Modify a supply into a regulated one ?

Papabravo

Joined Feb 24, 2006
18,770
Hi.
How would you modify a linear 15VDC 30A unregulated supply into regulated 14VDC 30A ?
There are at least two approaches:
  1. Because of the power levels, I'd use a buck converter because efficiency is everything.
  2. A linear regulator is possible but, tricky to manage if the voltage of the unregulated supply tends to drop precipitously as the current demand increases. If you can get to 50% efficiency, then your input power will be on the order of 840 Watts. Clearly you don't have that.
What kind of answer were you expecting? Something easy and straightforward perhaps?
 
Last edited:

Papabravo

Joined Feb 24, 2006
18,770
That would require about 93% efficiency and you're not likely to get there.

What is the ripple on the unregulated 15VDC?
A synchronous buck converter with some pretty fancy MOSFETS in parallel with some high current Schottky rectifiers might get the job done. I'm pretty sure it would cheaper to bin the unregulated supply and buy what you need.
 

dl324

Joined Mar 30, 2015
14,670
It is a non-regulated linear supply
A DVM won't tell you much about ripple voltage. If it's a couple volts above 15V, that makes what you're trying to do easier, but you're not likely to get 30A unless the unregulated supply current rating was conservative. With many unregulated supplies, the unloaded voltage is higher than the nominal rating and the voltage will drop under full load.
 

ci139

Joined Jul 11, 2016
1,854
what everything you need to regulate?
what is the full/main specs of your desired product (such as an adjustment range for the output voltage , drift , etc. ...) ?
 

LowQCab

Joined Nov 6, 2012
2,157
What makes You think that Regulating the Supply will fix your problem ?

Did the Transmitter have hum-issues when it was new ?,
or did You build it from scratch ?,
how old is the Transmitter ?, it may need a complete rebuild.

You are trying to fix what You "think" is the "problem"
before You actually know for a fact what the source of the "problem" is.

Don't think you're special, this type of repair pursuit is ubiquitous with all types of complex machines.

To fix this type of problem there are 2 approaches,
the "Shot-Gun" approach,
replace every Capacitor in the Transmitter, if that doesn't work, replace all the Resistors too,
and the "Methodical" approach,
which requires a thorough understanding of the operation of all sections of the Transmitter,
and requires the appropriate test equipment, and experience with it's use.

If it's a scratch-built-design, that's a different can of worms ..........
.
.
.
 

nsaspook

Joined Aug 27, 2009
9,992
Is the supply a simple, big-iron, 50-60hz transformer (not likely today) type? If so then AC side voltages adjustments are possible.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,874
Did the Transmitter have hum-issues when it was new ?,
or did You build it from scratch ?,
how old is the Transmitter ?, it may need a complete rebuild.

You are trying to fix what You "think" is the "problem"
before You actually know for a fact what the source of the "problem" is.

Don't think you're special, this type of repair pursuit is ubiquitous with all types of complex machines.

To fix this type of problem there are 2 approaches,
the "Shot-Gun" approach,
replace every Capacitor in the Transmitter, if that doesn't work, replace all the Resistors too,
and the "Methodical" approach,
which requires a thorough understanding of the operation of all sections of the Transmitter,
and requires the appropriate test equipment, and experience with it's use.
No hum reports when transmitting with battery.
No hum issues new or now unless that unregulated supply is in play.
What does special mean here ? Yes, I think that is the problem.
I will not do guesswork replacing capacitors and resistors.

Yes, 46 pounds of iron core transformer in it
 

LowQCab

Joined Nov 6, 2012
2,157
If the Transmitter worked, at one point in it's life, with no "Hum",
then replace ALL Capacitors,
this is a standard practice,
Capacitors don't last forever, and
can cause all sorts of odd problems.

( Resistors can also fail or change values with a lot of hours on them ).

Just because You get no "Hum-Reports" using a Battery,
doesn't mean that the Hum is created by the fact that the Power-Supply is not "Regulated".

Millions of Transmitters, and many other devices,
are very quiet without Regulation,
and some that are Regulated are noisy as hell.

The reasons for this are many ..............
.
.
.
 

bassbindevil

Joined Jan 23, 2014
454
You might be able to smooth it by adding L-C filtering. That'll be a big inductor, though, and copper isn't cheap. It could be better to adapt an ATX power supply, since those are cheap and plentiful.
 

dendad

Joined Feb 20, 2016
4,167
A great deal depends on the existing 15V supply. If it is just a bridge and capacitor, what is the unloaded voltage and the voltage at 30A?
And, when at the 30A current, does the ripple troughs go below the 15V?
It will be pretty hard to get a good 14V from the supply I imagine. Probably the ripple goes below that for a start. And, just 1V headroom for the regulator is pretty low.
Can you post a circuit of your supply and an oscilloscope display of the no load and full load output voltage?
 
Top