# Finding maximum current experimentally

How is the maximum current that a certain circuit can provide found experimentally? Say, that of a power supply circuit? Can it be done without destroying the circuit?

I'm looking at designing some circuits on my own and learning through experimentation. Current limits are clearly stated for ICs in their datasheet, but when it comes to discrete components, it's not so obvious. I can look at the maximum current provided by each component, but it would also help to have that confirmed through experimentation to make sure I actually understand what I'm doing.

• You haven't accepted any of the answers to your previous questions. You do realize that accepting a good answer works motivating for those who try to help you? Aug 12, 2011 at 4:25
• It hasn't occurred to me. I'll go do that. Aug 12, 2011 at 4:52

You need to note the difference between "can provide" and "is designed to provide". I'll call the latter "should" provide.

Many device can provide much more current than they should provide. Wll engineered devices will be equipped with protection to prevent attempts at overload causing damage. A typical example of this is a fuse - which "blows" when a circuit provides several times the current it should.

Whether something is damaged by current overload depends very much on the device and on the designers ability and/or the application. As a general guide, determining current by overloading is not likely to be wise and will often cause damage - often catastrophic but sometimes paertial. Partial damage may not be immediately obvious and may be cumulative.

Well designed power supplies will not be damaged by current overload. Maximum current is usually into a "short circuit" although some supplies use "current foldback" to reduce fault current until the overload is removed. More usual is for the supply to drop it's output to a low value. Or a fuse or circuit breaker may be fitted.

The maximum current that can be drawn from AC mains is set by design - usually breakers or fusing at each level. Without these a domestic outlet could provide 100+ amps and lots of smoke.

Small batteries may suffer a degree of damage if temporarily shorted. High energy batteries such as NimH and LiIon may melt or catch fire or come close to exploding - especially LiIon.

Shorting a car battery MAY be the last thing you ever do.

• I actually meant circuits that I designed myself. In any case, your answer's still informative. Should I take this to mean that it's not possible and I should refrain from trying? Aug 12, 2011 at 5:05
• @howardh - NO :-) !. take it to mean that situations vary widely and having more detail will help people provide a better answer. If you can give specific or more specific examples we can give good replies. As Steven says, many power supply IC's are protected against damage - but not all are. If you are taking output from a FET or other transistor then it will usually be able to be destroyed if you have not specifically added protection to prevent this. Do you have a circuit example? Aug 12, 2011 at 5:46
• I guess it was just a bad question then. I don't have a more specific situation yet. Aug 13, 2011 at 19:37
• Oh yes, now I remember. I was asking because I wanted to build a voltage regulator using a zener diode. I thought making the question a bit more generic could get me responses useful for other projects as well, but it looks like I was wrong. I'm following instructions from link. The website shows how it's calculated theoretically, but I'm interested in doing it experimentally.comments may only be edited for 5 minutes(click on this box to dismiss) Aug 13, 2011 at 20:37

Depends. Most three-legged regulators like the LM78XX have built-in current limiting, and often also overheating protection, so especially when you do it gradually you can find the limit experimentally.
When the maximum allowed current is reached the current limiter sets in and the output voltage will collapse. More advanced current limiting is foldback current limiting, where apart from the output voltage collapse also the current is decreased drastically, to prevent the power supply's overheating and protect the powered circuit from (further) damage.

Foldback current limiters have to be reset (simply switch off) before they operate normally again.