# 48V DC Power Supply Using LM7812 in Series

I want to create a 600W 48 VDC power supply with input taken from the line (220VAC) and so far the steps that I've had in my mind are:

1. Stepping-down the voltage to 15V using transformer with 3A current rating.
2. Rectify using bridge diodes.
3. Use 4 pcs. of LM7812 (linear voltage regulator with 12VDC output) to generate 4 12VDC voltage source.
4. Connect the output of LM7812 in series to produce 48VDC voltage regulator.

Will this method works or is there any flaws in the method I have? Is there any better method? Any suggestions are welcome.

Thanks.

• You would need four of everything (or a transformer with four isolated 15V windings, each appropriately rated for current). You can't get more than about 1A from a 7812 as @MattYoung points out, so maybe 50W on a cool day with the wind in the right direction. – Spehro Pefhany Nov 13 '14 at 16:53
• Step 1 is already problematic. 15V * 3A = 45W not 600W. – user_1818839 Nov 13 '14 at 17:05

No, what you proposed won't work, and here's why.

1. 15V at 3A is 45W, not 600W. Your input power is insufficient, so adding pass transistors will do absolutely nothing for you.
2. You cannot stack 4 12V regulators on top of each other unless they are isolated. With one transformer they would not be. Youu would be shorting the 12V output to ground.
3. The 78xx series' output current is 1A max. 600W would require 12.5A, 1250% over spec.
4. Let's pretend you could get it to work and it was 70% efficient (good for a linear supply). That means you would have over 200W of heat to get rid of. That would take some enormous heatsinks.

You really need to look at some sort of offline switching regulator for that kind of power.

• what if I add some pass transistors for each LM7812? – Antonius Perdana Renardy Nov 13 '14 at 16:52
• @AntoniusPerdanaRenardy See my edit. – Matt Young Nov 13 '14 at 17:02

No. For a start 7812s are limited to about 1.5 A (from memory) where you need 12.5 A. Even so to put the 7812-regulated circuits in series you would need a transformer with four separate secondary windings, four rectifiers, four reservoir capacitors and the like.

I would use a commercial switched mode supply. For example, RS Components own brand (Mean Well) supply is about 200 GBP. The switched supply will be much smaller and lighter than the linear supply and probably cheaper as well.

Get a professional to inspect and Portable Appliance Test your equipment before you power it from the mains. You are talking about a significant amount of power, here.

If you absolutely must have a linearly-regulated supply than I would take a look at the High Current Adjustable Regulator application circuit in National Semiconductor's data sheet for the LM117HV/LM317HV. You will need lots of heat-sinking and probably a fan.

How about a nice bench supply? Mastech will sell you 50V @20A for ~\$300