Power supply 5A max via step down converter: does max. current increase with stepping down voltage?

Thread Starter

peewee678

Joined Nov 21, 2018
3
Hi,

I have a 24V (switching) DC power supply 5A max.

If I connect it to a (switching) step down converter and set the output voltage of the converter at 12V, can I then connect a load that draws 10A (max) from the converter (if the converter itself can provide 10A)?
 

ebp

Joined Feb 8, 2018
2,332
Close, but not quite.

[EDIT to fix error]
The second converter won't be 100% efficient, but something in the range of 90-95% is not too unlikely if the design is good. In consequence, you can have 10 A at less than 12 V or 12 V at less than 10 A. You might get away with actually taking a full 120 W from the second converter and running the first with an overload. If your ambient temperature is moderate and the 24 V supply has good cooling, it may be OK. Although it is kind of a "cheat" you might also be able to turn up the output voltage of your 24 V supply. This is still a bit of an "overload" condition, but component stresses and aging factors trade off in complex ways in switchers, so this might be marginally less stressful than drawing higher current at 24 V.

Most ordinary switchers don't have accurate current limiting, so it will probably allow the overload. If long term reliability is important, I advise against operating beyond the manufacturer's specified rating. Be careful with low-cost switchers found on e-bay and the like. The power ratings aren't always realistic and they may run very hot at the full output power that is claimed.

Sometimes you can run into odd behavior with one switcher powering another. Usually things work OK, but not always.
 
Last edited:

Thread Starter

peewee678

Joined Nov 21, 2018
3
Close, but not quite.

The second converter won't be 100% efficient, but something in the range of 90-95% is not too unlikely if the design is good. In consequence, you can have 5A at a little less than 12 V or 12 V at a little less than 5A. You might get away with actually taking a full 120 W from the second converter and running the first with an overload. If your ambient temperature is moderate and the 24 V supply has good cooling, it may be OK. Although it is kind of a "cheat" you might also be able to turn up the output voltage of your 24 V supply. This is still a bit of an "overload" condition, but component stresses and aging factors trade off in complex ways in switchers, so this might be marginally less stressful than drawing higher current at 24 V.

Most ordinary switchers don't have accurate current limiting, so it will probably allow the overload. If long term reliability is important, I advise against operating beyond the manufacturer's specified rating. Be careful with low-cost switchers found on e-bay and the like. The power ratings aren't always realistic and they may run very hot at the full output power that is claimed.

Sometimes you can run into odd behavior with one switcher powering another. Usually things work OK, but not always.
Thanks but I'm a bit confused now. I understand the efficiency part (was expecting that) but you're mentioning 5A at 12V. According to crutschow's answer it could be 10A at 12V (he writes about 8.5a but let's forget about the efficiency for a moment).

And yet another person on Quora tells me it can never be more than the max. current of the power supply (5A in this case). If I'm correct that's what you're trying to say?
 
Last edited:

be80be

Joined Jul 5, 2008
1,944
If you get 5 Amps at 24 volts you would get 10 Amps at 12 volts but there is some lost so you'll be more like 8.5 to 9 amps.

But if there was 5 amps at 24 volts and you could drop it to 12 without lost it would be 10 amps.
You'll just have to see but most step down converter have loses so you'll more then likely end up short of 10 amps

24 volts 5 amps = 120 watts
12 volts 10 amps = 120 watt's

The kicker is what the supply load is if the load is 1.2 ohm at 12 volts you'll get the 10 amps.
 
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ebp

Joined Feb 8, 2018
2,332
I'm sorry, The correct output at 100% efficiency would be 12 V at 10 A, so at 90% you would expect, as crutschow said, 9 A at 12 V.

90% efficiency is probably more realistic that 95%, and it might be even lower. 95% is certainly possible these days with a very good design with carefully chosen parts.
 

Thread Starter

peewee678

Joined Nov 21, 2018
3
I'm sorry, The correct output at 100% efficiency would be 12 V at 10 A, so at 90% you would expect, as crutschow said, 9 A at 12 V.

90% efficiency is probably more realistic that 95%, and it might be even lower. 95% is certainly possible these days with a very good design with carefully chosen parts.
Thanks for the correction!
 
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