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noobie problem with voltage regulator simulation

Hi

I am trying to simulate a very basic +/-5V regulator circuit taken straight from the LM7905 data sheet.  I have 7805 and 7905 fixed regulators supllied by 9V batteries that should give me +5V and -5V output.  The +ve output is 5V as expected (channel D), but the -ve output is -5.6V (channel A).   (chanels B and C are the input voltages).   My mistake must be simple, but I can't spot it.  Screen capture is attached. 

thanks

George

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Message 1 of 5
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Try putting a load resistor from each output to ground.  Depending on the construction of the model for the regulator, it is possible that it needs a minimum load.

 

Lynn

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Message 2 of 5
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Thanks Lynn I tried that with a 1k load and that resulted in an output voltage of -5.3V, which is still completely unsatisfactory from a simulation package. 

 

I swapped the LM7905 for another -5V regulator, the UPC7905, which gave the correct output but failed after a few seconds.  The convergence assistant suggest to switch the integration to the Gear algorithm, which resulted in a stable and correct output. 

 

So I went back to the LM7905 but that still resulted in the incorrect output.

 

So Multisim cannot simulate the output of a garden variety, ultra common, component like the LM7905.  This is unacceptable.

 

Perhaps changes to other options in the "Custom Analysis Options" dialog will improve the convergence.  If anyone has suggestions I would be most gratefull.

 

Thanks

 

George

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Message 3 of 5
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By simple trial and error I was able to figure out that with  a 462 ohm load resistor the output of the LM7905 is -5V as expected, as to why this is and where this specific value comes from I haven't been able to determine that, but it doesn't seem to have anything to do with just a simple minimum load resistor, and I haven't seen  any particular option in the Custom Alaysis Options that should correct this.

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Message 4 of 5
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If you look closely at the datasheet for the LM7905, you will see that the nominal test condtions are at an output current of 500mA. The output regulation is specified for a current change from 5 mA to 1 A. The maximum value is 100 mV.

 

Although the datasheet does not specify or call for any minimum load, it also does not specify the performance at no load.

 

So what you are seeing at light load/no load in simulation may not be far from the behavior of the real circuit.

 

Lynn

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Message 5 of 5
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