Electrolytic capacitor ripple current rating. - Page 1
Forum section: electronics / beginners

author: anotherlin
author_rank: Frequent Contributor
timestamp: November 06, 2015, 07:31:25 am
content: Hi everyone,Just to double check before buying a lot of caps.My question is as follow : If I have regulated DC voltage in a circuit, do I have to consider AC ripple current for capacitor (on the regulated DC rail) inside this circuit ?Let's say I have circuit using 400mA at 5V and it takes 9V AC as input.The filtering capacitors in the power supply input must have at least 400mA ripple current rating.Then everything goes to a 7805 regulator. From now on, it supplies regulated 5V for all the circuit.In the rest of the circuit, do I have to care for current consumption for each part of it requiring a capacitor ?Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2015, 09:08:39 am
content: Actually it's more like 200mA, and probably more like 150mA RMS, for the input cap. The DC current draw is approximately the peak-to-peak current through the capacitor (work it out!).Regulated DC is just that, if it has any ripple it's probably not very good!Ripple rating will not matter, but ESR will still be important for dampening the regulator and load.Tim

author: elektroda
author_rank: Newbie
timestamp: November 06, 2015, 09:27:16 am
content: the most important is how big current download may from source , example transformer. later ,.Next important parameter is coefficient ESR . How big current may give back capacitor after bridge. If load is impulse , if quickly must give back big current , ESR must be small , then capacitor give back very quickly more curren at a little internal loss , If are a little small internal loss , then is small temperature. You must read what it is coeficcient ESR

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2015, 09:33:34 am
content: Behind a linear regulator Ripple current rating is usually not critical, unless you have a very extrem load that needs fast pulsed current like a Stepdown converter without the input capacitor.Usually there should be no big elektrolytic cap behind the 7805, but somthing like 100 nF-470 nF as a ceramic capacitor directly at the regulato. More small capacitors (decoupling) may be needed at the load. If electrolytics are needed to buffer puls loads, they are better close to the load.Ripple current at the rectifier very much depends on the impedance of the AC source and the size of the capacitor. With a small transformer and not so big capacitors somthing like a rippel current of 50% of the DC current sounds resonable. With a large, low impedance transformer (e.g. 200 VA torroidal) the RMS ripple current can be twice the DC current. Keep in mind that 400 mA DC current may need something like 600 mA of AC current from the transformer.

author: anotherlin
author_rank: Frequent Contributor
timestamp: November 06, 2015, 10:18:47 am
content: Ok, thank you for your answers.It's just that I bought some smaller in size Nichicon 10uF/16V caps, but found out that they have 20mA ripple current rating versus the 60mA I regularly buy.They are all to be used after the regulation, so it should be DC with almost no (residual) ripple. The load is audio (synth), not pulsed.I was a little bit concerned because, except when design the PSU, I've never really paid attention to the ripple current ratings.

author: mariush
author_rank: Super Contributor
timestamp: November 06, 2015, 03:02:42 pm
content: Use 25v or 35v rated capacitors at the input, at such low capacitance values the diameter and heights of capacitors are pretty much the same. But I suspect you'll need a lot more capacitance at the input.9v AC input is not such a good choice to power a circuit that consumes 400mA at 5v.You would have to rectify this 9v AC using a bridge rectifier, which means you'll have a peak DC voltage of 9v x 1.414 = 12.7v , minus the losses in the two diodes in the bridge rectifier that are always on (which would be about 0.7v for each diode), so you're left with about 11 volts.However, if the 9v AC is made using a plain transformer or a basic power adapter, that means the output voltage will often be much higher than 9v when the circuit is turned off or idle (when the current output of the transformer is very low), so you may actually have more than 9v AC coming out, could be even 10-11v AC. At the same time, there may be variations in the mains voltage which would cause variations in the output of the transformer secondary, making the voltage less than 11v DC peak, or higher than 11v DC peak.The basic idea is that you may have a DC voltage anywhere between 10v and about 13v at the output of the rectifier, so using a 16v rated capacitor is cutting it kind of close. It would be safer to use a 25v rated capacitor or higher.One more thing... like I said, after the rectifier you get a peak voltage, you need capacitors to smooth it out and get it closer to a straight DC output. The size of the capacitor can be aproximated and depends on current and the mains frequency and other things.In this case, let's say you're in Europe so you have a mains frequency of 50 Hz and the minimum voltage you'll get after the rectifier will be 10.5v (just in case you'll sometime have a mains voltage less than 220-230v) and you need at least 400mA so let's do the math for 500mA.The minimum capacitor size to keep the DC output above a minimum would be aproximated using this formula C = Current in amps / [ 2 x Mains Frequency x ( V dc peak - V dc minimum) ]Since you use a 5v 7805 regulator, that means you need at least 6.5v for the regulator to work, so your formula becomes C = 0.5 A / 2 x 50Hz x (10.5-6.5) = 0.5 / 400 = 0.00125 Farads or 1250uF minimum to keep the voltage to at least 6.5v at any time. So it would make sense to use a 1200uF 25v or something bigger there.You can change the 7805 regulator to something that doesn't need as much voltage about the ouput voltage (7805 needs 1.5-2v above), and then you could use a smaller input capacitor. For example, 1117 regulators need only 0.8-1v above output to work right.Anyway, the reason I say 9v AC is not a good choice is because the peak dc voltage would be a bit high for the input of the 7805 linear regulator, if your circuit doesn't always use 400mA.For example, if your circuit uses only 200mA and you use an input capacitor after the rectifier chosen like above, it means at only 200mA the minimum dc voltage will be much higher, probably at around 8-9v DC. In that case, the linear regulator 7805 will have to dissipate the difference between input voltage and output voltage as heat, so you'll get (8-9v in - 5v out ) x 0.2A = 3-4v x 0.2a = 0.6-0.8 watts, which means the linear regulator will be hot. Above around 1.2-1.5w, especially in an enclosed case with minimal ventilation, you'll need to put a heatsink on that regulator.Unless you have something that needs AC voltage, it makes more sense to use a 7.5v - 9v DC power adapter, at least you'll know the dc voltage will always be in that range no matter the current consumption of your circuit.