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Old 3rd October 2005, 04:29 PM   #1
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Default How do you measure ripple on the B+?

Ok, so I'm still learning this electronics stuff.
How do you measure ripple on the DC power supply?
This is high voltage, so I assume you can't just connect your probe from the scope to the DC, right? Are there high-voltage probes for doing this type of measurement?
I'm not sure my Tektronix 585a would like this much voltage
Thanks.
Glenn
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Old 3rd October 2005, 04:42 PM   #2
EC8010 is offline EC8010  United Kingdom
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Very carefully.

You first check the voltage rating of your probe. It's probably only 250V. At that point, you reach for your high voltage (2.5kV) probe and you switch the oscilloscope to AC coupling and measure away.

Of course, you might not have a high voltage probe. At that point, it gets rather more exciting. With the amplifier turned off, you solder one end of a 100nF 1kV capacitor to your 400V HT and the other end you take via a 1M resistor to 0V. You then power up and connect your probe across the 1M resistor and look at the hum. Alternatively, you take the signal across that 1M resistor to another audio amplifier and listen to the hum.

You all of this very carefully...
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Old 3rd October 2005, 05:43 PM   #3
dhaen is offline dhaen  Europe
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Quote:
Originally posted by EC8010
Very carefully.

SNIP.. You then power up and connect your probe across the 1M resistor and look at the hum. SNIP..
Note the order...
1 Power up
2 Connect probe (carefully)
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Old 3rd October 2005, 06:07 PM   #4
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Quote:
Originally posted by EC8010
Very carefully.

You first check the voltage rating of your probe. It's probably only 250V. At that point, you reach for your high voltage (2.5kV) probe and you switch the oscilloscope to AC coupling and measure away.

Of course, you might not have a high voltage probe. At that point, it gets rather more exciting. With the amplifier turned off, you solder one end of a 100nF 1kV capacitor to your 400V HT and the other end you take via a 1M resistor to 0V. You then power up and connect your probe across the 1M resistor and look at the hum. Alternatively, you take the signal across that 1M resistor to another audio amplifier and listen to the hum.

You all of this very carefully...
Thank you for that information.
Of course I don't own a high voltage probe , that's mostly why I asked the question. I assume you mean to connect the other end of the 1M resistor to ground when you say 0V, right?
Thanks again, I'll give it a try!
Glenn
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Old 3rd October 2005, 07:12 PM   #5
EC8010 is offline EC8010  United Kingdom
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Yes, ground = 0V.

Regarding order, it depends. An amplifier with silicon rectifiers can bump its HT around rather abruptly at switch-on, so I prefer to connect the probe when everything has settled down. You might not be so confident in your handling of a probe near high voltages and prefer to connect up, then power up. If you do, I suggest that you set your oscilloscope to 50V/div before switching power to the amplifier. In fact, I'd say as a general thing that it's a good idea to do that when poking around HT. Once everything is stable, increase the sensitivity of the oscilloscope. That way, you're less likely to blow up the oscilloscope's input stage (they tolerate overload, but maybe not all that much).
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Old 3rd October 2005, 11:29 PM   #6
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The quick and dirty way is just to use your DVM...set it for AC voltage and put it on the DC high-voltage rail.... This will give you a rough idea of the RMS ripple...which you can "very roughly" deduce the peak ripple.... The ripple will be 100Hz or 120Hz depending on where you are located.... The waveform is not always a true 50% duty cycle...due to the size of the input cap as well as the current demand ...this will alter the conduction angle the diodes are turning on ...So sometime it's better to see it on the scope...

Chris
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Old 4th October 2005, 02:52 AM   #7
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I'm with Cerrem on this, just use your meter. This is not a laboratory measurement, you just want to know how much ripple you have. This is close enough. If you view it on your scope, you can determine the amplitude of the ripple, but trying to come up with a reasonable equivalent voltage from the sawtooth waveform will be problematical. SO just trust your meter.
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Old 4th October 2005, 06:22 AM   #8
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Quote:
Originally posted by Enzo
I'm with Cerrem on this, just use your meter. This is not a laboratory measurement, you just want to know how much ripple you have. This is close enough. If you view it on your scope, you can determine the amplitude of the ripple, but trying to come up with a reasonable equivalent voltage from the sawtooth waveform will be problematical. SO just trust your meter.

... if you are SURE that meter can withstand the high DC on it's AC scale. There will be a blocking cap in the AC meter line. What is that cap's spec? 250V? 450V? Do you know, or you want to gamble?

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Old 4th October 2005, 08:08 AM   #9
dhaen is offline dhaen  Europe
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A meter will only give an approximate RMS value of ripple. They're designed to measure sine wave.
Of course it's fine for comparitive measurements.

Best also use it with EC8010's CR network.
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Old 4th October 2005, 11:51 PM   #10
Enzo is offline Enzo  United States
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If the input of my meter is rated to 600VAC or 1000VAC, then it shouldn't care if part of that is DC.
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