How to pick up cap for low voltage AC -> DC bridge rectifier? - Page 1
Forum section: electronics / projects

author: nardev
author_rank: Unknown
timestamp: June 12, 2019, 01:50:17 am
content: I have some power in the intercom locker which is like 24VAC, at the same time used for lock and intercom communication.I would like to add some more control to it with my board, which requires VDC ~(8V-12V) but i can't add normal 220VAC->12VDC so i would rather like to use what i already have 24VAC (which is usually like 16-18VAC due to other devices already powered there)So if i get "bridge rectifier" circuit before that, i'm wondering how to calculate adequate cap value in order to provide enough voltage and current for my controller board?My board already have decent buck converter and it behaves pretty ok with 8-12VDC range, even with some oscillation.What i'm also concerned about is the moment when the lock draws and spikes almost 0.5A, at that particular moment i wish to have 3-5 seconds of current and stable voltage so that my board doesn't reset but stays online.thnxp.s. I already tested some huge caps (forgot the values now), it was working pretty fine but i wish to be sure and make proper calculation to back my cap choice.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 12, 2019, 02:32:06 am
content: Quote from: nardev on June 12, 2019, 01:50:17 amThis is a textbook problem and has been addressed a zillion places:https://www.changpuak.ch/electronics/power_supply_design.phpAs far as the need to keep your board alive during the pulse..have a separate pathway feed your board through a blocking diode. with another cap and possibly a series resistor. Calculate cap/resistor value based on what your logic current draw is and your holdup time needs to be. When bridge cap drops because of lock current, blocking diode will prevent logic cap from dropping to feed it.

author: mariush
author_rank: Super Contributor
timestamp: June 12, 2019, 02:46:39 am
content: Ok....Let's go through some basics.Your transformer has an output voltage and some power rating, let's say 24v AC and 25 VAIn reality, your transformer will only output 24v AC with some significant load on it, and with the optimum input voltage.* At low low (for example with nothing connected to the transformer), it's typical for transformers to output up to 10-15% higher voltage maybe even more. So, your 24v AC transformer may output 26..27v AC, even more. This is normal and it's important to remember when choosing the capacitor.* You have to account for variations in AC voltage in your country. If the transformer is a 230v -> 24v AC one, it may designed to output 24v AC at a reasonable load with as little as 225v AC on the input. However, if the AC voltage goes to 220v or even lower, the output of the transformer may be lower, like let's say 23v AC.So keeping these in mind, it would be smart to treat your 24v AC transformer as a 23v...27v AC transformer.When you rectify AC to DC using a bridge rectifier, you get a higher DC voltage but less current.The amount of current can be approximated with the formula:Idc = 0.62 x IacThe 0.62 is a constant that works fairly well for 20..100VA transformers, maybe even higher. It's not perfect, but it's good enough.So in our example of a 24v AC transformer with 25VA rating, this means the transformer has an Iac = 25VA / 24v = 1.041 A. Using formula above, the maximum current would be Idc = 0.62 x 1.041 = 0.65AThe peak DC voltage will be equal to :Vdc peak = Vac x sqrt(2) - 2 x Vdiode, where Vdiode is the forward voltage drop on one diode that forms the bridge rectifier.So again, using our 24v AC transformer example, let's say you use typical bridge rectifier or 4 1n400x diodes, which have a forward voltage drop of ~ 0.8vUsing these numbers, you have the Vdc peak = 1.4142 x 24v - 2 x 0.8v = 32.3vHowever, let's go with the absolute minimum of 22v AC (100% load AND <220v AC) and absolute maximums of 28v AC (low load and >230v AC)22v AC : Vdc peak = 1.4142 x 22 - 2x0.8v = 29.5v28v AC : Vdc peak = 1.4142 x 28 - 2x0.8v = 38vThis absolute peak of 38v tells you that it's not a good idea to use capacitors rated for 35v, you should use capacitors rated for 50v or higher.Now you need to add capacitance after the bridge rectifier, in order to smooth out this shaky DC voltage... without capacitance, you have DC voltage, but it's like an 120 Hz AC waveform, only it doesn't go below 0v.An easy formula to use to approximate how much capacitance to use is this one :C = Idc max / [ 2 x Mains AC Frequency x ( Vdc peak - Vdc minimum) ]In the case of my 24v AC 25VA transformer, I determined above that it has a maximum Idc of around 0.65A, and that the peak DC voltage will be between 29.5v and 38v depending on conditions, with a typical peak voltage of 32v. You're in Europe where the mains AC frequency is 50 Hz, so knowing this, we have all the numbers to put in the formula above.Let's say you want the minimum output voltage to be 20v at any time. So, the formula becomes C = 0.65A / [ 2 x 50 Hz x ( 29.5v - 20v) ] = 0.65A / 950 = 6.8421e-4 Farads or 684uFSo a minimum value of 684 uF would kind of guarantee that you'll have 20v at any time on the output, as long as the minimum AC voltage is 22v or higher, and the current doesn't go over 0.65AAt lower currents like let's say 0.1A, the capacitor will discharge less, so the average minimum voltage will be higher than 20v.Anyway, 684 uF is not a standard value, in this particular example with this fictional transformer, you'd either go with 680uF or 820uF, which are standard values.You can go with bigger values, but there's downsides: bigger capacitor, more expensive, and bigger capacitors can also trip fuses (when you plug the device in mains, the empty capacitor draws a lot of current as it charges, so the fuse on the primary side could break if the current is too high for long enough time. Also, the bridge rectifier or the diodes in the bridge rectifier can be damaged. It's highly unlikely with a 25VA transformer, but it's important to remember if you're gonna work with bigger transformers in the future.

author: nardev
author_rank: Unknown
timestamp: June 12, 2019, 02:49:11 am
content: Oh man, you gave me a lesson. ThnxI was using triple the value, and unfortunately i didn't think much about the fuse. Thnx.