+5V and -5V Dual Regulated Power Supply

Thread Starter

Charlie Magne Aguda Ü

Joined Oct 24, 2017
33
I'm trying to simulate a power supply circuit that i have found online in multisim. The problem is that the -5 terminal is showing wrong value. What do you think is the problem here? (I'm a Beginner :) ) I have attach some screenshot and the file from my circuit in multisim.
 

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bertus

Joined Apr 5, 2008
20,041
Hello,

It looks like the negative regulator type is wrong.
Could a LM337 be used instead of the 7905?

Bertus
 

bertus

Joined Apr 5, 2008
20,041
Hello,

As for the pin-out of the regulators have a look at the following picture:

3-pin_regulators_pinout.png

Bertus
 

bertus

Joined Apr 5, 2008
20,041
Hello,

An other thing. Did you use the decoupling capacitors to avoid oscilations of the regulators?

Dual_supply_decoupling_capacitors.png

Bertus
 

bertus

Joined Apr 5, 2008
20,041
Hello,

The higher voltage could come from the missing decoupling capacitors.

Bertus
 

Alec_t

Joined Sep 17, 2013
10,350
Some regulators need a minimum load current in order to operate as per spec. Check the datasheets.
 

bertus

Joined Apr 5, 2008
20,041
Hello,

As @Alec_t said, the regulator might need a minimum load.
On the positive you have a 1K resistor, on the negative not.

Bertus
 

Thread Starter

Charlie Magne Aguda Ü

Joined Oct 24, 2017
33
Hello,

As @Alec_t said, the regulator might need a minimum load.
On the positive you have a 1K resistor, on the negative not.

Bertus
What should i add to fix this problem? i try adding a 1k resistor on the negative side but it's still the same output. i really don't know what to do here now. i did also try to add those missing decoupling capacitors, still no effect
 

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dl324

Joined Mar 30, 2015
8,849
What should i add to fix this problem? i try adding a 1k resistor on the negative side but it's still the same output.
Try increasing the load to 10mA. That seems to be a common requirement; even if not stated. You just need to dig it out of the datasheet.

EDIT: I just checked a datasheet and it seems that 5mA should be sufficient. But, I've had LM317 that wouldn't regulate unless the load was more than 10mA (which is the minimum stated in the datasheet). It was an out of spec part, but what could I do? Complain??

I'm surprised that MultiSim cares about minimum regulator load.
 

Alec_t

Joined Sep 17, 2013
10,350
I too would query the accuracy of your Multisim model. The 7905 output tolerance specified by Texas Instruments is ±4%, which your model clearly exceeds.
That said, the LTspice model gives Vout = -5.3V, which is also outside the TI spec :confused:.
 
Last edited:

absf

Joined Dec 29, 2010
1,942
What should i add to fix this problem? i try adding a 1k resistor on the negative side but it's still the same output. i really don't know what to do here now. i did also try to add those missing decoupling capacitors, still no effect
What are the voltages before entering 7805 and 7905? I guess it was too high...

My simulation with proteus shows reasonable voltages at the outputs even without the 1K resistors.

+_- 5V psu.PNG
Allen
 

Thread Starter

Charlie Magne Aguda Ü

Joined Oct 24, 2017
33
Thanks for all of your reply. I have found a solution i just add some 700ohms resistor in parallel with C8 the voltage seems ok now. thank you all :D
 

JoeJester

Joined Apr 26, 2005
4,077
That's one way to do it. Not a good way, but still, one way.

Any particular reason you want to drive those 5 volt regulators with 22V?
 
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