Designing a power supply...HELP!!!!

[LadyBugg]

Revision of Designing a power supply...HELP!!!!

Need to design a AC to DC power supply that uses bridge rectifier and slo-blo fuse in primary of transformer. Has to go from 120 V RMS to 24 Volts of Steady DC and have 1 Amp of Current and less than 300mV of ripple. I know it needs a capacitor and a voltage regulator. The Voltage regulator will be LM7824 and capacitor will be 28kmicroFarad. What other ratings do I need to consider with the bridge rectifier and the transformer. And is this the correct calculation of the capacitor?....
Here is some of the calculations I came up with...

120VRMS input thru 5:1 ratio transformer = 24VRMS - 1.4V drop thru bridge = 22.6V RMS /.707 = 31.92 V DC.

Capacitor value: 1Amp/ 120hz(300mV ripple) = 27778microF with a Vmax of 32V.

Voltage Regulator: LM7824 because + output and Voltage max of 38V and only 2 volt dropout....

Fuse Rating: (24Voutput + 1.4V drop) / 120 = 211.67mA = approx. ¼ amp fuse...

Does this make sense to anybody??????....LadyBugg.

 

[eblc1388]

Will be used in industrial applications of powering PLC type equipment.

Just buy one.

Don't mess with industrial application as PLC itself is many time the cost of your supply, unless you like to tell your boss that "I myself design the power supply, but I don't know why it fried the PLC."

 

[LadyBugg]

I am a first semester engineering student

I am a first semester engineering student and this is a class project. Just "buying one" defeats the purpose of my designing abilities....Thanks, I should of been more thorough in my first reply....LadyBugg....

 

[The Mad Professor]

Hmmm... "industrial " and "powering PLC equipment".. you say

I find it more than a little odd that the owners of this hardware would ask a person with little or no knowledge of electronics to build such a device.
You obviously have access to a computer and no doubt used a search engine
to find this forum, how you managed to miss the numerous websites that have extensive archives of circuits for free beggers belief but I suppose it could happen.

Anyhow, I digress and I feel sure many here including myself will all pitch in and lend a suggestion or three that will help you along.
You have allready told us the input voltage and the desired output voltages but have made no mention of the current (Amps) required for your project.
This information is crucial as it will determine wether your power supply is the size of a coke tin or larger than a truck.

Please note : MAINS VOLTAGES ARE DANGEROUS!!!

 

[audioguru]

Just make your power supply the size of a truck so it will be able to provide many amps and power nearly anything.

You mentioned you would like to use a zener diode for voltage regulation. A zener diode wastes the power supply's current if the load doesn't use it, so the zener diode heats with the amount of current. Therefore a zener diode is used if the required current is low. Additional or different parts are used if the required current is high.

 

[Hero999]

john,
Please start a new thread to ask this question rather than derailing an existing one.

LadyBug,
Do your own work.

Also you haven't said whether it needs to be regulated and what output current you require?

If it doesn't need to be regulated and you're not bothered about ripple then use a 18V transformer, with a bridge rectifier and huge filter capacitors.

 

[LadyBugg]

1 amp of current

Still again I should of been more thorough in my first reply...
One Amp of current is the requirement....LadyBugg.

 

[audioguru]

A zener diode would cook if it had 1A with 24V across it. Adding transistors would make it cooler but make it complicated.

Just use a 7824 1A voltage regulator IC. It has built-in current limiting and it will automatically shut down if it gets too hot. Its ripple output is nearly zero.

 

[Hero999]

Use **broken link removed** as a guide.

 

[LadyBugg]

Just an update on my project. It does work but had to add a heat sink on the voltage regulator and 2 more capacitors a 1 µF to the input side of the regulator and a 10 µF to the output side to bring down the ripple voltage. The voltage regulator also needs a fan to keep it cool in max load condition. I recieved an "A" for the class and really do appreciate everyones help. Thanks. LadyBugg.

 

[audioguru]

He/she got an "A" for making an overheating power supply with its filter capacitor way too big. The poor 24V regulator had an input voltage way too high at about 32V and with 1A though it needed to dissipate 8W.
The datasheet shows how big its heatsink needed to be and also shows the capacitors that needed to be added.