Conversion a DC capacitance value to AC. - Page 1
Forum section: electronics / beginners

author: pastecopy
author_rank: Newbie
timestamp: March 21, 2020, 04:16:44 pm
content: Hi, I came across this forum in regards of DC and AC capacitor.https://www.eevblog.com/forum/beginners/what-is-the-difference-in-an-ac-capacitor-over-a-dc-capacitor/msg1136967/#msg1136967I would like to use an AC to replace the DC capacitor but I have no idea what would be the equivalent value to use as DC has the tendency to have a lot bigger capacitance value than AC?Here's the value printed on the DC capacitor, 680 microF, 50V.What would be the equivalent AC capacitance? 680 microF AC capacitor is going to be very huge in sizes.Thank you.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 21, 2020, 04:29:04 pm
content: No, capacitance is the same. The thread you link discusses voltage ratings.Also note the maximum current the capacitor can take. AC voltage applied to a capacitor makes the current flow all the time.680uF capacitor capable of AC voltages - it has to be a film capacitor or a bipolar electrolytic - is a big one.What is your application, i.e., what are you actually doing? Is this for signal AC coupling?

author: pastecopy
author_rank: Newbie
timestamp: March 21, 2020, 04:39:35 pm
content: the electrolyte capacitor (DC) 680uF 50V is meant for small Fan Motor located inside a 1HP Air Conditioner.Newer model 1 HP Air Cond, only takes 1.0-1.5uF 450VAC to power up the Fan Motor. The physical size is already bulky if compared to the electrolyte cap.Can I use the 1.0-1.5uF 450VAC capacitor to replace the electrolyte 680uF 50V?I don't understand why a DC cap takes a lot bigger capacitance (680uF) compare to an AC capacitor (1.0-1.5uF) for the same power of a Fan Motor?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 21, 2020, 04:48:28 pm
content: The 680uF 50V electrolytic capacitor is connected to a completely different circuit, and doing something completely different than the "1.0-1.5uF" capacitor in another air conditioner.

author: Brumby
author_rank: Unknown
timestamp: March 21, 2020, 11:28:31 pm
content: Quote from: pastecopy on March 21, 2020, 04:39:35 pmYour question seems to indicate you feel there is a relationship between voltage rating and capacitance - and that this relationship has something to do with AC and DC.Quite simply: There isnot.A circuit will have the need for a specific capacitance in order for the required operation. Voltage does not come into this requirement.HOWEVER, in order for that capacitor tosurvivein the circuit, it must be rated appropriately (eg: voltage, temperature) AND be of an appropriate type - which includes bipolar (or not) as well as specific construction.QuoteAs has been said here:Quote from: Siwastaja on March 21, 2020, 04:48:28 pm

author: bdunham7
author_rank: Super Contributor
timestamp: March 22, 2020, 12:03:19 am
content: Quote from: pastecopy on March 21, 2020, 04:39:35 pmAssuming that you are correct and both of these capacitors are for the fan motor, they still are performing different functions. Assuming again that this is a split system and your fan motor is either the inside or outside (condensor) fan, you simply have one with a DC motor and one with an AC motor. You could, in theory, use a 680uF 450VAC capacitor in your DC system, but as you have figured out, it would be huge. The reason the DC system requires a larger value is that it is a reservoir of power while the AC system uses the capacitor for a phase shift--these are entirely different functions.

author: pastecopy
author_rank: Newbie
timestamp: March 22, 2020, 12:30:11 am
content: Thank you for the input.This Air Cond is already more than 20 yrs old, the possibility of being a DC fan motor is likely (will disassemble further to verify). The fan motor is meant for Indoor.Another question is that, ain't capacitor is used to start up a motor? It stores charges and fire up the fan motor before gaining constant rotation? Does this has any relation between a DC and an AC capacitor? A DC will need around 680uF to starts up meanwhile an AC cap takes only 1.0-1.5uF?I checked the back of the PCB and the fan motor has 3 wires connected to it. 2 of the wires are connected to this electrolyte cap. Meanwhile, the newer AC with 3 wires too, 2 of them are connected to the 1.0-1.5uF 450VAC.

