EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: pyroesp on August 07, 2016, 06:23:50 pm
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I made the schematic below in LTSpice XVII, and everything seems to work like it should, but there's still something I don't quite get in the simulation...
This schematic should be able to output 0-15V from a 0-5V signal and 0-5A current limit from a 0-5V signal.
Probably best to use more powertransistors in parallel, but this is just a test.
The schematic is set to limit the current at 500mA. With a resistive load of 10Ohms, the output voltage won't go higher than 5V.
The simulation is set to DC Sweep of the V_set voltage from 0 to 5V.
(http://i.imgur.com/c0u9lkr.png)
So here is what I don't understand. In the graph below you have Vfdb1 being the voltage at the output of U2 and Vfdb2 being the voltage after the voltage divider (1/3).
Why does the voltage at Vfdb2 start to increase after the 2.5V mark on the horizontal axis, if the voltage at Vfdb1 does not increase?
Am I just not seeing how the voltage at Vfdb2 can increase or is this a simulation issue ?
(http://i.imgur.com/9YVZ8qn.png)
Attached is the schematic, if you want to try it out.
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The schematic is set to limit the current at 500mA. With a resistive load of 10Ohms, the output voltage won't go higher than 5V.
That's what's supposed to happen with a 500mA limit.
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That's what's supposed to happen with a 500mA limit.
I was just stating the fact that the power supply schematic seems to work correctly. Sorry for the misunderstanding.
My question is about the voltage Vfdb2 in the graph.
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In that case U1 is struggling to keep both it's inputs at the same voltage, when the voltage difference gets big enough, current starts to flow in the input protection diodes.
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When going into current limit, U1 can not get both input at the same voltage.
I'll check the LT1002 datasheet to see if what you say is possible.
EDIT: Well... should have checked the datasheet beforehand. From the simulation it looks like the voltage at Vfdb2 starts to rise at around a difference of 1~1.2V.
The datasheet shows 2 diodes in series going from input to input, where current can flow through.
@StillTrying: Thanks for the help.
EDIT2: Added a diode between the voltage divider and negative input of U1. I can clearly see the voltage rising on the negative input and Vfdb2 staying at 1/3 of Vfdb1.
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Don't forget you can see the actual current flowing into an input by probing the edge of the op amp symbol where the input is. Now I've got the sim to run I can see up to 1.6mA flowing into +in and out of -in.