When to Use Does a bridge rectifier reduce voltage?？

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baltazar99

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Reducing voltage on a full bridge rectifier

« on: July 15, , 10:25:00 pm » 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 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.

bob

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Re: Reducing voltage on a full bridge rectifier

« Reply #1 on: July 15, , 10:36:10 pm » 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.

xavier60

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Re: Reducing voltage on a full bridge rectifier

« Reply #2 on: July 16, , 01:49:22 am »

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 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.

The rectified voltage will still be 26V RMS with no capacitors. But it's likely that the controller needs filtered DC. Show us its schematic. « Last Edit: July 16, , 01:56:06 am by xavier60 » HP A dso, Fluke 87V dmm,  Agilent UA psu,  FY function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.

bobbydazzler

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Re: Reducing voltage on a full bridge rectifier

« Reply #3 on: July 16, , 02:05:51 am » 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.

gbaddeley

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Re: Reducing voltage on a full bridge rectifier

« Reply #4 on: July 16, , 02:55:48 am »

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.

A 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). Glenn

bobbydazzler

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Re: Reducing voltage on a full bridge rectifier

« Reply #5 on: July 16, , 03:27:20 am » 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.

Anthocyanina

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Re: Reducing voltage on a full bridge rectifier

« Reply #6 on: July 16, , 03:31:31 am »

A 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).

wouldn'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

baltazar99

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xavier60

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Re: Reducing voltage on a full bridge rectifier

« Reply #8 on: July 16, , 09:24:51 am »

the controller is a Hakko T12 clone from AliExpress.

https://www.aliexpress.com/item/.html?spm=a2g0o.detail..27.24fN2DqGs&gps-id=pcDetailBottomMoreOtherSeller&scm=...0&scm_id=...0&scm-url=...0&pvid=a55b4fd8-b22e--831a-bf22aa&_t=gps-id:pcDetailBottomMoreOtherSeller,scm-url:...0,pvid:a55b4fd8-b22e--831a-bf22aa,tpp_buckets:668%230%%_668%%%234_%230%%230_%%%_668%%%_668%%%_668%%%_668%%%_668%%%230_668%%%&&pdp_ext_f=%257B%scene%%253A%%%257D

Only 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 LM regulator with input from the positive diodes and the voltage did not drop enough.

It 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?
HP A dso, Fluke 87V dmm,  Agilent UA psu,  FY function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.

Zero999

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Re: Reducing voltage on a full bridge rectifier

« Reply #9 on: July 16, , 09:48:35 am » 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

baltazar99

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Re: Reducing voltage on a full bridge rectifier

« Reply #10 on: July 16, , 09:49:16 am » 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.

xavier60

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Re: Reducing voltage on a full bridge rectifier

« Reply #11 on: July 16, , 09:52:46 am »

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

For 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
HP A dso, Fluke 87V dmm,  Agilent UA psu,  FY function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.

Zero999

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Re: Reducing voltage on a full bridge rectifier

« Reply #12 on: July 16, , 10:02:16 am »

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

For 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

It'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.

xavier60

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Re: Reducing voltage on a full bridge rectifier

« Reply #13 on: July 16, , 10:02:44 am »

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.

It 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. « Last Edit: July 16, , 10:08:44 am by xavier60 » HP A dso, Fluke 87V dmm,  Agilent UA psu,  FY function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.

PKTKS

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Re: Reducing voltage on a full bridge rectifier

« Reply #14 on: July 16, , 10:03:11 am »

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 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.

Just get one of these ...

** IT WILL SOLVE YOUR PROBLEM VERY NICELY **

You will have your bridge working at 36 with proper regulation
up to 24V at 5A should be more than enough

https://www.aliexpress.com/item/.html

Done a couple of these to myself
Affordable and reliable.

Paul

mikerj

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Re: Reducing voltage on a full bridge rectifier

« Reply #15 on: July 16, , 10:16:28 am »

I have built a full bridge rectifier using 4 diodes and a couple of uF capacitors in parallel.

...
I am trying to power a soldering iron controller with requires 26v DC with an output current of 6amps.

Which diodes have you used in your bridge rectifier?  They'd better be big high current ones.

baltazar99

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Re: Reducing voltage on a full bridge rectifier

« Reply #16 on: July 16, , 10:17:11 am »

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.

It 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.

Yes you are right, i have already fried a couple of cartridges getting red hot.

baltazar99

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Re: Reducing voltage on a full bridge rectifier

« Reply #17 on: July 16, , 10:20:42 am » i have used IN
https://components101.com/diodes/1n-power-diode-pinout-equivalents-datasheet

PKTKS

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Re: Reducing voltage on a full bridge rectifier

« Reply #18 on: July 16, , 10:23:22 am » get yourself a proper 5A bridge

https://www.aliexpress.com/item/.html

Mount on  a  good heatsink..

And hopefully your TRAFO can bear that..
Solder Irons pull huge spikes..

Paul

baltazar99

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Re: Reducing voltage on a full bridge rectifier

« Reply #19 on: July 16, , 11:51:44 am » 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

xavier60

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Re: Reducing voltage on a full bridge rectifier

« Reply #20 on: July 16, , 12:04:37 pm »

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

You 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.` HP A dso, Fluke 87V dmm,  Agilent UA psu,  FY function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.

