48 VAC - 24 VDC Power Supply 4

[Paddymagee]

Hi,

I am trying to convert the 48VAC I have from a transformer into 24V DC for running a PCB. The current requirement would be 200mA max.

I cant seem to find any off the shelf power supplies or transformers so I attempted to build a circuit by rectifing the 48V and then using the zener diode method to give 28V DC (the PCB can accept 28V max)

I used a 6w 120R resistor and a 5W 28V zener but the circuit ran so hot that It burnt my finger! So I assumed this was not the right solution.

Is it possible for me to use a 240/110 transformer except put 48VAC into the primary and get 24VAC out at the secondary. I know I can build a circut to get rectify the 24AC.

Any thoughts any one?

 

[itsmoked]

Paddymagee, Your scheme would be a huge waste of power, which shows up as heat in the elements you are using.. Why not add another transformer to drop the voltage down to 24VDC then run your method. BTW a 200mA supply can be a lot to get working reliably.

And Yes "that" transformer would probably work fine as long as it's rated for 200mA.

 

[bklauba]

For a "home-brewed" supply, a common approach is to use a 24 VAC trans, full wave bridge, and a voltage regulator, like an LM7824.

The reg can give you up to an amp. Suitable choice of the other components, and 200 ma is easy.

BK

 

[buzzp]

Things may run a bit cooler using a regulator as suggested. If you can, use a 24VAC transformer instead to reduce the heat.
Anytime a zener is used as a regulator, count on a lot of heat and not a very stable supply.

 

[Paddymagee]

Thanks All,

I there is only one transformer in the panel (its a lifting winch) which is 3-Phase 415V to 48VAC. There is no neutral in the panel. There is no room in the panel for anything but the smallest transformer.

I think I might use a transformer intended change 240 - 110 to change 48 - 24 instead.

 

[felixc]

Using the 220-110 transformer to reduce your AC down to 24 volts will work fine! If you can't use the casing as a heat sink for the regulator, you may want to add a series resistor between the rectified AC and the regulator, to absorb some of the heat.

 

[itsmoked]

Here is what I would do...

Use that transformer BACKWARDS! Use it to jump the 48Vac to 96Vac. Then use any of the many outstanding off-the-shelf power supplies available to efficiently create your 24VDC, filtered and without a bunch of heat u will be then forced to deal with, *after* it has caused other problems.

Example:

http://datasheet.astrodyne.com/MCA-10.pdf

You want the Chassis Mount Version.(last page of PDF) This puppy puts out 400mA with any input between 85Vac-265Vac, 47Hz-440Hz.

This will save you DAYS of screwing around. They cost about $80 bucks... This is a CHEAP solution. This will be far more reliable than your presently contemplated solution.

 

[Paddymagee]

Thanks all,

Thats a few ideas to get me started. Looks like the zener method is unsuitable for most projects.
A transmformer of some kind is in order.

Regards
Paddy

 

[itsmoked]

Indeed. Zeners for voltage regulation are like using a flint to light your stove....

 

[wrong100]

A TRANSFORMER WILL TAKE ABOUT 2*2*1 INCHES.
I DO NOT THINK YOU CAN GET A 48 TO 24 VOLT,
IF YOU CAN IT SHOULD BE 1/2 THE SIZE.
ANY 1 TO TWO IN REVERSE OR TWO TO 1
WILL WORK (BEST BET). ANY OTHER OPTION
WILL USE .2*24= 4.8 WATTS.
CALL IT 5 WATTS.
50 DEG C RISE = 75 DEG C.
5WATTS *132 PER WATT = 660.
IT SHOULD HAVE BURNT YOUR FINGER AND DIED.
50 /660 = .075 DEG C PER WATT NEEDED.

IF YOU ARE SO TIGHT ON SPACE
TRY USING A 100 OHM DROPPING RESISTOR
IN SERIES WITH THE REGULATOR (5WATT).
IF A LOT OF FREE AIR FLOW A 10 WATT
(RS10100) SHOULD GET TO ABOUT 112.5 C.
OR A RH10 SERIES WITH ENOUGH CHASSIS
MOUNTING WILL WORK (ABOUT 4 IN SQ).
THESE ARE GENERAL BALLPARK NUMBERS.