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using inline diodes to drop regulator input

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brownt

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I have a AMS1117 5 volt regulator powering a system that draws about 90mA.
The only option for input to the regulator is a 12V plug pack. The regulator is good for input voltage up to 15 volts, but the drop across the regulator is creating a lot of heat. Too hot to touch. The regulator is mounted on a small PCB with a copper pad that acts as a small heatsink.

I have put three 1N4001 diodes in series with the supply, dropping it to 9.9 volts, and the heat on the regulator is OK.

Is there a problem with dropping the voltage that way? Or is there a better solution given the circumstances.
 

Hi,

The overal generated heat will be the same.
You may use the diodes in series. You may use a suitable zener, but you may use a suitable (bigger) voltage regulator, maybe with heatsink.

The drawback with the series diodes is, that the range of input voltage is limited by the voltage drop.
In case the input voltage drops to 7V ... the diode solution will fail, the regulator_only solution will still work (depending on voltage drop and regulator type)

To reduce heat you may consider to use a switching buck regulator. This usually is my choice.

Klaus
 

Adding a series element isn't uncommon. Klaus covered many of the other options. A resistor could have worked too.

As klaus said total watts will remain the same no matter how you do it. It's probably ideal to have the right size regulator to begin with, on the other hand I do this (resistors usually) to get away using the same regulator in many places

Also look at these types of 'switching regulators':
https://www.recom-power.com/emea/products/switching-regulators.html

And there are now very many integrated switching regulators in chip sized packages like TI's LMZ series:
https://www.ti.com/ww/en/simple_switcher/power-modules.html
 

Is there a problem with dropping the voltage that way?
Unless the board has forced ventilation, you might to be aware that the spacing of the diode to the surface of the PCB have be at least 1 cm, as well as the spacing between the diodes and other heated devices need to be sufficient to one not overheat the others mutually.
 
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    d123

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Is there a problem with dropping the voltage that way? Or is there a better solution given the circumstances.

Yes, it is ok to distribute the total dissipation over 4 devices; but you do not gain anything in terms of efficiency. Total dissipation of (12-5)*0.09=0.63W is not much if your power source is the wall plug.

Perhaps you can use two regulators placed judiciously at two points such that each deliver about 1/2 the total load current.

But you still have to live with the waste heat.
 

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