I’m an electronics newbie. I did a very simple project. In order to convert 12v dc to 6v dc I used one of those step down converters. Unfortunately what I’m trying to power is an AM radio. It power on without a problem but is completely unusable because of the EMI - RFI. Take in consideration that I usually run it using its ac power adapter (without any EMI – RFI). My question is: is there is a way to convert 12v dc to (smooth as if coming from a battery) 6v dc? The solution must be able to supply 0.4 amps (6v). Thank you very much.

  • \$\begingroup\$ No. But you can get very close, for a certain definition of "close". \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 14 '15 at 23:22
  • \$\begingroup\$ Is a linear regulator not ok? \$\endgroup\$ – Jon Feb 14 '15 at 23:23
  • \$\begingroup\$ AC adapters are not "EMI/RFI free" either. It sounds like you simply have a very poor quality buck converter. \$\endgroup\$ – Nick Johnson Feb 15 '15 at 11:58

The easiest way is to use a series pass regulator, like a 7806, to drop the 12V down to 6.

You'll waste about P = (Vin-Vout) x Iout = (12V - 6V) x 0.4A = 2.4 watts, so you'll need a heat sink on the regulator.

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    \$\begingroup\$ This. Instead of fighting the EMI, simply don't generate it. \$\endgroup\$ – Brian Drummond Feb 15 '15 at 0:14
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    \$\begingroup\$ There are fancy circuits with zeners etc. but for raw simplicity the linear voltage regulators are hard to beat. For added noise protection, put a small electrolytic cap (100uF should do) on the input, and a 100nF cap on the output. Put it all in a box, the box can act as a heatsink. \$\endgroup\$ – Alan Campbell Feb 15 '15 at 2:47
  • \$\begingroup\$ @Joe Depending on your definition of "convert" this solution might not behave as you intended. This 7806 solution doesn't use any voltage above, it burns the excess. In other words throws the energy away as heat. \$\endgroup\$ – Pro Backup Apr 21 '15 at 18:23
  • \$\begingroup\$ @ProBackup: A 7806 needs a raw supply voltage of at least about 2 volts above its rated output voltage in order to regulate its output voltage properly, so it does: "use any voltage above" within limits. I think you missed the point, however, which is that a linear series pass regulator acts like a rheostat in series with the raw supply and the load, with its slider's position controlled by analog feedback in order to keep the voltage into the load constant. As such, it generates no significant EMI, if any at all, and should behave exactly as intended. \$\endgroup\$ – EM Fields Apr 22 '15 at 0:19

If you don't mind wasting a considerable amount of power, 2 resistors will do it.

12v -- resistor -- load -- resistor -- ground

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    \$\begingroup\$ No. That is not an acceptable way to generate a power rail. \$\endgroup\$ – Matt Young Feb 15 '15 at 0:20
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    \$\begingroup\$ How will that keep the voltage across the load constant as the current through it changes? -1 \$\endgroup\$ – EM Fields Feb 15 '15 at 0:22
  • \$\begingroup\$ Even if a series resistor were appropriate (and it's not), there is absolutely no reason to use two resistors and you would almost never put one on the ground side. \$\endgroup\$ – DoxyLover Feb 15 '15 at 2:49
  • \$\begingroup\$ @EM Fields - It won't. That's why it's a bad way to create a power supply. \$\endgroup\$ – Austin Feb 15 '15 at 4:08
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    \$\begingroup\$ @Austin Steiner: Here: \$\endgroup\$ – EM Fields Feb 15 '15 at 9:55

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