What bridge rectifier and capacitor do I need? - Page 1
Forum section: electronics / projects

author: Lado
author_rank: Contributor
timestamp: May 02, 2022, 09:30:10 pm
content: I have a transformer that's 230V with two secondary windings. One is center tap with thicker red, black, white wires and the other is with thinner black and brown wires.The transformer is outputting the following voltages:Black and white - 8VBlack and brown - 12VWhite and red - 16VBlue and brown - 19VThese are the specs I found:12.5V / 12A U at idle speed : 2 x 7.6 or 1 x 15.1 ∆U - U decrease to : 1 x 13.8 / 12.5 / 10.6 Load current I : 6/12/20I'm interested if both outputs have the same output power with the difference that one has lower voltage but more ampers and other has a higher voltage but less current. Which one would be better to be used?What are the specs of the bridge rectifier and the capacitor that I need to convert it to DC voltage?I would connect it to a buck converter so I'm not limited by the output voltage.Can I use a 600V 15A bridge rectifier with a 10000uF 35V 105°C ±20% capacitor?

author: MarkF
author_rank: Super Contributor
timestamp: May 04, 2022, 01:40:47 pm
content: What is the max voltage and current that you're trying to achieve?What are the input requirements of the buck converter?Let me try to re-state the problem:The transformer secondaries look like this:I don't really understand the transformer spec you stated. Probably because the wire gauge is in metric.You just need to determine the max current capability of each secondary winding.I would pick the winding that with the higher current capabilities (i.e. the white-black-red) if the voltage is high enough for you needs. You decide based on the input requirements of the buck converter you mentioned. Are both windings capable of the current you require?Just because a winding is capable of delivering a stated current doesn't mean you have to use it all.As far as diode bridge and capacitors, that bridge would be okay, but you would not need the 600V capability, and the capacitor would be based on current requirement and the AC frequency. 10000uF sounds small for a 15A load.Calculating Filter Capacitor:https://www.homemade-circuits.com/calculating-filter-capacitor-for/

author: tunk
author_rank: Super Contributor
timestamp: May 04, 2022, 02:07:11 pm
content: What will you be using it for, i.e. voltage and current?You info is a bit unclear, where's the blue wire?I guess the specs are for the red-black-white:- 15.1Vac at no load (or 2x7.6Vac)- 13.8Vac at 6A load- 12.5Vac at 12A load- 10.6Vac at 20A loadI think a rule of thumb is to use ~2000uF/A and that this will give you ~4V ripple.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 04, 2022, 02:33:18 pm
content: The current rating bridge should be at least 20% higher than the current in the circuit. It will get hot without some kind of heat sinking, about 1W per Amp dissipation.At the rated current the capacitor needs to be larger. Its value depends on how much ripple you can tolerate.https://skillbank.co.uk/psu/smoothing.htm

author: Lado
author_rank: Contributor
timestamp: May 04, 2022, 04:35:30 pm
content: Quote from: MarkF on May 04, 2022, 01:40:47 pmI don't know the max. current of the windings.I only have five wires. The blue and brown are connected (continuous) and the red, black and blue. Those three are thicker and all three are continuous.I wasn't able to find any specs on the transformer other than the ones I already posted.The buck boost converter is rated for max. 75W and it won't be run for extended periods of time nor will it be under the full load.The input for it is 6-36V which is great because I'm not limited by the needed voltage.I chose that bridge rectifier because it's the type with a hole in the middle which lets me mount it in the housing for cooling and it's easier to connect it since I already have crimp connectors on the wires.https://canada.newark.com/productimages/standard/en_US/10P1437-40.jpg

author: Lado
author_rank: Contributor
timestamp: May 04, 2022, 04:38:42 pm
content: Quote from: tunk on May 04, 2022, 02:07:11 pmPlease see the post above where I gave more info. I think the transformer is rated at 12V - 12A so that seems to fit.

