# Power Supply Diagram

Discussion in 'Power Electronics' started by MyTechtime, May 19, 2014.

1. ### MyTechtime

8
0
May 19, 2014
Hello, I need somebody who has built power supplies before to verify that this diagram will work. The transformer will supply 4A. This was designed for a custom purpose. Will this power supply work correctly? Thanks. (the bridge rec. is supposed to be connected to ground, forgot to draw that in)

2. ### KrisBlueNZSadly passed away in 2015

8,393
1,267
Nov 28, 2011
Hello and welcome to Electronics Point

1. You need to connect the negative side of the bridge (on the left) to the 0V rail. It looks like you just forgot to join the line up. Edit: Oh, you already said that.

2. You need a smoothing capacitor as well as the 0.1 µF decoupling capacitor from the LM338's input to the 0V rail. One convention is to use 1000 µF per amp of load current, so you would need at least 4000 µF. 4700 µF is a common value. It needs to be rated for at least 35V, preferably 40V or 50V. Here's a suitable part: http://www.digikey.com/product-detail/en/UVR1H472MRD/493-1115-ND/588856

3. You haven't given any part number(s) for the bridge rectifier. Here's a suitable one (it may need heatsinking): http://www.digikey.com/product-detail/en/GBU1004-G/641-1367-5-ND/2074832

4. The voltage setting resistors for the LM338 are wrong. Here's how to calculate R2.

There will be Vref (1.24V nominally) across R1 so the current will be I = V / R = 1.24 / 120 = 10.333 mA. The ADJ pin draws about 45 µA as well; adding this to the R1 current gives an R2 current of about 10.378 mA.

There will be (Vout - 1.24V) across R2. So the right value for R2 will be R = V / I = (24 - 1.24) / 0.010378 = 22.76 / 0.010378 = 2193 ohms, or about 2k2.

5. The LM338 will be dissipating around 20W at maximum load so it will need significant heatsinking.

Edit: For a rise of around 50 °C above ambient, at 20W dissipation, you'll need a heatsink with a thermal resistance of around 2.5 °C per watt. Assuming you use a TO-3 version of the LM338, this one looks suitable: http://www.digikey.com/product-detail/en/641A/345-1052-ND/340348 You might get away with a smaller one if the 4A output load is intermittent instead of continuous.

Last edited: May 19, 2014
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3. ### MyTechtime

8
0
May 19, 2014
Thanks for the help!
This will not be giving out 4A constantly. In a normal state, it will only have to give about .5 amps to power the supervisory circuits and status lights.

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