converting to 5V power supply

Question

Hi, I have a base board which offers 2.5V@8A, 3.3V@8A and 12V@1A power supply but my sister board need 5V. So I'm considering to convert 12V to 5V, can I just use two resistors (10k and 24k ohm) to do that? This is under assumption that the sister board's impedance is high so I'm not sure if this simple circuit works. It should work but I'm rly not sure.. first time for me doing this.

jw0752 · Accepted Answer

Hi Bin, This little DC / DC converter will work well for your application. I have used them in similar applications. http://www.newark.com/recom-power/r-78e5-0-0-5/dc-dc-converter-2-5w-5v-0-5a-sip/dp/62X9499?st=Recom%20R It will give you up to 500 mA. If you need more current there is also a version that will deliver 1 Amp. shabaz is correct, the resistors will not work properly and would not be stable or efficient. John

shabaz · Answer

Hi Bin, The calculation is not right, you can use this web page to do it (the formula is there too): Voltage Divider Calculator However, unless your sister board has very low current requirement (much less than the current through the resistors - you can type in the resistor value and the voltage at this page to find out the current, or just use Current=Voltage/Resistance : Ohms Law Calculator ) then the answer is you'll need something else, most likely a DC-DC converter. If you provide more information about the sister board, then it is possible to help more. Similarly, although you have not specified the base board (which makes it harder to help you again), it sounds like a computer power supply. If you are not going to use the other two outputs, then please confirm it will operate if you do not use the other two outputs at all because some older computer PSUs may not like this. Finally, you're referring to the sister board impedance, but I hope you did not just measure with a multimeter and make this assumption. Most circuits are non-linear with their supply demand, and it is not possible to measure the resistance and base the power supply decision around that.

genebren · Answer

Hi Bin,

Both shabaz and jw0752 have offered some sound advice (as they are known to do around here). Another simple and perhaps lower cost approach might be a simple 5.0V regulator. This is feasible is the supply currents are low enough that the regulator is not called upon to dissipate too much power (not a problem with the DC-to-DC convertors mentioned above). As an example, if the load current is 100ma, the regulator would dissipate (12v-5v) * 0.1A, or 0.7Watts, which could fall outside of the parts specification. As a side note, some DC-to-DC convertors can generate a lot of high frequency noise, that might cause other issues (on the main board and the sister board), so be aware, should you see any performance issues in your project.

Good luck and welcome to the Element14 community!

Gene

genebren · Answer

Regulators can be build that will handle several watts. In addition, heatsinks and good PCB layouts can extend the range by helping to dissipate heat into the air and/or other materials, so that the chip does not over heat. There are also other techniques, like adding inline diodes to the regulator input to reduce the voltage drop across the regulator (5Vin-0.7Vd- 2.5Vout) * 8A = 14.4Watts (will offering reverse voltage protection). These are all steps to finding and using regulators without damaging them.

Good Luck with your project.

Gene

converting to 5V power supply

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