Understanding amperage

[jamessinghal]

I am still a newbie, and I am in the process of learning about power supplies, and wanted to know a little more about amperage.

My current understanding is that a if a power supply says 10 amps, the maximum is 10 amps, and amperage depends on how much the device is using.

If somebody has a correction, or more information on this topic, I would love to hear it!

[Benta]

The term amperage is not really used in electronics.
You'll see it used in electric power generation and distribution to describe the current carrying capability of cables and switch gear.

[rstofer]

We tend to use 'current' and express it in amperes.

A circuit draws the current is wants up to the ability of the source to deliver.  Most power supplies are 'constant voltage' and 'current limiting'.  Very few are 'constant current' because that implies that the supply will deliver whatever voltage is required (at that instant) to keep the load current constant.  There are uses for 'constant current' but a bench supply is probably not one of them.

You will see people using 'constant current' when they really mean 'current limiting'.

[suicidaleggroll]

You will see people using 'constant current' when they really mean 'current limiting'.

I'm not sure I understand the distinction you're trying to point out.  What is the difference between constant current and current limiting in your view?  Because most bench supplies do switch to constant current mode when the load impedance drops below your set voltage / current.

[Vtile]

You will see people using 'constant current' when they really mean 'active current limiting'.

Would this be a better wording. It is a constant current source in the limits of the maximum output voltage (edit. but yes, used mainly for setting the maximum current output for mainly device safety reasons). We are getting a side track though, considering the wording of the original poster this propably will only put his head to spin.

[rstofer]

You will see people using 'constant current' when they really mean 'current limiting'.

I'm not sure I understand the distinction you're trying to point out.  What is the difference between constant current and current limiting in your view?  Because most bench supplies do switch to constant current mode when the load impedance drops below your set voltage / current.

My Rigol DP832 PS has both voltage control and current limiting.  It will put out the voltage I set up until the current reaches the limit  set (or the internal limit) and then it shuts off the outputs.  This is constant voltage - the voltage stays the same regardless of current up until it hits the limit.  This means I can set the output current for, say, 0.1A for a breadboard project and limit the damage that an 'oopsie' can create.  Constant voltage, current limiting.  Most power supplies work in this mode. 

Most regulators (eg LM7805) have voltage foldback current limiting for self protection.  If the current is excessive, the regulator reduces the output voltage to limit heating of the regulator.

Constant current means just what it says.  The power supply will try to keep the load current constant regardless of the output voltage.  If I try to pump 1A through a 1K resistor, the supply will try to produce 1000V to do it.  That may exceed its capability but it will try.  If I lower the resistor to 1 Ohm, the supply will only output 1V, still trying to push 1A.  With a constant current power supply, the voltage varies but the current stays constant.  Just what it says constant current.  Not many power supplies have this capability and I have yet to see a reason for it.

OK, it might be nice to have constant current for diagnosing control system current loops (4-20 mA) and it might be interesting for characterizing LEDs (push 20 mA, measure Vf) but these would be small supplies of limited use.

Some power supplies will hit a current limit and then begin backing off on the voltage (voltage foldback).  It is not the intent of the supply to create a constant current, it is simply current limiting by varying the voltage.  If the load backed off, the supply would raise the voltage back to the set point, it would not keep pushing the same current.  I consider these to be constant voltage current limiting, not constant current.  AFAICT, the DP832 just trips the output, it does not fold back.  It just hasn't happened yet!

I suppose if you took a PS with foldback limiting, set it at its maximum voltage such that the set point current would always flow, independent of load (within reason), it would look a lot like a constant current source.  Many supplies operate like this but not all.  The DP832 considers an overcurrent (or overvoltage) to be a shutdown type event.

[helius]

Constant current means just what it says.  The power supply will try to keep the load current constant regardless of the output voltage.  If I try to pump 1A through a 1K resistor, the supply will try to produce 1000V to do it.  That may exceed its capability but it will try.  If I lower the resistor to 1 Ohm, the supply will only output 1V, still trying to push 1A.  With a constant current power supply, the voltage varies but the current stays constant.  Just what it says constant current.  Not many power supplies have this capability and I have yet to see a reason for it.

