Need idea about voltage ranging

[joe_1]

I want to send different voltages to same device using 3 different 5V I/O from PIC.
For example, if user selected Range A, the PIC will send I/O #1, which will go as input into a circuit that I need idea to build, and then this circuit will output 40V, 1A
This is the table:

Range A --> I/O #1 selected 5V from PIC will input into circuit; 40V, 1A output
Range B --> I/O #2 selected 5V from PIC will input into circuit; 20V, 1A output
Range C --> I/O #3 selected 5V from PIC will input into circuit; 10V, 1A output

I tried to use resistor/transistors, but got no where.
If you have any idea, please let me know, meanwhile I will keep thinking.

Thanks.

 

[Sceadwian]

The first solution that comes to mind is using a variable linear regulator and simply using a few transistors to switch different resistances from the feedback line, the main problem is that linear regulator will have to dissipate 30watts of power at the 10v 1A (if fully loaded) setting which is going to require a good heatsink and cooling fan. You will also need to provide more voltage than 40V's to the regulator (generally 3 volts over is a good place to start)

A switch mode regulator would be a better idea as it will dissipate significantly less heat.

 

[alec_t]

I agree; SMPS is the way to go. There is sure to be a delay before the output stabilises at a new selected voltage, however. How rapidly do you need to change voltage?

 

[languer]

Are the +40V, +20V, +10V already available and all you need is to switch them? Or do you need to create the +40V, +20V, and +10V?

 

[joe_1]

Yes, these voltages are already available, and all I want is to use them.
How fast the switching? The speed is not really very important.

 

[languer]

You could add them using relays (see relay example).

You could use beefy transistors to switch them on (e.g. Load Switch; or something like Pushbutton Power Switch) - making sure you switch all voltages off before switching any of them on (and of course, no two can be switched at the same time).

I would have all three voltage inputs into whatever load it is going through diodes (so as to protect back-feeding into the supplies). I would also a fuse each of the inputs.

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