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[SOLVED] SMPS Power Supply Voltage drop Under load.

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RHeTTRoNiCS

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Hello Again to All...Please help me understands more. What could be the most possible root cause for the smps power supply voltage drop under load? Vout is measured 5V without load. But when load (2ohm) connected can get only 4.7V...This smps specs is 5V, 20Amp...Please share your knowledge...thanks.
 

Surender Reddy

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May be the rating is wrong...
5V, 20A means large current it has to provide.....but as per you, it is not.
The voltage will drop when the current is not enough for load.
Once again check out the rating....
 

RHeTTRoNiCS

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Thanks for the reply...the smps power supply specs is correct (5V, 20A). I put a load (2ohm heater element) for testing purposes only.
 

SunnySkyguy

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Is 2 Ohms the measured cold resistance or the calculated hot resistance which is 10x higher for tungsten light. If it is the cold value of Nichrome wire, then OK.

Where is the datasheet?

If this is the secondary of a Dual output supply, then the primary DC out must be loaded with 10% min current in order to regulate. on some Astech models.
 
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RHeTTRoNiCS

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The 2 ohms of chrome...this is single output 5Vdc. It has 2 dual +15Vdc also but all ok. This is very old power supply...Is it possible to fine tune this 5Vdc output to 5.3Vdc? As we know it will drop 0.3Vdc under load. So Vout can still get 5Vdc when load connected??? :) Thanks.
 

L_jack_xing

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Where did you test ? The output pins or the load pins? Because 20A would have a large voltage drop.
 

bill66656

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Hello Again to All...Please help me understands more. What could be the most possible root cause for the smps power supply voltage drop under load? Vout is measured 5V without load. But when load (2ohm) connected can get only 4.7V...This smps specs is 5V, 20Amp...Please share your knowledge...thanks.
As L_jack_xing advised, firstly tap the output pin directly using the volometer. But there isn't much space for this possibility for you tended to take care in your evaluation work I think.

Then check the feedback pin potential by directly tapping the pin, if it's some mV higher than the right ratioed VFB,the circuit will give lower output as you saw. Carelly check the connection, maybe there is some leakage from higher voltage to the feedback node.


Good luck!
 

dick_freebird

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The first question is "which ground, which 'output'"? Anything
but a Kelvin connection at the points declared by the mfr
is going to deviate by the I*R drops around the current loop.
 

SunnySkyguy

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Unless you have schematics, it is hard to see in my crystal ball ;)


In the old days ( and we used plenty of PM PSU's) they had differential remote sense inputs on the unit to correct for line loss. Does yours?
 

RHeTTRoNiCS

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Guys thanks a lot for all the responses,...really take note all of it...but my boss can't wait any longer so we decided sending it for vendor repair....have a great day to all.
 

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