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Old 30th May 2011, 06:55 AM   #1
Sotirio is offline Sotirio  United States
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Default No power output on new SMPS

I purchased a Meanwell S-350-27 from Hifimediy to power both T1 and T2 amplifiers. When I initially energize the power supply it gives a 27v output but the power quickly drops out. When I deenergize the power supply the output briefly goes back to 27v. I am feeding the amp 120v with the voltage selector switch in the correct position and no load at the output. What is the malfunction?
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Old 30th May 2011, 07:08 AM   #2
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SMPS often do not like to work with no load.

If you dont want to risk amp modules try loading it with a light bulb.
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Old 30th May 2011, 01:56 PM   #3
Sotirio is offline Sotirio  United States
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Does an SMPS need to be loaded to maintain an output voltage?
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Old 30th May 2011, 03:11 PM   #4
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Generally "YES"

Most SMPS are PWM (Pulse Width Modulated). The SMPS will try to regulate its output by increasing or decreasing the Pulse Width to maintain regulation. If there is no load then, even with the narrowest pulse, the output will rise above the cutoff threshold and the SMPS may shut down.

Try loading the PSU with lightbulbs and see what you get.

You can simply use 110V mains bulbs (within reason).
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Old 30th May 2011, 09:49 PM   #5
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Assuming the issue is what Andy mentioned (which is likely) I have a suggestion - I don't know anything about your amps, but I suspect when there is silence, not "sound" of music playing, that they will not place enough of a load on the power supply. Thus, you may need a dummy load, something that just uses enough power to keep the PSU in regulation. The typical method is a power resistor of the appropriate wattage across the PSU leads positive and ground OR positive to ground and negative to ground if a dual rail supply.

I'd go ahead and add the resistor to the PSU itself instead of the amp since it is the PSU not the amp that actually needs it, and once you have it, the PSU should stay on without it hooked up to the amp for testing.

How much load the PSU needs (current drawn) might be in the PSU spec sheet, or the manufacturer might be able to tell you, or if all else fails you can find the value experimentally with a rheostat or parallel or series resistors till you reach a minimum current level needed - aiming for the minimum so there is the least electricity and heat wasted.
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Old 31st May 2011, 04:03 AM   #6
Sotirio is offline Sotirio  United States
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I connected an amp module since the measured output peaked at 27.5v but this did not keep the power on. Ideally the power supply will stay regulated while playing music once everything is connected. I will be on vacation this week, but will report back on Sunday night.
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Old 31st May 2011, 08:58 AM   #7
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Looking at the specs of the PSU it should be happy with very little or no load.

I would suggest that you have a faulty PSU.
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