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mattress67

Power supply ignorance

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I just purchased a Mastech Variable Linear Lab Power Supply 30V 6A. Obviously this unit is built in China and is distributed to various U.S. companies who slap their sticker onto the unit. It's a regulated DC supply.

Sure enough, it produces 30 volts, and 6 amps of power -- just not at the same time. At 6 amps, voltage drops to about 1.5 volts as I power a motor. At 30 volts, I can get about 0.6 amps.

Really, at $150 the unit is fine. And believe me, I certainly understand the relationship between volts and amps.

My question: when looking for power supplies, what spec listed would have led me to understand that this unit could only produce 30 volts at minimum amperage, or max amps at minimum voltage?

Also, what would a lab rated power supply capable of producing 30 volts at 6 amps (and everything in between) cost? 

Let's face it, in some ways a 12 volt car battery (with some simple electronics) at $60 is just as capable as this power supply. Und, that it wouldn't be regulated, but I'd have all the amps I want!

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It's probably a SMPS so this is pretty normal.

The power supply has a maximum power rating, this means that at higher voltages the maximum current will be lower than at lower voltages.

The maximum power rating should be listed somewhere in the specification.

Having a maximum power rating rather than just a maximum current rating is a good thing: it makes the power supply more flexible. Your power supply has a maximum power rating of 18W, if they decided to have a fixed current limit then it would only be capable of 0.6A at any voltage.

If I was given the choice between this PSU and an SLA of the same price I'd go for the PSU for general purpose experimenting. If I just wanted to power a motor then I'd go for the SLA.

I haven't checked out the prices of lab power supplies recently but a 180W power supply won't be cheap. For higher current applications I recommend buying a 12V or 24V fixed supply which will give a better price per Watt. Lab supplies aren't really intended for powering large motors, they're made for electronic circuits.

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lab power supplies base on an SMPS donot have much of a surge rating for the current. if you drive a motor (especially a motor from a power tool) , the motor will appear as a short circuit for the supply and most of the time the supply goes into the current limiting mode.

To drive heavy inductive loads like motors u need an old fashioned transformer based linear suppl but they are expensive than the SMPS types.

some times SMPS based supplies can work as a low cost alternative. in my case i needed a high current supply for my digital projects (voltages of 5V for logic and 12V for relays, etc.) so i used a extra computer 400W SMPS that i had lying around (they are quite cheap these days). so now i have 5V at 30A and 12V at 25A. ;)

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To be picky, it's not the inductance that's the problem, it's the motor's inertia causing the high current surge. An inductor would draw a small current at first which would increase linearly until it's limited by the resistance.

As this particular supply can source higher currents at lower voltages, you could start at a low voltage and increase it until the motor reaches full speed. Obviously the power supply needs to have an adequate power rating to drive the motor which I think is the issue here.

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I've just Googled for Mastech 30V 6A power supply and I get lots of results.

If the origional poster had given the model number, then I might've been able to find it and check that it says the maximum power rating in the specification.

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