author: pastecopy
author_rank: Newbie
timestamp: March 22, 2020, 01:25:05 am
content: The physical size of an AC cap is a lot lot bigger than DC with the same capacitance.45uF 450VAC is already that huge, I couldn't imagine a 680uF an equivalent capacitance of the DC cap.Why is this so?

author: Nerull
author_rank: Frequent Contributor
timestamp: March 22, 2020, 05:15:51 am
content: None of those are "DC".The run capacitor is big because its a foil cap, not because its "AC". It would be precisely the same size in a DC application.

author: vu2nan
author_rank: Contributor
timestamp: March 22, 2020, 05:36:51 am
content: An electrolytic capacitor is polarised and intended only for DC circuits. It's possible to have high-capacitance, small-sized electrolytic capacitors.Non-polarised film capacitors, usable in AC circuits, have low capacitance values due to size limitations.Nandu.

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2020, 10:30:49 am
content: Quote from: Nerull on March 22, 2020, 05:15:51 amThe 680µF capacitor is polarised and thus DC, whilst the others are non-polarised and therefore suitable for AC operation.Quote from: pastecopy on March 22, 2020, 01:25:05 amThe difference is the voltage ratings.The physical size of a capacitor is dependant on its construction and the amount of energy it can store.E =1/2CV2Where:E = energy in JoulesV = VoltageC = capacitance in FaradsFor the 680µF capacitorV = 50VC = 680µF = 680×10-6FE = 0.5×680×10-6×502= 0.85JFor the 45µF 450VAC capacitorNow the voltage is a little more tricky, since the rating will be for the RMS (Root Mean Square) voltage, yet the actual peak voltage will be √2 higher than that. Look up RMS peak voltage for more information.V = 450√2 = 636.4VC = 45µF = 45×10-6FE = 0.5×45×10-6×636.42= 29.1JIt's not surprising it's much bigger!If you repeat the calculation for the 1.5µF you'll find it stores less energy than the 680µF capacitor, despite being bigger, but it's a film capacitor, which is a less energy dense type of capacitor, than an electrolytic.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 22, 2020, 10:34:02 am
content: Quote from: pastecopy on March 22, 2020, 12:30:11 amThis isn't accurate.It's that a single phase AC signal cannot create good torque: it's like a single cylinder engine. At a certain point in time, the magnetic field tries to push against the rotor "sideways". You can live with it if you add a flywheel; once it already runs, it keeps running. It still sucks, though.Two different AC signals, phased apart 90 degrees, like in a two-cylinder engine, can produce torque all the time. (Or, you can have even more phases, like you can have a six-cylinder engine.)But in residential wiring, we are stuck with the wimpy single phase AC.Traditional solutions include:1) Use a brushed motor. The brush mechanism internally generates multiple "phases" so that different motor windings can push/pull in different directions,2) Build a two-phase motor, and electrically create another phase.What you are seeing is #2, the capacitor is used to add adelayin the mains power: it stores energy, and releases it back at a different time. The first phase of the motor is powered by the mains directly; the second one is powered via the capacitor delaying circuit. This isn't perfect; you can't create the optimal 90 degree phase shift, it will be less.Now, to the details: there are two major classes of this capacitor circuit: either capacitor-start, or capacitor-run applications. Sometimes you don't need continuous good torque; you have inertia, and once it spins up, it works well enough with one phase only. In this case, the second phase, created with the capacitor, is turned off after the motor has spun up.In capacitor-run type, the second phase is kept connected all the time.In either case, the capacitor value and type needs to be carefully selected so it creates the desired phase shift, and is suitable for the current going through the capacitor so it won't die early due to overheating.The 680uF 50V electrolytic capacitor has some completely different purpose. They are nice and small, but come with limitations; one is they can't take reverse voltage, i.e., will only work on DC. They can't be used as motor start/run caps, but there is no need to, either; such high capacitance values are not needed in motor start/run caps. Really big motors are always truly multiphase and ran from either 3-phase power directly, or through an inverter.

author: pastecopy
author_rank: Newbie
timestamp: March 27, 2020, 04:07:40 am
content: Thank you for the explaining the physical size determination of a capacitor, it really helps.And also the motor starting inertia momentum explanation.