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Old 1st June 2013, 08:04 PM   #1
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Default All I've got is a scope - how do I measure PS stability?

I have the usual assortment of small test tools, but for big stuff all I have is a Tek 2465A. Have 1x and 10x probes. I'm trying to figure out how much I can do to measure PS stability using a scope.

I have dummy loads to get a PS to drive the expected current, but once I have that set up... what do I do? Just measure it DC coupled to see if the DC out is stable? Do some sort of examination AC coupled?
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Old 1st June 2013, 08:22 PM   #2
Elvee is offline Elvee  Belgium
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A possibility is to use a small auxiliary supply adjusted to the same voltage as the supply under test. You then use this supply as a ground for your scope, preferably set to the max gain with x1 probes, it will act as a magnifying glass during your tests, even in DC.
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Old 1st June 2013, 09:22 PM   #3
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Elvee, this sounds interesting. I do happen to have a HP adjustable DC lab supply , but I'm not sure I understand. Are you saying that I set that supply to +5V (that's the same as the PS I want to test) and then tie the ground side of the probe to the + side of that supply?
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Old 2nd June 2013, 06:44 AM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by tonyptony View Post
Elvee, this sounds interesting. I do happen to have a HP adjustable DC lab supply , but I'm not sure I understand. Are you saying that I set that supply to +5V (that's the same as the PS I want to test) and then tie the ground side of the probe to the + side of that supply?
Yes, just be careful that earthings do not come in the way of what you intend to do, otherwise you may short the supply.
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Old 2nd June 2013, 08:23 AM   #5
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Cant the scope be ac coupled ? Its bad practice to float the scope ground but i guess youll be fine if its only 5v

Can your electric loads do steploads?
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Old 2nd June 2013, 11:36 AM   #6
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Quote:
Originally Posted by Elvee View Post
Yes, just be careful that earthings do not come in the way of what you intend to do, otherwise you may short the supply.
Okay. So I have the scope probe + and - across the + and - of the DC supply I wish to test. I have the + of the floater supply connected to the - of the probe/DUT. What do I do with the - of the floater supply? Leave it disconnected?

I'm not sure what this will get me with respect to testing PS stability.

Quote:
Originally Posted by Tazzz View Post
Cant the scope be ac coupled ? Its bad practice to float the scope ground but i guess youll be fine if its only 5v

Can your electric loads do steploads?
Yes Tazzz, the Tek can be AC coupled. I have various resistors to allow for a check across various output currents.
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Old 2nd June 2013, 03:29 PM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by tonyptony View Post
Okay. So I have the scope probe + and - across the + and - of the DC supply I wish to test. I have the + of the floater supply connected to the - of the probe/DUT. What do I do with the - of the floater supply? Leave it disconnected?

I'm not sure what this will get me with respect to testing PS stability.
Connect the two (-) together, see schematic.

If your aux supply remains stable, you will see the behavior of the main supply with the max sensitivity of the scope (2mV/div f.e.).

Using AC coupling, you can do the same, but for AC only which is less practical.


The output loop has to be connected to a single earth point (probably the scope, otherwise you will short something
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Old 2nd June 2013, 04:32 PM   #8
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Quote:
Originally Posted by Elvee View Post
Using AC coupling, you can do the same, but for AC only which is less practical.
Thanks Elvee! But I'm not sure I understand this part.
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Old 2nd June 2013, 04:46 PM   #9
WSJ is offline WSJ  United States
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Transient Recovery Time. Sometimes referred to as recovery time, transient response time or response time. The time required for the output voltage of a power supply to come back to within a level approximating the normal DC output following a sudden change in load current.

Transient recovery time on Page 12, 109 & 110
http://cp.literature.agilent.com/lit.../5952-4020.pdf

Last edited by WSJ; 2nd June 2013 at 04:54 PM.
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Old 2nd June 2013, 05:14 PM   #10
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Got it. Thanks. Something that can't be measured in the AC realm.
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