Constant Current Source or Current Limiter?

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cspwcspw

Joined Nov 8, 2016
59
I'm working through some nice material from TI about comparators. This schematic is a simplified "innards" of a comparator. They refer to Constant Current Sources. But in LTSpice, a Constant Current Source seems to be a different thing: if I use a 1 amp current source across a 1Meg resistor, I get a million volts appearing in LTSpice. So these things seem not to be "sources" from what I understand, but limiters. Are the two ideas used interchangeably, or sometimes ambiguous? How would I simulate these in LTSpice? Just hook up one end of the current source to the 15V rail?

Thankscomparator.png
 

BobTPH

Joined Jun 5, 2013
2,255
No, it is a constant current source. You just picked an example that is far from what you find in that comparator.

Try 1 mA and 1K.

Bob
 

eetech00

Joined Jun 8, 2013
1,835
I'm working through some nice material from TI about comparators. This schematic is a simplified "innards" of a comparator. They refer to Constant Current Sources. But in LTSpice, a Constant Current Source seems to be a different thing: if I use a 1 amp current source across a 1Meg resistor, I get a million volts appearing in LTSpice. So these things seem not to be "sources" from what I understand, but limiters. Are the two ideas used interchangeably, or sometimes ambiguous? How would I simulate these in LTSpice? Just hook up one end of the current source to the 15V rail?

ThanksView attachment 198349
hi

those symbols usually represent current mirrors in an IC.

eT
 

crutschow

Joined Mar 14, 2008
24,330
But in LTSpice, a Constant Current Source seems to be a different thing: if I use a 1 amp current source across a 1Meg resistor, I get a million volts appearing in LTSpice.
Not different.
An ideal current source generates current independent of the load, so 1A through a 1Meg resistance is 1Meg volts.
 

ebeowulf17

Joined Aug 12, 2014
3,153
A real world current source is different than a simulated one, in the sense that real ones can't generate higher voltages than they're provided.

So, a current mirror that's powered from a 5V supply, and configured to deliver 1A, will "want" to deliver a million volts across a 1 Meg load resistor, but in reality it can only deliver the 5V it has to work with (probably slightly less due to losses.) That same circuit, provided a load of 5 ohms or less will deliver 1A as intended at whatever voltage it takes to accomplish that, but any load greater than 5 ohm will make the circuit fail to deliver its intended output.
 

Alec_t

Joined Sep 17, 2013
10,906
If you want to simulate a current limiter you can use a diode model with an ilimit value specified in its model statement, e.g. .model md d(vfwd=1m ron=1 roff=1Meg ilimit=10m).
 
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