinusoidal waveform? This may help understand the laws of physics and where you went wrong.
something must be heating up, either rectifier or transformer. the problem will be with the heating part
In that case I would suspect a ground loop is the cause of the hum. This could occur if the device under test (DUT) is also mains earth referenced. This could still be solved by floating the DUT instead of the scope.
It's not as much a matter of the actual voltage, rather whether it's isolated from the mains or not.
Basically, an isolation transformer prevents a current to earth as that no longer is the return path. That means that no current can flow from the circuit through the scope's earth lead to earth nor through you should you by accident touch a live part.
That is not to say you shouldn't be careful. Theoretically, one could electrocute oneself with an isolation transformer by touching one side of the transformer with one hand and the other side with the other hand. But during normal probing around, that's not very likely to happen.
A rather graphical demonstration that the voltage level is not that relevant when you're floating was posted in another thread, and I'll repost it here. Granted, the man wears a suit that acts as a Faraday cage, but I think that's mainly to accommodate the charging/discharging currents as the goes from the helicopter onto the live wires and back. Both he and the helicopter take on the same potential as the wires, but since they're floating, they can't get electrocuted.
The fact that it cuts out if the case reaches a voltage of merely 40 VDC or 28 Vrms with respect to earth to me only confirms that you should never float your scope. At least not with a regular isolation transformer.
It would seem you are correct in that the heating method is still used at rf and microwave frequencies.
Whilst there may be digital RMS to DC converters the ones I was thinking of are linear ic's.
The original one was the AD536 its most recent incarnation is the AD8361 which claims a 2.5Ghz bandwidth.
I expect the latter may now spell the ultimate demise of the heater method.
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Measure the dc voltage across the cap with no load. This will be very close to the actually supply voltage before it says from the load. And it should be 1.4 times the ac voltage of your transformer. But in your circuit your only using half the transformer so .7 times the tranny voltage. Is that what you measured?
Jitter, you have a lot of misconceptions about power supplies and your reference to the isolated man on the helicopter shows you do not understand what is going on here. In order to make these audio circuits safe, the ground of the signal circuits is connected to safety ground. Now the audio interconnects are safe to touch. The other reason for doing this is in case the transformer develops a short from primary to secondary. Now there is a low impedance path from to the secondary to the safety ground and the fuse blows. So in other words if you are poking around in the secondary section of an isolated audio power supply you are only safe if it is a low voltage power supply, the same danger of getting a shock from earth/safety ground is still there.
In an audio power supply the signal ground must be at or very near 0V safety ground. If it was not you could not connect your audio cables between different equipment as they would all be at different voltage levels. The electronic circuitry could actually float up to a much higher voltage potential from the leakage current of the transformer.
Look at the motherboard in your computer notice the mounting holes have large ground pads around them, this is to form a large ground plane referenced to safety ground. Now your computer can safely connect to an audio system.
Now if for some reason we had a power supply with a much higher voltage it would be almost as dangerous as a touching the mains. Let’s say you have a 350 vdc B+ supply. Once again the ground of that supply is connected to safety ground. If you touch the 350 vdc B+ and any part of your body is touching ground like the computer case or another stereo component case you will get a lethal shock even though you are powered through an isolation transformer. Technically you could still poke around in the low voltage section of this power supply without getting a shock because of the transformer. But personally when there is a dangerous voltage present on the secondary I treat the whole circuit as high voltage and think twice before I connect anything to it.
And BTW, you deserve a swift kick in the pants for dismissing Tektronix. I am just a schmuck and there is no particular reason for you to believe me. But Tektronix is recognized as one of the most knowledgeable bodies of test equipment in the world and they cannot be dismissed with a wave of your hand.
Never a good idea. Also, more Tek scopes have died of bad power transformers due to floating the scope
than for any other reason.
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