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# 28 volts/4amps Voltage Supply

Discussion in 'The Projects Forum' started by samrat, Oct 20, 2008.

1. ### samrat Thread Starter Active Member

Apr 2, 2008
30
0
I am currently trying to get a 4 amps output with 28 volts from a wall-mount rectifier(RW748U) that has ajustable current of 0-12 amps, and voltage without load of 54 volts and 48 volts with load. This rectifier has a power rating of 700W. I have burnt resistors trying to connect in parallel and then in series. This is a urgent project requirement that is due next week. The power out of this rectifier has to give power to the power amplifier that we are trying to build. Can a circuit diagram be given of the best calculatons of the resistance to use and what about the power ratin of the resistors? What sort of load can I test the power supply with? As far as the manual of this rectifier is saying, the current is adjustable therefore it means that it will only chew up the current required for the load.

2. ### alwayslearning Member

Feb 27, 2008
22
0
You want to build a load to handle a power of;
P = IV = 4 Amps * 28 volts = 112 Watts!
1)Would you have time to order a high power wire wound resistor from:

http://www.resistorsonline.com/housed-chassis-mounted-wire.html

2)How close are you to a vehicle salvage?

Parts are cheap-a used blower motor draws about 11 amps at 14 volts (put two in series). The blower motor resistor circuit is used to control the motor current. For example at 13.8 volts the blower motor current drops to 4.4 amps using the 1.7 ohm resistor or 3.2 amps using 2.7 ohms. This was measured on a 2001 Crown Vic blower motor with a 1975 dart blower resistor board. I have seen these resistors in 1995 vehicles-have seen them always in or around the blower motor units.

3) Do you a Princess Auto close?

4) If you can afford it? You could always purchase a resistive load device
designed to handle the power you need-high power halogen lamps for instance-two auto units in series? ...

Last edited: Oct 20, 2008
3. ### tmd_63 Member

Oct 20, 2008
13
0
Simple ohms law should provide you with the answers. But you are trying to dump a very high power in these resistors. You will get a very effective heater with that.
Load is 112W (28x4). Voltage drop is 20V (48-28). Loading resistor will need to be around 160W (to stop high temperatures causing problems, 80W will be dropped in it). Resistance is 5 ohms (20V/4A).

4. ### samrat Thread Starter Active Member

Apr 2, 2008
30
0
TMD 63, Does this mean that after the resistive load, I can then connect to my power amplifier?
Always learning, will look up some car salvage in the mornig. So this resistor loads will have to handle the high power coming through before I can add my power amplifier to the cct???I was using resistors with power ratings of 2 watts, no wonder they kept burning....

5. ### samrat Thread Starter Active Member

Apr 2, 2008
30
0
Thanks for the tips guys. Will keep yous informed of how I go.

6. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Trying to step down the voltage using resistors is terribly inefficient. Not only that, fixed resistors dropping voltage to supply a variable load (you say the load will be an amplifier, I'm assuming a music amplifier) the actual supply voltage will vary over a wide range depending upon the amplifier's current demand from moment to moment.

The manufacturer's efficiency rating on the supply is 85%. Trying to reduce the voltage even using a "buck" switcher would reduce efficiency quite a bit more. If you opted for a linear solution (like a high-power voltage regulator IC) your efficiency would be abysmal.

Why not take a look at replacing that unit with an RW324U? It's output is adjustable from 22V to 29V, current up to 10A, and is 85% efficient. More than twice the power you need, with no modifications or add-ons necessary.

Here's another (and arguably better) option for you; Marlin P. Jones & Associates has a 28v 5.5A 150W industral power supply for \$40.75. These folks ship fast, and have good prices.
http://www.mpja.com/prodinfo.asp?number=16033+PS
If you order it right away, you should have it in time.

If you want a linear supply, MPJA has those, too. \$85 for a 28V 4A open frame linear supply.
http://www.mpja.com/prodinfo.asp?number=6629+PS

7. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
I tried to make a somewhat lengthy post, but for some reason it's not showing up.

You really need a different power supply. Trying to use resistors to drop the voltage will result in wildly fluctuating voltages at your power amp's supply input. A blower (or pair of them in series) would make things much worse; it would introduce a tremendous amount of electrical noise due to the brushes on the commutator making/breaking the circuit.

A linear regulator would be much better, but still would dissipate far too much power as heat.

A "buck" switching regulator would be feasable (and far more efficient), but your supply is 85% efficient as it is. Adding another 85% efficient buck regulator would reduce your efficiency to 72% in the supply alone.

If you started off with another switching supply that was rated for your project requirements, you would be far ahead. Marlin P. Jones & Associates has 28V 150W 5.5A industrial power supplies in stock at reasonable prices. I've bought from them a number of times in the past, they ship quickly.

8. ### samrat Thread Starter Active Member

Apr 2, 2008
30
0
Thanks Sgt Wookie. Buying one will be my best bet. I have learnt so much from just trying to make a power supply though.cheers

9. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Learning is good.

Please don't take my input as attempting to discourage you from learning more about power supplies. You have very specific requirements, and a very short amount of time to complete your portion of the project.

For this project, you simply don't have the time to design/order/build/troubleshoot a "do-it-yourself" supply. You need a COTS (Commercial, Off-The-Shelf) solution.

One caveat may be power supply noise. I do not know what your requirements are for supply ripple. However, supply ripple can be reduced quite a bit by adding a pi filter to the output of the supply I mentioned in my prior post.

A pi filter is a capacitor to ground, then an inductor in series, then another capacitor to ground. However, wait until you get the supply and can see how noisy it actually is under load; it may be clean enough for your purposes.

Last edited: Oct 20, 2008