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Power conversion

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vipsam10

New Member
Hi.
I recently got a project about power conversion. I have an equipment which requires +24V DC. I am currently using two 12V batteries to get the required voltage. I need to replace the two batteries which get damage too often with an alternate source. I have as option to convert the output of a rectifier (230VAC to 48VDC) to +24V. The issue here is that the +ve terminal of the rectifier output has been earthed and only the positive terminal is accessible. As such i need to convert -48V to +24V. Note that i cannot use an additional power supply, i cannot use another rectifier and i cannot use an inverter as well. Is such a project feasible ladies and gentlemen?
Any proposal will be most appreciated.
 

Hero999

Banned
Where did he say that?
 

vipsam10

New Member
Thx hero and 3vo. In fact i forgot to mention that i should use only the -48V supply available. Opamps require +/- 15V supplies, so i should think in another direction. 48 to 24 dc-dc may work, but i need additional triangular supply to achieve PWM and then control the switchings of my MOSFETS. Yes i do need a max of 1.2Amps for my circuit. The main problem is that the positive terminal of my source has been earthed for practical reasons and that earth should remain there. So i only have access to the -48V terminal w.r.t earth. I have to invert the polarity from -48 to +48 and then use a potentiometer to get +24V....how to invert the polarity guys??? Its from -ve to +ve! Thx
 

tcmtech

Banned
Most Helpful Member
I would check out one of the countless isolated DC to DC converter units at one of the many on line electronics supply company's. I know they make ones with 24 - 72 volt input and multiple isolated outputs. Digikey carries a great selection but so do most of the other on line places too! They are designed just for what your working with.
 

Torben

Well-Known Member
For the problem of needing a positive supply and only having a -48VDC supply available, I'm going to stick my hobbyist neck out and suggest this (if I'm wrong I hope I'm quickly corrected, but this is what I would try):

Voltages are relative, not absolute. You seem to know this since you mentioned specifically: ". . .-48V terminal w.r.t earth." So if you think about it another way, you could say that the one you're calling "earth" is actually +48V w.r.t. the one you're calling "-48V".

Measure the voltages across the two lines, but put your meter's ground lead on the line you're calling -48V and the +V lead on the line you're calling "earth". You'll see +48V.

In short, you can use those lines as your power supply, just use the "earth" as your +48V line and the "-48V" line as your ground. i.e. just swap the polarity.

Then you can use a DC-to-DC converter like tcmtech mentioned to step down efficiently to your required voltage.

Make sense?


Regards,

Torben
 

mneary

New Member
Since -48VDC is the common rack voltage for telecom, "bricks" that convert -48VDC to just about anything are about as popular as those with 120VAC or 240VAC inputs.
 

vipsam10

New Member
Thx tcmtech and torben. If i apply +48V (earth w.r.t -48V), then i should be measuring my o/p w.r.t -48V itself because DC-DC converters do not provide isolation.Plz correct me if am wrong.This means that safety will not be taken into consideration because normally, my casing is connected to earth. So, i will no longer have earth at my o/p.
 

crutschow

Well-Known Member
Most Helpful Member
Thx tcmtech and torben. If i apply +48V (earth w.r.t -48V), then i should be measuring my o/p w.r.t -48V itself because DC-DC converters do not provide isolation.Plz correct me if am wrong.This means that safety will not be taken into consideration because normally, my casing is connected to earth. So, i will no longer have earth at my o/p.
Most DC-DC converters have the input isolated from the output. Thus you can select either terminal to be ground/common for both the input and the output. It's switching regulators that are usually not isolated.
 
Last edited:

Hero999

Banned
Use a buck boost converter to get +24V from -48V.
 

vipsam10

New Member
Thx loadz..i got a few schematics from MAXIM. They are not totally what i want. I requested for alternatives. A buck boost topology will require a PWM and as such a triangular/ramp supply. I do not have the option of using any additional supply. My aim is to replace 24V batteries that are getting damaged too often. Using an alternative supply would mean coming back to the starting point. Any suggestion will be most welcomed.

Thx
 

crutschow

Well-Known Member
Most Helpful Member
If you preclude any additional power supplies, inverters, or rectifiers to do the conversion (and I don't see a good reason for that prohibition unless this some type of academic exercise) then, to answer your original question, I believe the project is not feasible.
 

Hero999

Banned
I don't think you understand.

A buck boost converter gives the opposite polarity output. You don't need another supply, use a buck boost converter to convert -48V to +24V.
 

vipsam10

New Member
Yes i can use a buck boost converter....but how will i control the MOSFET? How will i determine the duty ratio? Is there any IC + circuit to do the job directly with only -48V as input? The switch used in buck-boost converter chould be controlled how??? Thx for ur support once again
 

vipsam10

New Member
On top of that, my aim is to use the already available -48V to get +24V. I already have 24V batteries but i want to replace them because they are getting damaged too often. So, crutschow, using additional supplies would mean coming to the starting point again. Thats why i cannot use additional supplies. Well, Mr. Hero, i need a PWM for my buck-boost. For the PWM i need an IC which requires a VCC or a Vref or something like that....where can i get it?
 

vipsam10

New Member
Thx hero, the parallel universe of negative input voltages is really interesting....it may help me a lot...thx again
 
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