Regardless of what some folks may tell you, NEVER EVER turn on a Switching Power Supply without some kind of a load. Granted, there are some that you can safely turn on without a load, but when testing Switching Power Supplies, ALWAYS assume that these units need some kind of a load! It is simply much safer that way.
I had preached this in the classroom for years, and one year one of my students wanted to prove me wrong with a Switching Power Supply he had turned on & off a few times. He brought it over, set it up, and while about half of the class was watching and with a big smile on his face he turned it on with no load. Unfortunately, his smile disappeared when smoke and fire exuded from the unit (while those in the class watched). I then said, "I guess you need to repair the damage. You're married to it until it's fixed". He was not a Happy Camper.
For the material presented here, it would be helpful to print out the Switching Power Supply Schematic ( 76 Kb pdf file).
Switching Power Supplies can be divided into 3 Physical Parts:
- The Primary Section
- The Secondary Section
- The Control Circuit(s)
Other Considerations Safety Troubleshooting Procedures
The Primary Section: (Note: The Blue Highlighted areas here are not Links, but are references to the Schematic)
The Secondary Section: (Note: The Blue Highlighted areas here are references to the Schematic)
The Control Circuit(s): (Note: The Blue Highlighted areas here are references to the Schematic)
The first thing to really consider here, is that the front end of a Switching Power Supply is usually directly connected to the AC Power Line, with no isolation of any kind. This simply means that where we would think it insane to deliberately grab hold of the HOT side of an AC Power Line, that same individual might very well dive into an open Switching Power Supply, without realizing the immediate danger involved. Note that there are two different grounds... they are not the same, and you should never assume that the Gnd #1 is actually Ground... It is actually SUPPOSED TO BE NEUTRAL, but it may not. In any case, treat it as a potentially hot wire. I can't tell you how many times various folks have started to see if they could find out what's wrong with their Switching Power Supply, and hooked their Oscilloscope Ground to somewhere in the Primary side of the system. One of these things usually happens: Even with an Isolation Transformer connected as an input, you still need to remind yourself that between those two AC Line Inputs is a potentially dangerous situation, and due care and caution must be taken. What the Isolation Transformer does do is to allow you to hook the ground lead of the Oscilloscope to Gnd #1 in the Primary Side, and make observations, .......BUT do not make observations in the Secondary Side of this circuit with your Oscilloscope while you are Grounded to Gnd #1!!!! When you want to look at the Secondary Side of this circuit, move the Oscilloscope Ground Lead to Gnd #2. If you are taking voltage measurements with an Isolated Meter, then as long as you avoid making any body contact with any portion of the circuit, you can safely take measurements just about anywhere, but again ... don't try to take measurements across the Primary/Secondary boundry, as they need to remain totally isolated from each other.
Troubleshooting Methods and Procedures:
"Isolation" between the AC Power Lines with an Isolation Transformer is a must! An Isolation Transformer is simply a 1:1 transformer that effectively disconnects any "direct hot wire" to the AC Power Source. As mentioned above in "Safety", any point in the front end of that circuit may be hot, unless you use an Isolation Transformer. Be sure to follow those Safety Rules described above.
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SwPwrSup.html - SfE-DCS, ddf - 08/19/2001