Author Topic: Safely and reliably step down 12V 5A source to 5V or just buy a 5V power supply?  (Read 1308 times)

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Offline plastygrove

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Hi all,

I have one of those inexpensive 220V to 12V 5A (60W) switching power supplies. I'm working on a project which requires 5V at at least 4A current (RGB LED Matrix).

Is there a safe and reliable way to step down from 12V to 5V at high currents?

I thought of the 7805, but 4A might be pushing the 7805 to its limit (not to mention the large amount of heat).

I also thought of the LTC3780 on ebay/aliexpress, but from the feedback I got on the net, it's not particularly reliable (reports of it just not working, dying etc). Will be great to hear if anyone had good experience with this.

It might even be cheaper to just buy a 5V high power supply online for this project.

Any suggestions? I've never worked with high current before so, I just want to be as careful as possible.

Thanks,
PG
 

Offline Skimask

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Why didn't you just use a cheap PC power supply and tap off the 5v rail?
Would've been a lot cheaper...not to mention easier.

You got 12v, need 5v @ 4amps.  Assuming a linear regulator (eg. 7805), you've got 20watts (5v @ 4amps) going out, 48 watts coming in (12v @ 4amps).  You have to dissipate over 28 watts of power.  A ridiculous amount of heat in anybody's book.
« Last Edit: March 27, 2016, 03:22:36 pm by Skimask »
I didn't take it apart.
I turned it on.

The only stupid question is, well, most of them...

Save a fuse...Blow an electrician.
 

Offline mariush

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You don't want to use 7805 because it's a LINEAR regulator, which means the difference between input voltage and output voltage is dissipated as heat. No only is the 7805 capable of only 1.5A, it can only dissipate about 15w with proper cooling and (12v-5v) x 1.5A = 10.5w so you'd need a lot of heatsinking.

You can however go with a switching regulator dc-dc converter, which has a much higher efficiency, usually above 85% .. the cheap 5A (or higher) dc-dc converters on eBay will work just fine to drive leds.

Here's one example, or another or another.
 

Offline Hobby73

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You indicated that your RGB LED Matrix "requires 5V at at least 4A current".  But I thought these types of LED arrays require constant current regulation, which is a more stringent requirement than making some minimum amount of current available.  (You may well already know this, but I wasn't sure since it wasn't explicitly stated.)
 

Offline peter.mitchell

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How about you just change the feedback resistors on the 220-12v supply so it instead outputs 5v?
 

Offline plastygrove

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Why didn't you just use a cheap PC power supply and tap off the 5v rail?
Would've been a lot cheaper...not to mention easier.

You got 12v, need 5v @ 4amps.  Assuming a linear regulator (eg. 7805), you've got 20watts (5v @ 4amps) going out, 48 watts coming in (12v @ 4amps).  You have to dissipate over 28 watts of power.  A ridiculous amount of heat in anybody's book.

I'll need to buy the PC power supply as well (also on the list :D). But yea, the heat dissipation is the worry, 7805 might be okay for something low power in a pinch, but not really heavy duty.

You can however go with a switching regulator dc-dc converter, which has a much higher efficiency, usually above 85% .. the cheap 5A (or higher) dc-dc converters on eBay will work just fine to drive leds.

Here's one example, or another or another.

Nice, thanks for those links. I think I'll get one of those cheap step downs, that might do the job! I think I'll get the LTC3780 as well. After looking online a bit, it seems that it's a bit pricey is because it can do both step up and step down which will be useful for me to build a bench power supply.

You indicated that your RGB LED Matrix "requires 5V at at least 4A current".  But I thought these types of LED arrays require constant current regulation, which is a more stringent requirement than making some minimum amount of current available.  (You may well already know this, but I wasn't sure since it wasn't explicitly stated.)

I figure if I get a power supply with at least 5A, it should be sufficient for my needs? Most of the documentation seems to suggest that 2A is also good enough, but better have at least 4A if you intend to use all the LEDs. The RGB matrix I have is similar to Adafruit's RGB LED matrix. Do you think this might not work?

How about you just change the feedback resistors on the 220-12v supply so it instead outputs 5v?

I could try that, but I have no idea where to begin :). I checked online there isn't too much info on how to do this for my power supply, do you have any suggestions? In any case, the 5V 10A power supply is only 9 bucks with shipping, so I figure I might as well buy it.
 

Offline timgiles

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Hi,

I am in the process of trying the following ICs from TI and MPS (Monolithic Power Solutions):

     TI TPS56628
     TI TPS54628
     TI TPS54627

     MP8666

All the above are happy to supply 5A and give reasonable low to high output efficiency, depending on which one around 85-94% stepping down from 12 to 5V. I am looking to drive a high power USB 3 hub and would like high efficiency from 0.1A up to 5.5A.

I am waiting for some PCBs before I can test each one, mostly interested in the 56628 and the 8666. I have put together a rather large reference design PCB and also a much smaller board layout to see the effect on ripple and efficiency.

Not due to get my board until mid April but will post the results if you are interested.

Regards Tim
 

Offline xyrtek

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I will assume you want to learn electronics and use what you have on hand.

A 7805 would work just fine as long as you use the right application

Check this datasheet for several configurations including high current voltage regulator.

https://www.fairchildsemi.com/datasheets/LM/LM7805.pdf

This is basic stuff and useful to know. Old datasheets use to have typical applications, don't just google 7805 and go with the "first" link.
 


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