# The Naked Scientists Forum

### Author Topic: How does my generator control the magnitude of current?  (Read 3901 times)

#### manjit

• First timers
• Posts: 7
##### How does my generator control the magnitude of current?
« on: 19/10/2008 18:19:39 »
Hi

I have a square wave generator that can produce upto 26VAC. But the amount of current that is available is only 125 miliamps. I wonder if it controlled by a fuse or the internal impedance of the generator is quite high so that it does not allow more current than 125 miliamps. Also what would be the difference if one increases the voltage to 36VAC, but still the upper limit of the current is 125 miliamps?

Thanks for any input!

manjit

Mod edit - Formatted the subject as a question - please do this to help keep the forum tidy and easy to navigate - thanks!
« Last Edit: 28/10/2008 11:07:27 by BenV »

#### lyner

• Guest
##### Re: How does my generator control the magnitude of current?
« Reply #1 on: 20/10/2008 11:43:33 »
It could be limited simply by resistance. You could find out by measuring output volts for a range of currents. It would yield a straight line graph. The slope of the graph would give you the source resistance.

The output stage transistors are probably arranged to go into current limiting - a regulator circuit to limit the dissipated power. Your graph would then show a more or less constant output volts, followed by a sharp drop as the current exceeds 125mA.

The Collector of a transistor behaves like a 'constant current source' under a large range of conditions - until the transistor is near saturation- so it is easy to limit the current.

#### manjit

• First timers
• Posts: 7
##### Re: How does my generator control the magnitude of current?
« Reply #2 on: 21/10/2008 20:48:29 »
Hi!

Thanks a lot! That is a good explanation!  I will try measuring current.

manjit

#### lyner

• Guest
##### Re: How does my generator control the magnitude of current?
« Reply #3 on: 21/10/2008 23:05:19 »
And volts!

#### techmind

• Hero Member
• Posts: 934
• Un-obfuscated
##### Re: How does my generator control the magnitude of current?
« Reply #4 on: 26/10/2008 23:57:31 »
You might find that the output is buffered by an op-amp and has a seemingly low output impedance for "allowed" loads, ie very little change in voltage as you load it from 0 to 125mA ... but that if you draw more than 125mA for more than a few 10's of seconds that the op-amp overheats and burns out. It all depends what your source is and whether you expect it to be designed to be (relatively) indestructable or not.

I'm saying that internal power-dissipation might be the limiting factor. For example, 26V at 125mA is just over 3watts...
« Last Edit: 26/10/2008 23:59:42 by techmind »

#### lyner

• Guest
##### Re: How does my generator control the magnitude of current?
« Reply #5 on: 27/10/2008 10:31:59 »
An op amp output stage can still be made to behave as a constant current source, though, by appropriately feeding back the current output. By measuring the V/I characteristics you can't tell what, exactly, is in the box.
Don't forget, the output stage of an op amp will consist of two complementary bipolar transistor collectors or FET drains (current sources), the low output impedance is achieved by internal feedback.
Feedback can make anything happen, almost.

#### techmind

• Hero Member
• Posts: 934
• Un-obfuscated
##### How does my generator control the magnitude of current?
« Reply #6 on: 28/10/2008 20:38:52 »
Don't forget, the output stage of an op amp will consist of two complementary bipolar transistor collectors or FET drains (current sources), the low output impedance is achieved by internal feedback.
Feedback can make anything happen, almost.

Indeed - until the smoke comes out, at least!

#### The Naked Scientists Forum

##### How does my generator control the magnitude of current?
« Reply #6 on: 28/10/2008 20:38:52 »