Reducing voltage on a full bridge rectifier - Page 1
Forum section: electronics / beginners

author: baltazar99
author_rank: Newbie
timestamp: July 15, 2021, 10:25:00 pm
content: Hi,please bear with me, i am still trying to learn the basics.I have built a full bridge rectifier using 4 diodes and a couple of 1000 uF capacitors in parallel. the secondary AC voltage of the transformer I am using is 26v which gets converted to around 38v DC with the rectifier.I am trying to power a soldering iron controller with requires 26v DC with an output current of 6amps. What are my options to reduce the voltage to 26v DC with 6 amps current?NB: I have tried a voltage divider with 10 k resistors as R1 and 25k as R2 which gave me the correct voltage but as soon as I plugged in the load the output voltage dropped to 3v.Thanks in advance for your help.

author: bob91343
author_rank: Super Contributor
timestamp: July 15, 2021, 10:36:10 pm
content: A dropping resistor is always a poor solution, as is a voltage divider. A regulator is better.If you analyze your circuit, you will find that the output voltage without a load is about the peak voltage of the winding, less the diode drops. If you want to load this with a few Amperes, the output drops to a value that depends on the size of the transformer, diodes, and capacitors. The analysis isn't simple but is covered in most basic electricity or electronics textbooks.If you want to use a voltage divider, your choice of resistor value is very crucial. In your case, you raised the source impedance to such an extent that you couldn't get useful output current. Check out the Thevenin theorem.There are many solutions to your problem but each has its own flaws. I won't make a list here, but the very fact that you asked the question indicates that you need to learn some stuff.

author: xavier60
author_rank: Super Contributor
timestamp: July 16, 2021, 01:49:22 am
content: Quote from: baltazar99 on July 15, 2021, 10:25:00 pmThe rectified voltage will still be 26V RMS with no capacitors. But it's likely that the controller needs filtered DC. Show us its schematic.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: July 16, 2021, 02:05:51 am
content: 6amps you will probably need some kind of switching power supply, also possible you need more capacitance on the output to the rectifier(to maintain the output voltage above 26v). Resistive divider isn't going to work nor is some kind of linear psu because of the high current.

author: gbaddeley
author_rank: Regular Contributor
timestamp: July 16, 2021, 02:55:48 am
content: Quote from: bobbydazzler on July 16, 2021, 02:05:51 amA linear regulator will work, and is simpler than smps regulator, but is less efficient. If you can tolerate a few tens of watts of heat dissipation, use linear.Alternatively, use a slightly lower voltage tranny, and bigger filter caps (10,000uF or more).

author: bobbydazzler
author_rank: Regular Contributor
timestamp: July 16, 2021, 03:27:20 am
content: Well it's for a soldering station, I'd assume the iron is the only thing that will actually run at 26v(the board most likely has a linear regulator onboard for the electronics). In that case a linear supply with a giant heatsink/fan isn't practical I'd think, doubtful the soldering iron would care about a noisy 26v rail from a smps.

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: July 16, 2021, 03:31:31 am
content: Quote from: gbaddeley on July 16, 2021, 02:55:48 amwouldn't a linear regulator dissipate about 50-60W from dropping the voltage and feeding 6 amps to the load? this sounds like the job for a switched dc to dc converter

author: baltazar99
author_rank: Newbie
timestamp: July 16, 2021, 08:59:56 am
content: the controller is a Hakko T12 clone from AliExpress.https://www.aliexpress.com/item/1005002380823696.html?spm=a2g0o.detail.1000014.27.24f75426N2DqGs&gps-id=pcDetailBottomMoreOtherSeller&scm=1007.13338.177756.0&scm_id=1007.13338.177756.0&scm-url=1007.13338.177756.0&pvid=a55b4fd8-b22e-4051-831a-b532999f22aa&_t=gps-id:pcDetailBottomMoreOtherSeller,scm-url:1007.13338.177756.0,pvid:a55b4fd8-b22e-4051-831a-b532999f22aa,tpp_buckets:668%230%23131923%2399_668%23888%233325%234_3338%230%23177756%230_3338%233142%239890%2310_668%232846%238112%231997_668%235811%2327173%2311_668%236421%2330821%2393_668%232717%237562%23462_668%231000022185%231000066058%230_668%233422%2330698%23515&&pdp_ext_f=%257B%2522scene%2522%253A%252223416%2522%257DOnly a single 26v voltage source is needed to power both controller and iron. the problem is that the maximum voltage it can handle is 30v which is less than the output from the bridge rectifier. I tried a LM7805 regulator with input from the positive diodes and the voltage did not drop enough.

author: xavier60
author_rank: Super Contributor
timestamp: July 16, 2021, 09:24:51 am
content: Quote from: baltazar99 on July 16, 2021, 08:59:56 amIt actually states that it has a 30V 60A MOSFET, so that rules out supplying it directly with unfiltered 26V RMS pulsed DC which it could have so easily been designed to accept. I would rather power it from a conventional transformer instead of SMPS.A linear regulator might be practical. The filtered DC will drop somewhat when loaded and what exactly is the current draw of a T12 cartridge?

author: Zero999
author_rank: Super Contributor
timestamp: July 16, 2021, 09:48:35 am
content: What's wrong with using an SMPS? Mains frequency transformers are big and bulky and linear regulators horribly inefficient.Use a switched mode power supply. Something like this.https://www.meanwell-web.com/en-gb/ac-dc-single-output-power-supply-with-pfc-3-stage-enp--180--24

author: baltazar99
author_rank: Newbie
timestamp: July 16, 2021, 09:49:16 am
content: i am trying to use the controller with a C245 cartridge which draws 6 amps on 26volts. The controller can support many types of soldering tips. The power supply itself used to be for an old soldering station.

