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Old 12th December 2012, 03:16 AM   #1
weinstro is offline weinstro  United States
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Default Is this acceptable?

I need +- 24 Vdc and perhaps 700-800 ma.

Can I take two laptop power adapters rated at 24 Vdc and connect the outputs in series? Should I avoid providing an earth ground connection at the center node?

Thanks,

Rob
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Old 12th December 2012, 07:15 AM   #2
palstanturhin is offline palstanturhin  Finland
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Is quite likely to be possible.
Depends on the supply type.
If output is floating you are good to go...

Measure it with DVM between both supplies. If you get 0 reading, you most likely can serial connect them...

(Been there, done that...)
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Old 12th December 2012, 08:03 AM   #3
jan.didden is online now jan.didden  Europe
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All adapters I have ever used had fully floating secondaries, they have to for safety reasons.
And if you use the ones with the 2-pin mains lead, it's even impossible for them NOT to be floating.
So yes, just put them in series and ground whatever node you want.

jan
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Old 12th December 2012, 11:53 AM   #4
DF96 is offline DF96  England
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Unless the two adaptors are identical it might be good to put a diode in series too. Under normal conditions this will just drop 0.7V. On switch-off or some fault conditions it may stop one supply from driving current backwards through the other one, which could cause damage.
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Old 12th December 2012, 12:34 PM   #5
FoMoCo is offline FoMoCo  United States
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Every laptop adapter I have used has had the barrel (usually "minus") connected to earth through the ground pin on the AC plug. This would make it impossible to easily series them.

Most likely the secondary of the SMPS is indeed isolated, and you could just not connect the ground pin, but it's one of those "if you have to ask, you shouldn't try" kind of things.

So. The short answer is "No."
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Old 16th December 2012, 09:56 PM   #6
weinstro is offline weinstro  United States
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Default Power Supply Question

Thanks for the previous answers.

I've located two 24 Vdc power supplies which are likely to support a series connection. These are inexpensive SMPS, rated at about 2A each, and are a open frame rather than wall wart style case with barrier strips instead of barrel connectors.

However, they are described as "constant voltage, LED power supply". How is that different than a "DC power supply"?

I do need constant voltage rather than constant current. Application is for a small Class A headphone amp.

Also, for Class A, current load presented to the power supply should be constant, therefore step load response should not be a major concern, correct?

Thanks and regards,

Rob
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Old 17th December 2012, 09:16 AM   #7
gmphadte is offline gmphadte  India
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Your application will suffer with these SMPSs. Better construct a linear power supply with some low noise voltage regulators.

Gajanan Phadte
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