So if you set your power supply to 1000V and 1A, what do you expect to happen?
You do have a point insofar as most supplies with CV/CC are not especially stiff current sources. The performance of voltage source mode is the main  spec sheet criteria. But this doesn't mean they aren't able to be constant current, they just are not optimized for that.

[suicidaleggroll]

Constant voltage, current limiting.  Most power supplies work in this mode. 

...

With a constant current power supply, the voltage varies but the current stays constant.  Just what it says constant current.  Not many power supplies have this capability

Apart from literally every bench supply I've ever used.  From cheap chinese hunks of junk to expensive Keysights, all 3 of the supplies I have at home, all 4 of the supplies at work, all of the supplies I used in the lab in college, etc.  I've never even seen a bench supply that simply shut off the outputs when the set current limit was reached, all of the ones I've ever used will switch to constant current mode when that happens.

and I have yet to see a reason for it.

OK, it might be nice to have constant current for diagnosing control system current loops (4-20 mA) and it might be interesting for characterizing LEDs (push 20 mA, measure Vf) but these would be small supplies of limited use.

Testing voltage drop in cables and charging lithium ion batteries are two of the things I regularly use it for.

I suppose if you took a PS with foldback limiting, set it at its maximum voltage such that the set point current would always flow, independent of load (within reason), it would look a lot like a constant current source.

It is constant current, many supplies even have a "constant current" or "CC" led or message that pops up when it's doing this.  Most bench supplies have this functionality.

[Rick Law]

You will see people using 'constant current' when they really mean 'current limiting'.

I'm not sure I understand the distinction you're trying to point out.  What is the difference between constant current and current limiting in your view?  Because most bench supplies do switch to constant current mode when the load impedance drops below your set voltage / current.

Current limit = giving you what you need up to the specified current.
Constant Current = giving you the specified current all the time.
Constant Voltage = giving you the specified voltage and current is whatever the device takes
Of course, the qualifier "within the power supplies' capability" applies to all of the above.  At constant voltage of say 10V, in theory, the supply will keep it at 10V regardless of your device connected.  But, if your device draw more current than the supply is capable of supplying, the system breaks down and it can no longer give you the voltage you specified.

Using LiIon charging as an example since it goes both CC (constant current) and CV (constant voltage), and as example a charger with current limits of 500mA, and the battery is the typical "charge to 4.2V" type (so charger is voltage limited to 4.2V).

When the battery is low, it is in the CC mode.  It limits the current to 500mA.  It does that by adjusting the voltage.  If it is bigger than 500mA, it lowers the voltage, if it is less than 500mA, it increases the voltage (within limits).  So it does what it needs to ensure the supply is giving out 500mA.  Even while the battery can take more but supply will not give it more.  Thus, this is "constant current" (CC) mode at 500mA.  Supplier will change the voltage to keep the current constant at 500mA.

When the battery is near full, full enough that it cannot take 500mA any more at 4.2V.  Now, the charger now is in CV mode.  Voltage is kept at 4.2V constant (CV).  The battery draws whatever it draws (which would be below 500mA or it could not get to this mode).  When the current gets very low (low as defined by the battery type and size, say below 100mA or below 50mA), it is full.

Now if the charger (ie: the supply) is designed to be constant current all the time (no real design will actually do that.  This is just theoretical so as to understand), when the battery is near full, the charger can keep increasing the voltage as much as it can to force the current at 500mA.  Increasing voltage for LiIon will kill the battery or even cause a fire, but (in theory) you want constant current, the only way to make current go up is to push up the voltage.

Charger or a plain supply, V=IR by ohm's law.  The higher the current (I), the higher the V for a given R.

Charger or just plain PSU, as you can see a power supply cannot increase the voltage indefinitely.  Thus, in real life, it is often a current limit rather than a true constant current.  Conversely, you cannot have really constant voltage.  There is a limit to the amount of current it can produce.  So, beyond a certain point, voltage will drop because it can give you no more.  [edit: missing the word no earlier]

Hope this helps.

[helius]

....
Hope this helps.

That was a marvel of declarification, as Click and Clack used to say.

Riddle me this: Why would a bench power supply (of modest size, say 0.5 A at the wall plug) come with a shorting link across its + and - outputs? Should you be extra careful to remove it before turning it on?