author: mariush
author_rank: Super Contributor
timestamp: May 04, 2022, 04:39:49 pm
content: Ignore the winding with thinner wires, as it's probably an auxiliary power supply which can only deliver a small amount of power (a few watts)From your text, I see it as red- black - white as the main secondary winding, with black as the center tap.Then you have this :QuoteWithout any load on it, it's normal for transformers to output up to 10-20% higher voltage, so you're measuring 8v AC instead of 7.6v AC they say.When you convert the AC to DC using a bridge rectifier, you get a peak DC voltage equal to sqrt(2) x Vac - 2 x (voltage drop inside bridge rectifier)So for example, assuming 15v AC under load, your peak DC output after rectification will be Vdc peak = 1.414 x 15 - 2 x 0.8v = 19.6v ... let's go with 19v DC peak voltage .Now you need to add capacitance to smooth this and get the voltage to be at least some minimum amount all the time.You can approximate the capacitance required with formula :C = DC current / [ 2 x ac frequency x ( Vdc peak - Vdc minimum desired) ]The specs are a bit unclear ... normally the transformer would say something like 15 Vac 100VA which means AC Current = 100VA / 15 Vac = 6.66 AThat's AC current - only valid with resistive loads like for example an incandescent bulb.When you use a regular bridge rectifier and convert the AC to DC, the voltage goes up, but the DC current will be lower than the AC current - there are some losses in the bridge rectifier after all.You can approximate the DC current with Idc = ~ 0.62 x Iac (0.62 is a constant that works well with 50VA and higher transformers)So in my example of transformer with 15vAC and 6.66A AC, you would expect a peak voltage of 19.5v DC and ~0.62 x 6.66 = 4.13A - with A LOT of capacitance you'd have 19.519.x4.13 = 80 watts (you lose 2 x ~0.8v x current Watts in the bridge rectifier)Anyway going back to your transformer, the quote mentions 6 / 10 / 20 A ... let's be conservative and say the transformer can maintain 15v AC at 10A DC so your info would beVdc peak = 19v AC Frequency = 50 Hz or 60Hz , Current = 10ALet's say you want at least 16v all the time ... you put the data in formulaC = 10A / [ 2 x 60 Hz x (19v - 16v ) ] = 10 / 120*3 = 1 / 36 = 0.027777 Farads so you'd need at least 28000 uF of capacitance to have the voltage at least 16v all the time, when your devices consume 10A of DC current.

author: Lado
author_rank: Contributor
timestamp: May 04, 2022, 04:44:34 pm
content: Quote from: Terry Bites on May 04, 2022, 02:33:18 pmThanks for the info. I tried to figure out what values I need but I didn't know many values needed for the formulas.You can see in the post above why I chose that bridge rectifier and other additional info. I don't know if the higher voltage rating on the rectifier could be a problem.

author: Zero999
author_rank: Super Contributor
timestamp: May 04, 2022, 04:52:33 pm
content: What else are you using it for, apart from the buck converter?What output DC voltage and current do your require?My advice is ditch the transformer, bridge rectifier, capacitor and buck converter and buy a proper switched mode mains power supply with the appropriate rating. It'll work out much cheaper, lighter and efficient.A buck converter has a constant power characteristic, so will need a much larger smoothing capacitor, than a resistive load, drawing the same current.

author: MarkF
author_rank: Super Contributor
timestamp: May 04, 2022, 05:02:42 pm
content: Quote from: Lado on May 04, 2022, 04:35:30 pmThis makes no sense. Where did all the voltages you stated in your first post come from?With only five wires, are you sure the thin wires aren't the primary and the thick wires the secondary?What does "continuous" mean?A picture might help...You haven't said what voltage and current you want to generate.Are you making a general purpose power supply?Is this what you have?

author: Lado
author_rank: Contributor
timestamp: May 04, 2022, 05:39:00 pm
content: Quote from: mariush on May 04, 2022, 04:39:49 pmYou helped me a lot. Thank you. Some things are clearer now.

author: Lado
author_rank: Contributor
timestamp: May 04, 2022, 05:48:05 pm
content: Quote from: Zero999 on May 04, 2022, 04:52:33 pmNothing else besides the buck converter. I plan to convert a UPS to a mini lab power supply. It already has a transformer and the housing so it's convenient to just add a variable buck boost converter (that I also already have). It's one of those little ones that has a LCD, few buttons and a knob. I don't need high power output because I'll use it for smaller repairs where it's nice to have the convenience of a variable power supply.The output is rated for 75W max., 0-36VDC and 5A max.The output is more precise than cheaper lab power supplies and with analog potentiometers and there are more options. I also like it because it's a fun project and I'm recycling.I was thinking about using something like thishttps://bdspeedytech.com/index.php?route=product/product&product_id=3608I think they have a 4A and 6A version.It's a more expensive solution. I would have to take the transformer out of the housing and figure out the spacers for the PCB, maybe additional cooling etc.I was also thinking about using an old computer PSU to use it with the buck converter but then there are other issues. It wouldn't fit all in the PSU housing, the housing is more inconvenient for use on the desk, it's louder, more prone to gathering dust etc.

author: Lado
author_rank: Contributor
timestamp: May 04, 2022, 06:00:23 pm
content: Quote from: MarkF on May 04, 2022, 05:02:42 pmIf you mean the rated voltages at certain loads, I found them googling the markings on the transformer. I got the output voltages by measuring them myself.Here are the pictures:https://ibb.co/3Wfc5KKhttps://ibb.co/ftrGWhqWhat I meant with continuity is that those wires are connected when I check them in continuity mode on the multimeter.The lower black and white wires are the primary widing for 230V and the other side has two windings. One is center tap with red, black, white and the other is brown and blue.

author: MarkF
author_rank: Super Contributor
timestamp: May 05, 2022, 09:46:03 am
content: Your transformer has one center tapped secondary winding and another non-center tapped secondary.I would use the heavier white-red wires since they will give you the highest current capability.The other winding (blue-brown) you could possibly use as a second lower current fixed output (maybe a fixed 12V output).As far as the diode bridge you showed, it will be fine, easy to mount and the transformer already has spade lugs. I've used bridges like that in several of my power supplies.