Need a transformer to boost 5VDC from a SMPS to 8V, what do I need?

[fastline]

I have a multi output PS for a machine that is real close to what we need. One of the cars supplies 40A of 5V so I want to supply my 5V directly from that as well as parallel a boost transformer to get my required 8V from it as well. Other voltages include -5 and 24V.

I want to boost the 5V so even in a worst case, I will never over volt the 8V side. What kind of transformer do I need? I will need no more than 10A of 8V and will probably add my own fuse in the system since the 40A supply is extreme in this case.

 

[ben7]

I have a multi output PS for a machine that is real close to what we need. One of the cars supplies 40A of 5V so I want to supply my 5V directly from that as well as parallel a boost transformer to get my required 8V from it as well. Other voltages include -5 and 24V.

I want to boost the 5V so even in a worst case, I will never over volt the 8V side. What kind of transformer do I need? I will need no more than 10A of 8V and will probably add my own fuse in the system since the 40A supply is extreme in this case.

well I suppose you could use the +5v and -5v to get 10v, just need to drop two volts then, or use a switching regulator on the 24v rail to give you 8v. With 24v input and 8v output a linear regulator wouldn't do so well.

I would use my first suggestion unless the -5v rail is not capable of handling that amount of current required(A load ? unknown).

-Ben

 

[crutschow]

You could use a boost switching regulator from the 5V to get the 8V (you need more than a transformer). But remember that 10A for the 8V will require more than (8/15) * 10 = 16A (@100% efficiency) from the 5V supply.

You could also use a buck regulator from the 24V which would require more than (8/24) * 10 = 3.33A from the 24V supply

 

[crutschow]

well I suppose you could use the +5v and -5v to get 10v, ...

That only works if the 8V circuit does not share a ground with rest of the circuits.

 

[fastline]

I have 2 5V cards rated at 40A each. I have to invert one when I chain them to get my -5 reference since each card is isolated. I guess my concern with using a higher voltage was what IF it shorted and ran the higher voltage out? This is a pricey CNC machine supply so I want to make sure that can't happen but just maybe regs and such do NOT fail shorted.

I am not much familiar with switching regs but certainly open to discuss ideas here. We are actually not totally sure what all the 8V rail supplies but know it most likely powers basic components like the keyboard and some common switches. The voltages are checked by the control boards for presence and if they are missing, the machine will not boot up completely.

I certainly am open to solid state voltage tuning here. There is actually another 12V card in the supply but when powered, a pack of resistors starts smoking and I have not figured it out and thought I would just use another card. Guess I could also try to fix the 12v card but I am a little lost in it. I recently just replied in the repair section with pics of that card. I do have options here, just need help deciding the safest route to go.

 

[Diver300]

What is the load that needs 10A at 8V?

At that sort of power, a separate power supply seems the easiest.

Also, if you are needing -5V, how much current is needed there? 40A at -5V seems like a lot.

 

[fastline]

I don't need anything close to 40A. Actual values are still unknown because we don't know what all is powered by what voltage. The books for the machine are MUCH LESS than helpful. A rough guess based on the previous supply would be no more than 10A for each of the 5 voltages and probably not even that.

I am hoping I can repair this 12V supply and use a regulator on it but that still brings up the question, can they fail shorted in a manner that would send the full 12V to the field?

 

[crutschow]

What happens when a power supply fails depends upon the design.

Certainly fuses can protect against a short circuit current.

But, depending upon the design, a regulator can fail such that it applies the full input voltage to the output. For such an over-voltage you can add a crowbar circuit that clamps the output (typically using an SCR) if the voltage rises above a preset value. At that point the fuse will blow.

 

[fastline]

Relative to the design of this PS, I feel pretty comfortable with its designed voltages but I am just worried about the voltage that I generate or create off another output. IE creating 8v from 12v. There are tuning pots on each output and in playing with it, if it gets more than .5v out of spec, the output ceases. I can only guess a feedback circuit is in place to sample the voltage to protect devices