Problem with simulation

[azeemakhlaq]

i am trying to make a Variable DC voltage supply ranging from 0V to 32V with a fixed 1A of current. i have a schematic googled from a thread but it will give to 30V. unfortunately i can't simulate it with Multisim. Please help design and schematic is attached. i have tried my best but again and again convergence simulation error appears. i didn't know what happen to the circuit? please help ASAP! please made any changing to make it a DC variable voltage supply 0-32v,1A 😞 😞

[johnsold]

I do not have Multisim so I cannot look at your simulation.  The comments below apply in general to SPICE analysis. Some specific items amy be different for Multisim.

The first thing that occurs to me is that you need to do a DC or transient analysis on all of the circuit except the AC input. When you do a DC or transient analysis, all the AC sources are set to their DC values, which typically are zero. If you have connected a source like V1 1 2 AC 35 at the AC input, you will not have any DC. The source connected to the AC input should be a SIN source, something like Vin 1 2 DC 0 SIN(0 35 50 0 0), where the parameters are Vo = offset voltage, Va = amplitude = Vrms*sqrt(2), Freq (Hz) = 50 (or 60, depending on whereyou live), TD (delay), and Theta (damping factor). Note that the op amps will not have any power applied until the sinusoidal signal builds up. Depending on how the model is set up that lack of power might be a consideration in the convergence errors.

Another thing is to make sure that the Vin source is not connected to the negative output node. I would make the negative output node ground or node 0.

If those ideas do not solve the problem, try disconnecting US1, pin 11. Add a DC source at the maximum rated voltage for the LM324 between US1-11 and US1-4. This will provide power to the op amp at all times. With the op amp active at the beginning of the simulation, it might converge.  From looking at the schematic of the LM324 it appears that it will not function until the power supply voltage is at least 4*Vbe for the transistors. The datasheet specifies Vcc minimum of 3 V.  The model may not accurately simulate the behavior for voltages less than the specified minimum.

Lynn

[azeemakhlaq]

This is what i am trying to simulate. i can't point out what is the problem with the circuit..

[johnsold]

If V1 is an AC source and Multisim is doing a DC or transient analysis, your circuit has no power.  AC sources are only used for small signal ac analysis.

Lynn

[Tien_P]

Hi Azeemakhlaq,

I've looked at your Multisim circuit and here are a few issues I want to point out:

1. A lot of parts you selected are rated parts, this means they blow up when power apply to those parts exceed a certain limit. I suggest you avoid using rated parts, there is extra modelling in the background that makes them blow up and this adds another level complexity to your circuit. There is resistor and capacitor family in the Basic group that are ideal, use those parts.

2.  You bridge diodes are not correctly wired, you should flip the diodes D4 and D6 horizontally

3.  The schematic you posted doesn't look right to me. The rail voltage for the opamp US1A is connected in reversed.  The negative rail is connected to the positive side of the rectifier, and positive rail is connected to the negative side.  Like Lynn, I suggest you place a DC source for the op-amp rail.

4.  The schematic shows a max input AC voltage of 25V, you placed a 225Vrms voltage source in Multisim. The components selected in the schematic BOM may not be appropriate for 225Vrms.

[azeemakhlaq]

I have done what you have said. still facing simulation convergence error. plz take a look at schematic... please made it a 0-32 V, 1A variable DC supply plz..... 😞 😞

[Tien_P]

Hi Azeemakhlaq.,

The attached circuit is simulating. I adjusted the circuit spacing to get more room and noticed there were some wiring errors; sorry I don't recall where it was. 

The opamp is a multi-section component, if selected all the sections from one IC you only have to wire the rail voltage to one section, the other three sections will automatically connected to the rail as shown by the x in the attached circuit.  In your circuit you used four different ICs, three of them are connected to the DC supply and one is connected to the bridge (U1A).

[azeemakhlaq]

didn't get your solution. the output i want isn't coming out. i want to build a variable DC supply with 1A fixed current and 0 to 32 adjustable voltages.. can you give me a final solution for this problem? i am tired of simulationg it but couldn't get the output.. if you have any better schematic than me! so suggest me plz... 😞 😞

[johnsold]

Are you saying that you want a 1 A constant current output, so long as the load resistance is between 0 and 32 ohms?  That is a very different circuit.  It is not a voltage regulator.

Please tell us what you really want. Do you need a circuit design as opposed to just a simulation?

What is the load resistance? How does the user control the circuit? What kind of safety or protection features do you need?  What is the exact output (voltage and current) that you expect for a particular input condition?

Lynn

[azeemakhlaq]

i will let you know my complete requirement. i'm not getting my desired schematic from google. here i mentioned what i want:

- A Variable DC Voltage Supply whose output voltages ranges from 0v to 32v and at all voltages the output current must be constant at 1A.

-Use Potentiometer to change Voltages at constant current 1A.

- Input source must be a 220v AC source (Vrms) which is to be step down by a transformer and rectified by a Bridge Rectifier..

-Supply Protection Circuit i.e. a Fuse must be present in the circuit

-Output Voltages Must be on 7-segment LED display and Constant output current i.e. 1A must also be shown on 7 segment LED display.

The above desired circuit must be made of simple component which are easily available in the market. Use what ever the model of the components.

please help as soon as possible i am trying my best for this but continously getting failure.. 😞 😞 😞