[jamessinghal]

So if I use a transformer to convert the 110vac to 30vac, will it be able or will it force the extra current through

[xrunner]

So if I use a transformer to convert the 110vac to 30vac, will it be able or will it force the extra current through

Nothing is "forced" through. The load takes whatever current it may draw - depending on the load - up to and beyond the ability of the power supply to provide, in which case, either a fuse would blow or the magic smoke is released.

[rstofer]

That depends on the load.  If you have a 10 Ohm resistor and you feed it with 20 VAC, you get 2 amps.  If you feed it with 30 VAC then you get 3 amps.  Assuming that there is no AC voltage control.  Ohm's Law still applies E = I * R or, in this case, I = E / R.  If you increase E, for a given R, you will see an increase in I.

This also assumes a transformer VA rating of 30 VAC * 3 Amps or at least 90VA.  As your current gets close to the VA size of the transformer, you can expect the voltage to sag.

The load is in control, the transformer doesn't force any particular current.  It provides a voltage and the load determines the current.  For simple resistive loads you can expect the current to vary indirectly with resistance (increase resistance, decrease current).

[IanB]

I am still a newbie, and I am in the process of learning about power supplies, and wanted to know a little more about electric current.

My current understanding is that a if a power supply says 10 amps, the maximum is 10 amps, and the actual current depends on how much the device is using.

If somebody has a correction, or more information on this topic, I would love to hear it!

Yes, this is true. Although as others have said, the better word to use is electric current (as substituted above).

Constant current means just what it says.

Current limit = giving you what you need up to the specified current.
Constant Current = giving you the specified current all the time.
Constant Voltage = giving you the specified voltage and current is whatever the device takes

Constant current and current limited are qualitatively the same thing, any differences being quantitative differences in the tuning of control loops, the response time, and the regulation.

It does apparently take some people by surprise to realize this.

Consider:

If you have a constant voltage supply, 10 V, capable of up to 5 A, then: This supply will maintain an output of 10 V as you decrease the load impedance until the current tries to exceed 5 A. At this point, the voltage will drop below 10 V, either by control or by exceeding the supply capability. It is constant voltage, current limited.

If you have a constant current supply, 5 A, capable of up to 10 V, then: This supply will maintain an output current of 5 A as you increase the load impedance until the voltage tries to exceed 10 V. At this point, the current will drop below 5 A, either by control or by exceeding the supply capability. It is constant current, voltage limited.

Note the symmetry here. No supply can provide infinite current, physical limitations prevent it. No supply can provide infinite voltage, physical limitations prevent it. All supplies are either voltage limited, or current limited, or both.

More specifically, all "constant current" supplies are in fact "current limited" supplies. There is no conceptual difference.

[Brumby]

So if I use a transformer to convert the 110vac to 30vac, will it be able or will it force the extra current through

ABLE is the answer.

The other points about constant current and current limited will become clearer in time - but don't get too caught up with this at the moment.


Here's a response to a previous, somewhat similar, question:

A load and a supply are different....

This is a basic bit of knowledge that needs to be well understood - but it is not hard.

• A power supply that is rated at 12V, 5A will provide a fixed 12 volts and be capable of providing up to 5 amps of current at that voltage.
• A load that is rated at 12V, 5A will require a fixed 12 volts and 5 amps of current at that voltage.
• In all cases, the voltages must match and the current capabilities of the supply must always be equal to or greater than the demand of the load.

Examples.  Let's continue to use the 12V 5A supply for these examples.....

- A circuit (load) requiring 12V 1A will work happily.  (In fact, you could run 5 such circuits.)
- A circuit (load) requiring 12V 200mA will work happily.   (In fact, you could run 25 such circuits.)
- A circuit (load) requiring 12V 5.5A (it's just a little bit over) continuously^** would be a problem.  It might work, but the supply is overloaded.  Depending on the supply, this could cause overheating, a drop in voltage or activation of protection mechanisms and your load may not function correctly.  If you have a supply that can still deliver the power, then it will be under a greater stress than it was designed to handle - and it may suffer a shortened life.
- A circuit (load) requiring 12V 20A continuously^** will not work.  The supply will just cave in with either overheating and a drop in voltage or activation of protection mechanisms.  Depending on the supply, this could be a serious fire risk.

