Hi guys, i have a question that's been bugging me for a while.

*An Adapter puts out a fixed voltage e.g 24v(laptop adapter) and it says 3.4A.
I know the Volts are CONSTANT, but are those 3.4A also constant or not?
Is it a "can supply UP TO 3.4A"?
Because im struggling with ohm's law here. BECAUSE IF the Amps are constant, and i put a 10ohm resister as load and noooooo other stuff in for extra resistanse, ohms law only states 24v/10ohms=2.4A now if 3.4A is a constant what the now? do i add 2.4 and 3.4 together for total current in circuit or what?
I take it the 3.4A is only what it can max out as, right? or ohms law will make shit blow up, right?

Must i simply treat the adapter as a battery in the circuit? .i.e Draws as much current as you need but there IS a max where beyond which bad stuff will happen?

 

Hi Rick,
It is: Is it a "can supply UP TO 3.4A"?
The connected load will determine the current drawn,
E

What happens when you attempt to draw more than 3.4A is determined by the PSU design.
It either current limits at 3.4A or the output voltage falls,
What type is the PSU.?

 

Generally speaking, a typical wall adapter is a voltage source not a current source. This means the device will output the rated voltage until the rated current is reached, after that, the voltage will drop or the (properly designed) device will enter a fail-safe mode intended to prevent short circuits. The actual function will depend on the topology of the device because a Switched-Mode-Power-Supply behaves a lot different than a linear power supply. In any case, the device is only capable of outputting so much power measured in watts.

Using your example of a typical 24V / 3.4A power source (81.6‬W) for various loads, the remaining available current (at 24V) will be:

1) 3.4A - (24V / 100Ω) = 3.4A - 0.24A = 3.16A
2) 3.4A - (24V / 10Ω) = 3.4A - 2.4A = 1A
3) 3.4A - (24V / 5Ω) = 3.4A - 4.8A = -1.4A

Therefore, the device will not be able to supply the rated voltage to a 5Ω load. Again, what happens in this case depends on the topology of the power source.

Another way of looking at this is to determine the minimum load the power source can supply:

?Ω = 24V / 3.4A = 7.1Ω

So, it doesn't matter if the there are 10 loads or just 1 as long as the total resistance doesn't exceed 7.1Ω.

It's important to note these calculations are only true for ohmic loads. If the load has capacitance or inductance (or both), the calculations change but don't concern yourself with this until you nail down how ohmic loads function.

 

Generally speaking, a typical wall adapter is a voltage source not a current source. This means the device will output the rated voltage until the rated current is reached, after that, the voltage will drop or the (properly designed) device will enter a fail-safe mode intended to prevent short circuits.

Yes; this is accurate.

Here's how that might look:
Suppose you have a resistive load; 24V ÷ 10Ω = 2.4A draw. The supply can deliver up to 3.4A.
Change the load to 5Ω. 24V ÷ 5Ω = 4.8A. The supply can't deliver that much current. The voltage will drop.
Ohm's Law predicts that a 5Ω load drawing 4.8A will require 24V but the supply can't sustain that much current. So at 3.4A on a 5Ω load the voltage will drop to 17V.

 

I know the Volts are CONSTANT, but are those 3.4A also constant or not?
Is it a "can supply UP TO 3.4A"?

Yes, the spec is a little ambiguous on that point, but most power supplies have a constant-voltage output (except some of those for LED lighting which have a constant-current output), and the current is determined by the load.
So your statement "can supply UP TO 3.4A" is correct.

 

w

Yes, the spec is a little ambiguous on that point, but most power supplies have a constant-voltage output (except some of those for LED lighting which have a constant-current output), and the current is determined by the load.
So your statement "can supply UP TO 3.4A" is correct.

Unless it is an ancient unregulated supply, which might supply 30V with no load, 26V with a 1A load, and 24V with a 3.4A load.

 

Hi guys, i have a question that's been bugging me for a while.

*An Adapter puts out a fixed voltage e.g 24v(laptop adapter) and it says 3.4A.
I know the Volts are CONSTANT, but are those 3.4A also constant or not?
Is it a "can supply UP TO 3.4A"?
Because im struggling with ohm's law here. BECAUSE IF the Amps are constant, and i put a 10ohm resister as load and noooooo other stuff in for extra resistanse, ohms law only states 24v/10ohms=2.4A now if 3.4A is a constant what the now? do i add 2.4 and 3.4 together for total current in circuit or what?
I take it the 3.4A is only what it can max out as, right? or ohms law will make shit blow up, right?

Must i simply treat the adapter as a battery in the circuit? .i.e Draws as much current as you need but there IS a max where beyond which bad stuff will happen?

Actually "it depends". As has been presented quite well already, the voltage is the defined specification and the amps is the "Up to" variable. This holds very well for all REGULATED power supply devices.
For the non regulated devices it gets a bit goofy, because with most of them the voltage will drop as the load current increases.
So for a NON-REGULATED supply it would be more correct to say 24 volts AT 3.4 amps, because the effective internal resistance drops the voltage to 24 volts at 3.4 amps. The result is that a non-regulated supply will deliver only that voltage at that load current, which is usually the design maximum. At a lower current the voltage will be greater.
And it is ALMOST NEVER that such a supply is a constant current device.

 

thank you guys so much. i now understand, as my suspicions were set straight. thank you all.

i will start new thread with new questions

 

Certainly there is quite a bit to know about the application of power supplies, even if you will only be using well designed commercial models, and not designing your own supplies.