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Old 5th August 2012, 09:15 PM   #1
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Default How to build 24v 2A Single Rail?

I have done a lot of searching but it is surprisingly hard to find specific links on how to put together a single rail power supply. I need anywhere from 19-26v at 2-2.5A.

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Old 6th August 2012, 04:04 AM   #2
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19-26v at 2-2.5A

2.5A x 1.8 bridge rectifier and capacitor multiplier = 4.5A needed.

24vdc common voltage x .707 = 16.968vac min voltage. 24vdc was picked because the transformer will have a higher voltage with no load.

Voltage drop of bridge rectifier about 1.4 vdc.

Standard transformer = 18vac.

18vac x 1.414 = 25.452vdc

25.452vdc - 1.4vdc rectifier drop = 24.052vdc, now you know why 18vac is a standard transformer size.

4.5A needed x 18vac = 81 watts, sort of equals VA.

Antek - Transformers - Grid View[]=0

look for 18vac transformer at least 80VA.

100VA is standard size and 50VA is way to small. More VA is fine.

AN-1218, 18vac 100va, no shield.
AS-1218, 18vac 100va, with shield.

With or without shield is application specific.

The bridge rectifier should be over sized in case of brief short circuit and because they cost very little. Basically cheap insurance.

This is cheapest overkill chassis mount bridge

GBPC1204 Fairchild Semiconductor | Mouser

400 volt 12 amp chassis mount bridge rectifier.

This is cheapest bridge rectifier fairly oversized and PCB mount. 100 volt 6 amp.

BR61 GeneSiC Semiconductor | Mouser

Capacitor rule of thumb, 1000uF per amp minimum. 2000uF per amp is plenty of extra without over taxing the bridge rectifier.

2.5 amps max x 2000uF = 5000uF

Capacitors should have 20% safety margin for voltage.

24vdc x 1.2 = 28.8vdc, 30VDC is a standard size. Any voltage capacitor 30vdc or higher will work. Capacitors are expensive and higher voltages cost more. 35 and 50 volts are the most popular voltages and will give you the lowest cost.

There are capacitors that are leaded and for a premium price with screw terminals. The type depends on how you are building this.

Here is an example of specs for caps.

Look at 2200uF at 35VDC it has a ripple current rating of 1260mA = 1.260A. Two of these caps in parallel would give you 4400uF and 2.52 amps ripple rating. Since you need a maximum of 2.5 amps from your power supply this would give you a very long capacitor working life.

These also have the coveted 105 deg C rating for even longer life. - 55 C to + 105 C.

EEU-HD1V222 Panasonic Electronic Components | Mouser

This is the minimum to get a power supply up and running. For a practical supply you would put a .1uF and a .01uF across the 2200uF and maybe some bleeding resistors.

The input side of 120 or 240vac would have MOV surge protection and maybe a .1uF safety cap, and a fuse.
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Last edited by powerbob; 6th August 2012 at 04:06 AM.
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Old 6th August 2012, 04:40 AM   #3
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Awesome. Thanks PB!
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Old 6th August 2012, 07:13 AM   #4
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24V 90W switching supplies are common, cheap, and relatively compact.

The plus side is they're regulated, the negative is some high frequency ripple on the output.
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Old 6th August 2012, 08:12 AM   #5
troystg is offline troystg  United States
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Powerbob, Outstanding reply. All the answers and information one could ask for.

theAnonymous1, Short and sweet. :-)
Thinking positive doesn't make things better, it makes you a better person.
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Old 7th August 2012, 12:06 AM   #6
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This ebay board seems decent, although I suppose I could breadboard?

Click the image to open in full size.
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Old 7th August 2012, 02:47 AM   #7
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Originally Posted by wushuliu View Post
This ebay board seems decent, although I suppose I could breadboard?
There are no physical dimensions so you can not know for sure if your components will fit. Also it holds more caps than you need so it is much larger than it has to be.
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