author: xavier60
author_rank: Super Contributor
timestamp: July 16, 2021, 09:52:46 am
content: Quote from: Zero999 on July 16, 2021, 09:48:35 amFor me it's the possible EMI which would be a real problem for me. For most users it wouldn't matter.I will try something like this some day.https://www.rchelicopterfun.com/t12-soldering-station.html

author: Zero999
author_rank: Super Contributor
timestamp: July 16, 2021, 10:02:16 am
content: Quote from: xavier60 on July 16, 2021, 09:52:46 amIt's only a soldering iron, so I don't see how EMI is an issue. There are quite likely other things in your lab which emit more EMI such as LED lighting, mobile phones, Wi-Fi etc. A decent, SMP with good fitering, isn't very noisy.

author: xavier60
author_rank: Super Contributor
timestamp: July 16, 2021, 10:02:44 am
content: Quote from: baltazar99 on July 16, 2021, 09:49:16 amIt still can be done. Problem is, get it wrong, series pass transistors break down, controller's MOSFET then breaks down causing the iron to turn red hot before burning out.

author: PKTKS
author_rank: Super Contributor
timestamp: July 16, 2021, 10:03:11 am
content: Quote from: baltazar99 on July 15, 2021, 10:25:00 pmJust get one of these ...** IT WILL SOLVE YOUR PROBLEM VERY NICELY **You will have your bridge working at 36 with proper regulationup to 24V at 5A should be more than enoughhttps://www.aliexpress.com/item/1000004290003.htmlDone a couple of these to myselfAffordable and reliable.Paul

author: mikerj
author_rank: Super Contributor
timestamp: July 16, 2021, 10:16:28 am
content: Quote from: baltazar99 on July 15, 2021, 10:25:00 pmWhich diodes have you used in your bridge rectifier? They'd better be big high current ones.

author: baltazar99
author_rank: Newbie
timestamp: July 16, 2021, 10:17:11 am
content: Quote from: xavier60 on July 16, 2021, 10:02:44 amYes you are right, i have already fried a couple of cartridges getting red hot.

author: baltazar99
author_rank: Newbie
timestamp: July 16, 2021, 10:20:42 am
content: i have used IN5408https://components101.com/diodes/1n5408-power-diode-pinout-equivalents-datasheet

author: PKTKS
author_rank: Super Contributor
timestamp: July 16, 2021, 10:23:22 am
content: get yourself a proper 5A bridgehttps://www.aliexpress.com/item/4000945803833.htmlMount on a good heatsink..And hopefully your TRAFO can bear that..Solder Irons pull huge spikes..Paul

author: baltazar99
author_rank: Newbie
timestamp: July 16, 2021, 11:51:44 am
content: i managed to get the voltage down to 26v using 2 zener diodes in series but they got way too hot.i will see what options people have been suggesting to be tried. It looks like an IC Regulator with variable output might be the best option. thanks to all for helping out

author: xavier60
author_rank: Super Contributor
timestamp: July 16, 2021, 12:04:37 pm
content: Quote from: baltazar99 on July 16, 2021, 11:51:44 amYou need to understand the relationship of voltage current and wattage.To safely dissipate the approximately 72 watts of heat produced by the difference between 38 and 26 volts at 6 amps, you will need 4 large TO-247 power transistors on a large heatsink. Something like large audio amplifier output transistor on a heatsink from a large audio amplifier.The fact that the iron will be powered at some low duty cycle most of the time, makes things a bit easier.`

author: Zero999
author_rank: Super Contributor
timestamp: July 16, 2021, 12:37:45 pm
content: Quote from: baltazar99 on July 16, 2021, 08:59:56 amA transformer and rectifier is a bad idea. The output voltage is unregulated and it's big and bulky. You'll be fine with any power supply between 24V and 28V, rated to at least 6A.Here's another suggestion.https://www.amazon.co.uk/Meanwell-Medical-Adapter-GSM160B24-R7B-Switching/dp/B07G59T56W

author: tunk
author_rank: Super Contributor
timestamp: July 16, 2021, 12:52:06 pm
content: An old laptop PSU/charger would probably work fine.It is ~19-20V, but will work unless you need a very high power solder station.

author: tszaboo
author_rank: Super Contributor
timestamp: July 16, 2021, 01:06:35 pm
content: The proper way of writing it is either:FULL BRIDGE (shaky camera) rectifierorbridge rectifierThere is no in-between.

author: Zero999
author_rank: Super Contributor
timestamp: July 16, 2021, 01:27:42 pm
content: Quote from: tunk on July 16, 2021, 12:52:06 pmThat will work, but bear in mind that, if it's designed for 26V, running at 19V, will only give you just over half of the maximum rated power. P = V2R, so 192/262= 53%.

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 16, 2021, 04:06:15 pm
content: It may work just fine as is. The 36V will drop down significantly when you draw 6A. There would be significantly ripple if you draw 6A and with only the 1000MFD capacitor for filter.

author: Zero999
author_rank: Super Contributor
timestamp: July 16, 2021, 04:20:47 pm
content: Quote from: BeBuLamar on July 16, 2021, 04:06:15 pmThe problem is, the switching MOSFET is only rated to 30V and the voltage will rise, when the temperature controller turns it off.Using a transformer, bridge rectifier and capacitor to power this circuit is a recipe for unreliability and magic smoke.

author: baltazar99
author_rank: Newbie
timestamp: July 16, 2021, 05:56:29 pm
content: after considering everybody's recommendation i will use a switching power supply which outputs the exact 26v at 6amps. I can use the transformer for other projects i am sure. thanks everyone.