Help with dual output power supply

[keiththevp]

Hello,

I am trying to create a dual source power supply that on one side can supply 0-150v DC low amps and on the other side 0-20v DC high amps. I have attached a VERY basic schematic that shows all the parts I am planning on using. Now here is my problem... both sources use the same output to load, therefore I am worried if one side is on and the other is off that the side that is on will back feed power into the side that is off and create a problem. If you wouldn't mind checking the attached drawing and letting me know if it is OK or if I need to add some more protection to stop the two sources from interfering or damaging each other. The only abnormal thing in the wiring is the transformers on the 0-20v DC side. Their DC ground output is on the transformer and NOT coming off the bridge rectifiers. This is the part that worries me. I believe that the bridge rectifier on the 150v side would stop any back-feed of current but since it can not be wired that way on the transformer side I am worried they will be damaged.

Thank you in advance for any help you electronic experts can offer!!!

Thanks,

Keith

 

[MikeMl]

Your diagram makes no sense. You are trying to build two power supplies: a 150V low current, and a ~20V high current. That means you will effectively have four output terminals which I think you are trying to combine. Lets call them 150+, 150-, 20+, and 20-. Are you asking if it is OK to connect 20- to 150- together to create a Common node, so that you effectively have three output terminals: +20V,+150V, and Common? The answer is yes, provided that both supplies are "transformer isolated" between their respective 120VAC line input and their rectified outputs.

Looking at your very confusing block diagram (why didn't you just post a schematic?) I do not see any transformer in the 150V supply, which means no isolation from the 120VAC to the rectified output, which means it cannot be combined with the 20V output at all. This creates a shock hazard and an instant explosion if either wire from the 150V supply touches either wire from the 20V supply.

A solution is to get a 120V-120V isolation transformer, and add that to the 150V supply.

 

[keiththevp]

Well on the 150v side there is a variable transformer(variac) 110v ac in and 0-130v out then it goes to the BR and then to the load. If I knew how to make a schematic I would but the free programs I tried were all too confusing or didn't have the right component library. Basically the only reason I wanted the outputs to be on the same terminals was so I could put a common shunt on them for the amp and volt meter. So if I am using a variac on the 150v dc side then can I have a common ground but no common positive? Forgive my very limited knowledge in this area but that is why I am asking before I build and have a problem.

 

[keiththevp]

Or what about just adding a second ac solid state relay to the neutral wire between the bridge rectifier and the variac. Or is it better to just get two meters for voltage and amperage? and use four output terminals?

 

[MikeMl]

A Variac is a three terminal device (autotransformer) which provides no isolation between the Line side and the load side. To get isolation, you use the Variac to control the voltage to the primary of an 1:1 isolation transformer, and then connect a FWBR to the secondary of the isolation transformer. This gives you an isolated output which can be combined with the other transformer-isolated supply.

Do you have a scanner? Draw a schematic using a 2B pencil and then scan it.

 

[MikeMl]

Here is what happens with no isolation:

Note that the differential voltage V(Plus)-V(Minus) is the expected DC voltage with some ripple. However, look at V(Plus) and V(Minus) individually. With respect to earth ground, they both have the full line voltage superimposed on them.

 

[keiththevp]

Mike first off thank you for your replies and schematic. I see what you mean about the isolation on the 150v side, I have seen other people do this with no isolation so I didn't think it was a problem. But for safety it makes a lot of sense in case the BR goes bad.

So if I get an isolation transformer for the 150v side and cut the hot and neutral wires coming off the variac (going to the isolation transformer and the other three transformers) with a DPDT switch to completely isolate the two power supply circuits will this allow me to have only two output terminals?

Thank You for all the help. I really want to get this right before I build so there is no danger to me or the components.

Keith

 

[MikeMl]

No, you cannot parallel a 150V supply with a 20V supply. The best you can do is to have three output terminals: +20V, +150V and Common, as I said previously.

 

[keiththevp]

Can you explain why you can not just use a DPDT switch to control the two power supplies? What parts will be damaged and why? What would happen if you charged a capacitor to 150v dc then hooked this capacitor to a 12v dc power supply?