# High current, low voltage capacitors

I'm putting together a high current (200A+), low voltage (<5v) rectifier circuit.

I have sufficient bridge rectifiers, but to smooth out the waveform I need a strong enough capacitor bank. Besides opting for capacitors rated at a voltage of 10vdc (double the 5v just to be safe), what other stats or capacitor specs should I be looking for? Just get three one with the highest max operating temperature? The AC wave will be 50hz.

• Low-voltage high-amperage power sources are not designed as plain AC rectifiers. They are made as "AC-DC converters", where AC gets rectified to high-voltage DC, the DC gets converted into high-frequency "chopped" signal, and then transformed into low-voltage, and then rectified using synchronous active rectifiers. The smoothing caps then operate at a very high frequency and don't need to be of extremely high values. You need to seriously reconsider your design approach, unless you are making some welding apparatus. – Ale..chenski Feb 19 '19 at 1:46
• In addition to what Ale..chenski said, you have discovered that at high currents and low frequency you need an unreasonable amount of smoothing capacitance since the charge consumed is so high but the refresh rate is so low. The ripple will be enormous even with massive caps. Refresh a few thousand times faster and your capacitors can be a few thousand times smaller. Capacitors have a limit on ripple current anyways and such high currents and low frequencies will produce enormous ripple currents that will overheat and blow caps. – DKNguyen Feb 19 '19 at 1:54
• what is your ripple spec 10% V? with 200A RMS – Tony Stewart Sunnyskyguy EE75 Feb 19 '19 at 3:39
• @SunnyskyguyEE75 haven't considered that, just want to minimise what ripple there is. If it's not worth the effort, I may just leave it as AC. The purpose is to heat a piece of carbon or stainless to serve as a heating element. Was considering rectifying the AC signal to minimise any eddy currents. – Chris Feb 19 '19 at 3:42
• You can use a 6V 700A CCA lead acid battery – Tony Stewart Sunnyskyguy EE75 Feb 19 '19 at 4:18

First make a rough spec for ripple from C and ESR.

For $$\C=I_C*dt/dV\$$
with Vc=5V and let ripple=5% or dV=250mV(pp) and dt= 10ms ( 100Hz) with Ic=200A

$$\C=200A*10ms/250V=8mF\$$
let dV=2% 100mV from cap
ESR=V/ESR=0.1V/200A=500 μΩ

For a total ripple of 250mV+100mV=350mVpp

ESR*C=T= 500 μΩ * 8mF = 4 μs

Now check Electric Double Layer Capacitors (EDLC), Supercapacitors specs. anything close rated for >5V

8.2uF 50mΩ TOO HIGH by 100x even at \$16

Now you know this concept is going to be more expensive than a 1kW PC SMPS PSU.