Connecting several power regulators to a single voltage input

Question

I am trying to make a power source to feed the following:

• 3x DC motors, that need to work under changeable voltage; not supposed to operate over 2A RS385-ST
• 1x Servomotor 12V not over 1.5A
• 2x 5V 1A for this L293D
• 1x 9v 0.1A (for control unit)

I am using a LM317 to control the voltage each motor receives; so I am using three of this regulators, one for each motor. The picture shows the circuit. The motor loads are modeled with resistances. Circuit in picture does not include other regulators:

As noted there is also a couple of 7805 for the 5V outs, one 7812 for the 12V servomotor and a 7809 for the control unit's power. They are all part of the 78xx family. All the regulators are connected to the bridge's output.

My problem is that no matter what capacitor I use for smoothing the bridge's output, the simulation fails. I am simulating in Multisim.

I think that my problem is that the input voltage is too high for some of the regulators. All ideas, from changing the circuit to finding a different duty, are welcomed.

Answer 1

Two things to look at (but I don't promise either one is the solution):

• Most simulators like every node in the circuit to have a dc path to ground. In your circuit, the primary side of the transformer is fully isolated from ground. You can solve this by arbitrarily connecting one of the two sides of the ac source to ground, or by connecting one of them to ground through a high-value resistor.

• 430 mF is a very large capacitor. This value could result in an ill-conditioned problem for the simulator. When the capacitor is charged up (assuming you're doing a transient simulation and the capacitor starts out at 0 V) there will be very large currents, which could lead the simulator to giving up on the grounds that the results are unreasonable. Try reducing this value or adding a small series resistance (like 1 ohm or so, maybe less) to C1.