# Power supply fluctuations. Bypass capacitor needed?

I am powering up a 5V device using a 12v DC 1A SMPS power supply. The 12V supply is connected to the 5V device through a 7805 voltage regulator. The device draws about 30mA when idle, but when it transmits a signal it draws about 100mA to 115mA.

When the device is about to transmit the signal, the input voltage of the device drops below the minimum voltage required for the device to function (3V DC). This is caying the device to restart. I am guessing this is because the 12V power supply is unable to keep up with the sudden power requirement.

Do I need a bypass capacitor? If so what rating capacitor should I use and where do I place it (near the input of the 5Vdc device, near the output of the 12Vdc power supply, or at the input of the 7805 voltage regulator?)

• By "Do I need a bypass cap" do you mean you currently don't have any? Jul 7, 2017 at 10:06
• I do have a .33uF capacitor on the output of the voltage regulator and one on the input as well. Jul 7, 2017 at 10:08
• These are Electrolytic capacitors by the way. Jul 7, 2017 at 10:12
• Add a schematic. Jul 7, 2017 at 10:59
• Something doesn't make sense here. Even with only 0.33uF input & output capacitors (which I doubt are electrolytic, despite the comment saying that), I haven't seen a 7805 which would respond to a 70mA (30mA -> 100mA) load step by causing the output to drop to 3V if the input is a genuine 12V 1A power supply. My hypothesis is that something we are being told as "fact", isn't true e.g. power supply can't actually supply 1A, or load step is actually much larger (but may not show on measuring instrument e.g. DMM), or 7805 capacitor locations are wrong etc. etc. Much more details needed! Jul 7, 2017 at 13:55

You say "When the device is about to transmit the signal" and use the word "sudden" which implies things are going fast, so we can rule overheating.

Check 7805 regulator input and output voltages with a scope while doing your transmission test.

• Output voltage falls, but input voltage stays strong: add more capacitance at the output.
• Input voltage falls too: add more capacitance at the input.

Some modern fast LDOs are happy with 1µF ceramic at the output. But 7805 is quite an old design, and as such it is slow to respond to fast current changes. Thus it needs capacitance on the output to smooth things out. Maybe your DC-DC is slow to respond also, but this is less likely (do the above measurement).

Since a 100µF 25V capacitor costs 5c you might as well put one on both sides.

You'll want to keep ESR under 2 ohms to reduce voltage sag due to 100mA current spike to a reasonable 0.2V maximum. Since most general purpose caps have tandelta around 0.1-0.2, this means 100-200µF or more. A 10µF cap would have too much ESR to be of any use on the output.

You could use a low-ESR cap if you have'em, but then you'd have to worry about stability and such, so basically just reach into your parts bin and grab any general purpose electrolytic that's big enough and fits. It won't hurt if it has more capacitance than required.

Also if the thing will sit outside in the cold, remember that capacitor ESR goes through the roof at low temperatures, so in this case you might want to oversize the cap or use something a little more evolved like Panasonic FC. Or just stick a 10µF ceramic in parallel instead of your puny 330nF.

Do I need a bypass capacitor?

If, at the end of the day, adding capacitors at the input/output to the 7805 doesn't solve the problem, then consider using a buck converter instead of a 7805 linear regulator. It will provide 5 volts at (say) 1 amp and take 5 watts (plus maybe another half watt) from the 12 volt supply. This means that the current taken from the 12 volt supply is 5.5/12 = 460 mA and importantly NOT 1 amp. Maybe one of these: -

Plus it's got the added advantage that it'll still produce a regulated 5 volt output when the input is as low as 5.6 volts. If you need a little more current then try this: -