SMPS feedback loop for voltage regulation

i purchased a surplus SMPS Intergraph PN CPWS14003. when i place load on 12 vdc output, the existing measured 12 v drops to 0. i suspect it has something to do with feedback from output is missing. however, i don't have schematic or any other info.

so any thoughts such as where to possibly buy schematic, how to tell by visually looking at circuits where to introduce feed back, etc. thanks.

Perhaps you're activating a current overload protection circuit--in other words, your load is demanding more current than the supply is designed to deliver.

Another possibility is if it has sense [S] lines, and they are floating (i.e. not connected to anything) that might do it (though that is more likely to cause the power supply voltage to go up rather than down, but who knows how they will behave if not connected).

The purpose of sense lines is to make it possible to sense the voltage out at the load (assuming the load is far from the supply). The supply lines will tend to drop voltage, causing a loss of regulation. By sensing voltage out at the load (the "feedback" you mentioned), the supply can up the voltage to compensate for line loss. If this feature is not being used, the sense lines usually need to be jumped to the supply lines on the terminal block (back at the power supply).

There is usually a positive sense terminal, and a negative sense terminal. The positive sense terminal needs to connect to the positive supply line, either at the terminal block or out at the load. The same for the negative sense terminal (it goes to the negative supply line).

thanks Steve. according to the power specs on the label, it should handle the load i placed on it. so your other hypothesis is most likely the case. however, the output connectors don't indicate which would be the sense line. so i will check for one of the output connectors that doesn't have voltage and then risk connecting to the 12 volt line in which i am interested.

i did find some tutorials online that speak to the IC that has to do with chopping voltage to the inductors and capacitors for the purpose of regulation. the IC did have a pin labeled feedback. so if this is a standard design i may be able to trace one of the unmarked connectors to this IC..

i appreciate the response.

If it's any help, here are some thoughts:
# assuming this power supply has a terminal block, the sense terminals are usually next to the power terminals, to make it easy to jump them when the sense lines aren't used.
# If you know this supply only has one voltage out, yet there are 4 (rather than 2) outputs, then that is a good indication that there _are_ sense lines.
# Usually there is hook up information printed somewhere on the supply casing. Have you looked for that?
# Also, you said you connected a load to the supply--how did you know where to connect it if the terminals aren't marked? Perhaps, the reason the supply went to zero when you connected the load, is because you didn't get the load across the supply lines (but instead across one or more of the sense lines).

hi Steve. i have been checking out some of your suggestions. the only info on case of SMPS is voltage and current capacity and total watts for each combination of outputs. secondly, i tried both 12 volt connectors with same result, 120 ma load triggers current lockout and the 12 volts connectors are rated at 6 amps. thirdly, the various voltages available is marked on the case. i used voltmeter to determine which was which.

here is further info. there are nine output connectors as follows: P1 +and - 5V, P2 +5, P3 +5, P4 +5, P5 +3, P6 +5 and +12, P7 +5 and +12, P8 +5 and +12, and P9 + and +12.

the power ratings are: -12@.5 amps, -5@ .5amps, +12@5 amps, +12@5 amps, +3.3@19 amps and +5@30 amps.

i am in the process of learning more and will update as i go along. thanks.

COMMENT: the SMPS has 9 connectors, each connector has multiple pins (obviously manufactured for a specific application). the case of the SMPS has written on it the available voltages and current capacity of each (-12V@0.5A, -5V(_at_)0.5A, +12V@5.0A, +3.3V@19.0A, AND +5.0V@30A). i measured voltage a each connector (no load). some connectors have more than one voltage available as some have up to 8 pins. some of the pins are common(neg,black) and is where i attached the neg probe of the vm.

will try to upload pic. thanks

The voltages you listed on those nine outputs, are those voltages that you measured? And if so, with or without a load? And how come P1, P8 have two voltages? And, it looks like you left out a number on P9.

Any chance you can upload some photos?

Steve, see comment in my last post. wasn't sure how to respond to your last post (email alert)

this SMPS came with 115 power cord and the option to use 208. also some info i found on the net, it is either flyback regulator or forward regulator type (allows multiple outputs and input to output isolation). all other types mentioned (buck regulator, boost regulator and cuk regulator) are limited to single outputs. don't know if that info helps any.

You say there are nine connectors and you list them as P1 - 9. And then there are numbers [P3 +5, P5 +3, etc.] and some of them have a " + and -" [P1 +and -5V] and one of them have to positive numbers [P6 +5 and +12] and one of then, P9, has " + and +12".

What does it all mean ??? How can a terminal have more than one voltage? One Under load & one with no load? But, then, why aren't they all like that? And how could there be a positive no load reading with a negative under load reading (or vice-versa)--which, to me, blows my theory or load/no load readings...

I'm SO confused!!! ;^)

hi Steve. i am sure that i am contributing to the confusion. i was able to upload pics (see above image codes). notice in the top most pic the label on the outer case. these voltages appear on one or more of the 9 connectors. also note that each connector has several pins and each connector may have +5, +12 etc. i am not sure how voltages connect within the assembly but in the case of the common (neg, black) lines they all terminate on the same terminal block. i assume that all +5 voltages for example originate from a common terminal block and fan out to the various connectors.

one of the ICs on one of the circuit boards is an LM339N which is a 4 comparator chip. this may be where the comparison between DC output level and design are compared.

i will continue learning about this device and posting info. i am thinking about removing the PCBs from the metal enclosure so that i can better examine structure. thanks.

Ah HA! Now it all makes sense. This is an ATX PC power supply. If i remember correctly, there are a couple of unusual connections that have to be made for it to function properly. And I seem to remember there's more than one configuration. Check out these sites for more info:

* http://www.mbeckler.org/powersupply/
* http://www.alexwhittemore.com/atx-bench-power-supply-mod/
* http://jumperone.com/2011/06/atx-power-supply-tutorial/
* http://www.youtube.com/watch?v=z2oSFpKh_Uw

And gobs more behind a Google search on: "using a ATX supply as a bench supply" or words to that effect. Maybe, also try: "Converting ATX power supply to bench supply"

Also, consider this: There was a run of bad capacitors (some manufacturer cut back on materials which made the caps prone to early failure). A lot of power supplies and mother boards died a premature death because of this.

Also, in this video, he talks about a capacitor that tends to fail, that is right next to a heat sink in a PS that looks very much line the one you have:

* http://www.youtube.com/watch?v=Ei1HZ9kkXYo

hi Steve. congratulations. your recognition that the device i have Is ATX type and your links to ATX info led to successfully firing up the SMPS. there were two requirements mentioned in the links. green wire jumped to ground and a certain amperage load on the each of the 3.3, 5,and 12 volt dc outputs. one or the other may have been sufficient but to play it safe i did both. once fired up, i connected an AC/DC TV that draws about 3 amps to the 12 vdc and operation was normal.

the first time i tried to connect the TV, the SMPS immediately shut down. that is when i generated my post asking for assistance. never having experience with the ATX SMPS, i am not sure that i would have been able to use the SMPS. i bought 2 of them for $10.00 each from a web site named ELECTRONIC GOLDMINE, a dealer in surplus products. so now i have a POWER SUPPLY for my projects.

so many thanks for your help in successful conclusion to my post for assistance.

Awesome--glad I could help ;)

Yeah, Electronic Goldmine is one of my favorites

just as a follow up, i tried powering up the SMPS only green connected to black. no go. connected load to 3.3 volts and SMPS powers up. so apparently takes both. thanks again.