# power supply max current

suppose you have a power supply but don't know what is the maximum current it can deliver. how would you find out what the max current is?

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Warren Thai wrote:

Do you mean a multi-megawatt generating plant, or an AAA battery, or something else?
The term "Power supply", even assuming an electrical one as it was posted here, covers far too wide a range of possibilities to give a meaningful answer.
In general, you ask the manufacturer. Or you use experience based on knowing the output capabilities of other power supplies of similar design. Or you open it up, identify the components used and make an educated guess - usually allowing a large factor of safety. Or you take measurements under light load and use a mathematical model of the system to determine what its operating conditions will be under heavier loads. Or you look at the rating of any circuit protective devices. Or, etc, etc, etc..
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Sue

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Short the output and measure the current!
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Peppered_Porcelain wrote:

I would suggest making it clear that you aren't seriously suggesting this as a general method.. If the OP had to ask the question, he may not have the level of knowledge needed to appreciate the answer..
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Sue

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Yeah, ok, didnt think about that. It was a joke.
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| Peppered_Porcelain wrote:
|> |> |>>suppose you have a power supply but don't know what is the maximum current |>>it can deliver. how would you find out what the max current is? |>> |> |> |> Short the output and measure the current! |> |> | I would suggest making it clear that you aren't seriously suggesting | this as a general method.. If the OP had to ask the question, he may not | have the level of knowledge needed to appreciate the answer..
How about putting incrementally larger loads on and measure the voltage drop and calculate the system impedance?
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@ipal.net wrote:

The nature of the power supply is unspecified. It may have output voltage stabilisation that will compensate for changing loads.
Without an expansion of what is meant by the OP by the expression "power supply" (or indeed "maximum current")....
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Sue

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Palindr?me wrote:

a general answer to a general question: assuming the product specifications are unknown, examine the key components i.e. power transformer, rectifier, pass elements, fuses, etc. most often the power transformer is the limiting factor.
a power supply that runs on 120 VAC and has a 1A line fuse you know will only draw 120W before the fuse opens. if its output is 12 volts you know Imax will be 120W / 12V ( - losses) so something less then 10 amps. likely the real value will be much less.
some supplies are current limited. for these the "crowbar" testing method is valid. you place a ammeter across the output and read the result. most supplies however will emit heat and/or smoke and/or flames and/or blow fuses when this is attempted.
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To the question of what I mean by power supply, I meant something like a mobile phone charger or lab voltage supply (+-30V) that kind of stuff.

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Warren Thai wrote:

In the cse of the phone charger, or anything similar that has been designed for a specific purpose, there is often a label on it that gives its designed output voltage and current. Because it has been designed with one very specific application in mind, it will not normally have characteristics that make it automatically suitable for other things. For example, its output may be very unregulated and thus vary greatly with load current. Because they have been designed with a specific load in mind, the design may treat any other load as a fault - and clear the fault current using a non-resettable device within the unit..Re-deploying such units for other applications where the power needed is not insignificant, needs a great deal of thought.
Lab supplies and the like are designed for a wide range of loads. They generally are fitted with re-settable protection devices so that they will not develop a permanent fault if overloaded. However, there are exceptions. They will normally have a label, or have other means of showing the maximum output capability.
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Sue

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what if its like a ac plug pack, the ones you use to power electronic keyboards or computer speakers etc. The label has been removed a long time ago. I know the voltage by measuring with a multimeter. I'm after an experimental way of determining its maximum output current.

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| what if its like a ac plug pack, the ones you use to power electronic | keyboards or computer speakers etc. The label has been removed a long time | ago. I know the voltage by measuring with a multimeter. I'm after an | experimental way of determining its maximum output current.
It's probably going to come down to something like this. Measure drops in voltage at various levels of current load and figure out the system impedance from that. Then figure out the power loss at various levels. Then figure out the heat dissipation from the enclosure and calculate the equalibrium temperature for a standard ambient temperature. Find a heat rise that is within the norms of cheap components (probably no more than 20C above ambient and no more than 50C total ... and maybe even less) and limit the current at that level. Even this is not perfect as the internal heat distribution could be uneven. But it's a start.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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Warren Thai wrote:

First, I would just buy a plug pack suitable for whatever you need. Makes mores sense to ma than fiddling with an unknown device. That said, here's how you can do what you asked, if you must:
If it is an AC plug pack, you'll need to add a bridge rectifier and a large capacitor to make the output DC.
Feed the DC output of the plug pack into a variable current control. (10K Pot, 330 ohme 1/2 watt resistor, PNP power transistor.) Connect a 10 ohm 25 watt resistor from the output of the current control to ground. Put a voltmeter across the resistor. Increase the current until the voltage won't rise any more. Divide the voltage reading by 10 - that's the maximum current. In use, you want to limit the current to less than that maximum. 20% lower would likely be a safe margin. The final test for safe maximum is monitoring the temperature of the plug pack in use over time. It's ok if it gets warm, not ok if it gets hot. The circuit:
From Plug PNP Power Pack Transistor + ---+--- -------+ | \___/c | [330R] | | | [330R] | | | | +-----+ | | | / | <--------+ 10K \<---+ | | / | [10R] [Voltmeter] | | | | Gnd --------+----+----+ <--------+
Most plug packs are < 1 amp. If yours is physically large, it may go higher. The above circuit is good to a little over 2 amps. If you go higher, you'll need a larger wattage 10 ohm resistor, and the transistor will need a heatsink.
Ed