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.

 

Hello,

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

Bertus

 

Hello,

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

Bertus

Do you think i have connected the terminals on the right wire in my LM7905? I'm trying to copy the circuit i have attached.

 

Hello,

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



Bertus

 

Hello,

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

View attachment 137866

Bertus

Thanks Mate!

 

Hello,

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



Bertus

 

D1 and D3 anodes are shown connected to ground in multi.png and m.png. They shouldn't be.

 

Hello,

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

View attachment 137875

Bertus

I didn't try it yet. But i will put it on my circuit. Thanks!

 

D1 and D3 anodes are shown connected to ground in multi.png and m.png. They shouldn't be.

Thank you mate. The problem is fixed. is it normal that the reading in multimeter is -5.591 instead of -5 volts something ? if not. how can i fix this?

 

Hello,

The higher voltage could come from the missing decoupling capacitors.

Bertus

 

Some regulators need a minimum load current in order to operate as per spec. Check the datasheets.

 

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

 

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

 

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.

 

It would seem there's a problem with the LM7905 model.

 

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 .

 

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.


Allen

 

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

 

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?

 

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?

I'm also trying to build it with a variable 0-30 volts.