Oscilloscope

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rperkins
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Re: Oscilloscope

Post by rperkins » Sun Sep 22, 2013 2:37 pm

SkyNorth wrote: When connecting the two secondary windings together , one is the source , the other is the load. Each has its own internal resistance.........When this happens you get a voltage divider,........with equal voltage drops across each. , this means the power transfer is split evenly between the two.
-Brent
thanks. I understand now why in this situation only 50% of the power is usable. It also got me thinking along these lines.
"In a normal situation , when a transformer is operating with its rated load on the secondary, a high percentage of the available power is transferred to the load because the internal resistance of the secondary windings is very small when compared to the resistance of the load."

Kilrah wrote: Best is to use 2 probes in differential mode to measure waveforms across components.
SkyNorth wrote:To use two probes , keep them referenced to circuit ground , and measure both sides of the part in question ,one probe on each side. Use the scopes "subtract" function, to subtract the lower probe from the upper probe , and this will give you the reading across the part in question.
-Brent
This was very helpful. thanks again. Being the impatient type I did not wait for the isolation xformer to arrive and used the tip you guys gave me. I was able to probe the circuit. It was very cool to see the bridge rectifier working properly. I could see the positive half on ch 1, the negative half on ch2, and the computation of the measurement between the points with the math function. If I wanted to I could see them all on the screen at the same time in different colors :) Was able to determine that the switcher was attempting to start and then shutting down. Started focusing on the output side of things and removed about 4 fast switching diodes for test. one of them was shorted. ( -15v line ) put it back together using a diode that isnt utilized in this configuration ( +12v) and got the power supply working :)

Still gotta wait on some parts before I can button it up because I damaged the input filtering inductor and the bridge in my initial (short circuit) testing. Jumpered out the input filtering inductor and used a bridge that would work but is physically too large to fit to continue troubleshooting.

Showmaster: It is under the math function that I found the 'A-B' setting for the scope. I havent used the math function much. Initially investigated the 'fft' feature a little, mainly because it got such negative reviews online. Didnt even go very deep into it so cant comment on the fft feature personally.


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ShowMaster
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Oscilloscope

Post by ShowMaster » Sun Sep 22, 2013 5:51 pm

Keep the good user info coming!
Suddenly I have 4 scopes.
A JDR 2000, a Tek 2246 the Rigol 1102 to replace the first 2, and a now a 465.
2 good friends passed away this year and the wives were going to throw the scopes out if I didn't want them.
The 2000 and the 2246 have some minor issues and I wasn't going to fix them. Too soon for the 465 to know. Having the Rigol and all it's features I'll maybe try and fix the others. I'd like the 2246 or 465 as a backup for the next years until I cut the cord on the older scopes as I appreciate my 1102 more and learn to use its features.
There are active Arduino user groups in my area so that may be where they end up.
Most local RC people I've run into aren't technical enough to use them.

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ShowMaster
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Re: Oscilloscope

Post by ShowMaster » Tue Sep 24, 2013 4:49 am

I used the Rigol 1102 tonight to measure the CPPM output of a Frsky receiver and I'm more impressed every time I use it. I even stored the waveform and recalled it without the manual.
I compared it to a Tek 465 that I mentioned I just was given and the Rigol really looks good now. The 465 is nice but I don't regret the Rigol purchase.

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rperkins
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Re: Oscilloscope

Post by rperkins » Sat Sep 28, 2013 2:15 am

rperkins wrote:Cheaper than ruining my scope
http://www.ebay.com/itm/321143753473
rperkins wrote: Being the impatient type I did not wait for the isolation xformer to arrive and used the tip you guys gave me. I was able to probe the circuit.
Well good thing I didnt wait. I got the power supply all fixed up without using the isolation xformer and thought I would test the isolation transformer before leaving positive feedback on ebay . It doesnt work.

It's sorta wierd, it didnt blow a fuse when I plugged it in but when I connect my scope ground to the hot or common, It sparks (drawing current) and my overhead light dims. I disconnect it before it blows a fuse .The Green ( earth ground) wire on the input reads zero resistance to all 3 of the output connections. :( .

EDIT - see post below
Last edited by rperkins on Sun Sep 29, 2013 5:45 pm, edited 2 times in total.