^** Often there are circuits that perform different operations at different times and their current requirements can fluctuate.
One example is a temperature controller with a heating element.  When monitoring, it may draw very little current which is well within the capabilities of the supply - and during these periods, all will be well.  But when the element is switched on, the current draw will go up dramatically.  If the total current required by the load now exceeds the capabilities of the supply - problems will arise.

[Rick Law]

....
Hope this helps.

That was a marvel of declarification, as Click and Clack used to say.

Riddle me this: Why would a bench power supply (of modest size, say 0.5 A at the wall plug) come with a shorting link across its + and - outputs? Should you be extra careful to remove it before turning it on?

Why would a bench power supply (of modest size, say 0.5 A at the wall plug) come with a shorting link across its + and - outputs?

No idea.  Never got a "real" bench power supply.  I would imagine it is to ensure there is no static electricity creating some kind of current that could hurt.

As to "declarification", nah, not for that.  I was hoping I expressed it in a way to aid his understanding.  (And hoping I did not misunderstand the whole thing myself.)

Seeing your note, I reviewed what I wrote and I noticed I committed one of the sins of typos I was exactly fearful of.  In the very last sentence when I wanted to say "voltage will drop because it can give you more.",  I was missing a NO there.  It should be "because it can give you NO more."

I better edit that and add the word NO in there.

Thanks, I otherwise would not have noticed.

[IanB]

Why would a bench power supply (of modest size, say 0.5 A at the wall plug) come with a shorting link across its + and - outputs?

No idea.  Never got a "real" bench power supply.  I would imagine it is to ensure there is no static electricity creating some kind of current that could hurt.

It is so you can set the current in constant current mode. You set the voltage with the output open (infinite impedance) and you set the current with the output closed (zero impedance).

To set up the power supply this way you install the shorting link and adjust the current dial to the current you need. The voltage will be (about) zero at this time due to the short circuit. Then you remove the shorting link and adjust the voltage dial to the voltage limit you want to be applied. That gives you a constant current supply subject to an upper voltage limit.

[Rick Law]

Why would a bench power supply (of modest size, say 0.5 A at the wall plug) come with a shorting link across its + and - outputs?

No idea.  Never got a "real" bench power supply.  I would imagine it is to ensure there is no static electricity creating some kind of current that could hurt.

It is so you can set the current in constant current mode. You set the voltage with the output open (infinite impedance) and you set the current with the output closed (zero impedance).

To set up the power supply this way you install the shorting link and adjust the current dial to the current you need. The voltage will be (about) zero at this time due to the short circuit. Then you remove the shorting link and adjust the voltage dial to the voltage limit you want to be applied. That gives you a constant current supply subject to an upper voltage limit.

Geeze...  Now I understand the word "cheap" in "cheap eBay power supplies".  My $10 cc power board from eBay doesn't give me a wire but just had the instructions on setting current written on the item description page.  I had to get my own wire to short the damn thing to set the current!

Damn cheapies...

Good thing I upgraded to $13-$16 boards from AliExpress...

(Back to being serious, thanks IanB - I learned something new...)

[helius]

No idea.  Never got a "real" bench power supply.  I would imagine it is to ensure there is no static electricity creating some kind of current that could hurt.

OK. An honest answer merits a serious response:
Most (almost all) lab power supplies are CV/CC, meaning they can operate as either constant voltage sources or constant current sources. There is no switch to select either mode; instead, the supply switches at the "crossover point" where the load resistance \$ R = {V_0 \over I_0} \$ (for the V and I settings on the supply). In other words, it switches automatically to maintain instantaneous \$ V \le V_0 \$ and instantaneous \$ I \le I_0 \$. This also makes the supply overload and short-circuit protected, since a short circuit will supply the programmed current I across nearly zero volts, which is effectively zero power.
Having a shorting link across the + and - outputs is what you would do to set the current control (or current limit; they are the same). Just like leaving the outputs open is the way you set the voltage control (or voltage limit; they are the same). The term for this open/short relationship is that CC and CV are dual to one another.
The characteristics of a supply are also dual to those of a load: a load can be "constant current" or "constant voltage", too. The V/I graph has four quadrants, two of which supply power (I and III) and two of which consume power (II and IV). Some instruments can operate through all four quadrants, meaning they automatically cross over from supply to load the same way that the common PSU crosses over from CV to CC.