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Kilrah
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Re: Oscilloscope

Post by Kilrah » Sat Sep 28, 2013 7:06 am

Why would you connect your scope ground? Isn't the whole point to have that one floating?


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rperkins
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Re: Oscilloscope

Post by rperkins » Sat Sep 28, 2013 8:47 am

maybe I shoulda said 'probe ground'. I was thinking with the isolation transformer I would be able to probe across any 2 points in a circuit without shorting out the mains power

SkyNorth
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Re: Oscilloscope

Post by SkyNorth » Sat Sep 28, 2013 10:50 am

You use the isolation transformer, to power the scope and float the scope ground.
The isolation transformer output has no ground current path , so you cant electrocute your self touching the scope case and ground

-Brent

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rperkins
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Re: Oscilloscope

Post by rperkins » Sat Sep 28, 2013 1:23 pm

Thanks for the reply. I will have to do more reading. This guide to working on SMPS said to put the isolation xformer on the device under test, not the scope.
http://www.repairfaq.org/sam/smpsfaq.htm#smpstste

In any event should the green wire on the input of the xformer read 0 ohms to the hot and neutral of the secondary ? I'm surprised it didnt trip the breaker. I never connected a load to the secondary though, well except for the scope.

EDIT - see post below
Last edited by rperkins on Sun Sep 29, 2013 5:45 pm, edited 1 time in total.

SkyNorth
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Re: Oscilloscope

Post by SkyNorth » Sat Sep 28, 2013 10:04 pm

Yes floating the unit under test is better , if the isolation transformer is large enough wattage to handle it.

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rperkins
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Re: Oscilloscope

Post by rperkins » Sun Sep 29, 2013 5:14 pm

After doing more research it seems there is not a clear consensus on how to troubleshoot live AC mains powered circuits with regard to the fact that the oscilloscope probes ground connection is tied to earth ground. All methods have safety concerns and it boils down to the troubleshooter being aware of the implications of what he is doing. I am not advising anyone on how to proceed on this issue. Take the time to learn about the risks and dangers and decide for yourself. You are responsible for your own well being and the environment around you. I am not responsible for your safety.

There was nothing wrong with the isolation transformer. For safety reasons they tie the neutral to ground within the transformer. Watched a video where he creates a 'tech isolation transformer' by modifying a typical isolation transformer. Followed his procedure and even had the reported 'ghost voltage' . Had to make test line with a light bulb to verify that there was no real potential after making the change.
http://www.toddfun.com/2011/04/30/isola ... nsformers/
( man I dont like video tutorials. Could have read this information in 15 minutes instead of the 50 minutes of this video, plus the other 20 I spent trying to get it to run at 2x speed. Shouldnt complain about how good free information is delivered so my rant is over )

Also noticed that the schematic for my isolation transformer does not show the neutral tied to earth ground on the secondary side of the transformer. It does show that earth ground is tied all the way through though.
http://www.alliedelec.com/images/produc ... 009038.pdf
20130928_214421.jpg
shows earth ground tied to neutral on secondary side of Allied isolation transformer
Left the earth ground attached to the case of the transformer but removed it from the secondary output. The case could have been connected to the earth ground of the DUT (device under test) or the earth ground of the mains, but not both. I chose the mains but also realizing that by touching the frame of the DUT and the case of the transformer a voltage difference could occur. Made a note on the transformer exactly what is isolated so when I pull it out in 6 months to use it that I am reminded of the risks.

I like the idea of using 2 probes ( with each ground at earth ground ) in A-B mode. Nice and simple with no isolation or ground lifting needed. Probably would have never bought an isolation transformer as that fit my needs. Since I bought one for my bench it needed to meet my needs.

SkyNorth
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Re: Oscilloscope

Post by SkyNorth » Fri Oct 24, 2014 8:41 pm

Hi,
Check out the new Rigol Scope DS1054Z
http://www.rigolna.com/products/digital ... Z/ds1054z/

At $400 , This is the best deal out there for a 4 channel scope.

-Brent

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ShowMaster
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Re: Oscilloscope

Post by ShowMaster » Sat Oct 25, 2014 1:11 am

Nice.


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