Zero999

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Re: Reducing voltage on a full bridge rectifier

« Reply #21 on: July 16, , 12:37:45 pm »

the controller is a Hakko T12 clone from AliExpress.

https://www.aliexpress.com/item/.html?spm=a2g0o.detail..27.24fN2DqGs&gps-id=pcDetailBottomMoreOtherSeller&scm=...0&scm_id=...0&scm-url=...0&pvid=a55b4fd8-b22e--831a-bf22aa&_t=gps-id:pcDetailBottomMoreOtherSeller,scm-url:...0,pvid:a55b4fd8-b22e--831a-bf22aa,tpp_buckets:668%230%%_668%%%234_%230%%230_%%%_668%%%_668%%%_668%%%_668%%%_668%%%230_668%%%&&pdp_ext_f=%257B%scene%%253A%%%257D

Only 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 LM regulator with input from the positive diodes and the voltage did not drop enough.

A 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

tunk

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Re: Reducing voltage on a full bridge rectifier

« Reply #22 on: July 16, , 12:52:06 pm » 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.

tszaboo

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Re: Reducing voltage on a full bridge rectifier

« Reply #23 on: July 16, , 01:06:35 pm » The proper way of writing it is either:
FULL BRIDGE (shaky camera) rectifier
or
bridge rectifier

There is no in-between.

Zero999

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Re: Reducing voltage on a full bridge rectifier

« Reply #24 on: July 16, , 01:27:42 pm »

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.

That 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%.

Learn about the full bridge rectifier – how to convert AC alternating current into DC direct current. Using capacitors to filter the rippled DC wave into smooth DC with experiments.

Remember, electricity is dangerous and can be fatal. You should be qualified and competent to carry out any electrical work.

Scroll to the bottom to watch the YouTube tutorial.

The most common method used is the full wave bridge rectifier. This uses four diodes nodes. The AC supplier is connected between diodes one and two, with the neutral between three and four. The DC positive output is connected between diodes two and three, and the negative between diodes one and four. In the positive half of the sine wave, the current flows through diode two, through the load, through diode four, and then back to the transformer.

In the negative half, the current flows through diode three, through the load, through diode one, and then back to the transformer. So the transformer is supplying an AC sine wave, but the load is experiencing a rippled DC waveform because the current flows in just one direction. In this example circuit, we can see that rectified waveform on the oscilloscope, but this is not a flat DC output, so we need to improve this by adding some filtering.

Using a rectifier will result in a ripple in the waveform. To smooth this out, we need to add some philtres. The basic method is to simply add an electrolytic capacitor in parallel to the load. The capacitor charges during the increase in voltage and stores the electrons. It then releases these during the decrease.

This therefore reduces the ripple. The oscilloscope will show the peaks of each pulse, but now the voltage doesn’t decrease to zero. It slowly declines until the pulse charges the capacitor again. We can further reduce this by using a larger capacitor or by using multiple capacitors. In this simple example, you can see the LED turns off as soon as the power is interrupted.

But if I placed a capacitor in parallel with the Led, it remains on because now the capacitor is discharging and powering the LED during the interruptions. In this circuit, I have a lamp connected as the load. The oscilloscope shows the rippled waveform. When I add a small ten microfarad capacitor, we see that it makes very little difference to the waveform. When I use a 100 microfarad capacitor, we see the dip is no longer down to 0 volt.

For more information, please visit Does a bridge rectifier reduce voltage?.

At microfarads, the ripple is very small. At microfarads, it’s nearly completely smooth. This would be fine to use for many electronic circuits, we could use multiple capacitors also. Here we have a 470 microfarad capacitor, which has made some difference. But if I use two capacitors in parallel, we see the waveform is much more improved.

When using a capacitor, we need to place a bleeder resistor across the output. This is a high value resistor, which will drain the capacitor when the circuit is off to keep us safe. Notice with this circuit that when I switch it on, the capacitor charges quickly to over 15 volts. But when I switch it off, the DC output is still at 15 volts because there is no load, so the energy is still stored in the capacitor.

This could be very dangerous if the voltage is high. In this example, I place a 4.7K Ohm resistor across the output. We see the capacitor charges up to 15 volts and when I switch it off, the capacitor quickly discharges. The electrons are flowing through the resistor which discharges the capacitor. We can also see that without a capacitor, the output voltage is lower than the input voltage because of the voltage drop of the diodes.

Here we have a simple full wave bridge rectifier on the input. We see there is 12 volts AC on the output. We have ten 5 volts of DC. The voltage on the output is lower because of the diodes. Each diode has a voltage drop of around zero 7 volts.

If we look at this circuit with a diode and an Led, we can measure across the diode to see a voltage drop of around zero 7 volts. The current in our Fullbridge rectifier must pass through two diodes on the positive half and two diodes on the negative half. So the voltage drop combines and is around one four to one 5 volts. So that is why the output will be reduced. However, if we were to connect a capacitor across the output, we will see that the output voltage is now higher than the input voltage.

How is that possible? That’s because the AC input is measuring the RMS voltage or the root means squared. This is not the peak voltage. The peak voltage is 141 times higher than the RMS voltage. The capacitors are charged up to the peak voltage and then release.

There will be a small voltage drop because of the diodes, so the output is less than the peak input, but it will still be higher than the RMS input. For example, if we had 12 volts RMS on the input, the peak voltage would be 12 volts multiplied by 1.41, which is 16.9 volts. There will be a 0.7 volt drop here and here. So 16. 9 volts subtract 1.4 is 15.5 volts. The capacitors are charged up to this voltage. This is only the approximate answer, though. The amount of ripple and the actual voltage drop of the diodes will cause it to be slightly different in reality, but we can see that the output is higher than the input. Another common filter is placing two capacitors in parallel with a series inductor between these.

This is used for circuits with larger loads. The first capacitor smooths the ripple. The inductor opposes the changing current and tries to keep it constant, and the second capacitor, which is much smaller, will then smooth out the final remaining ripple. Additionally, we can also connect a voltage regulator to the output. This is very common and allows some variation to the input, but it will provide a constant output voltage.

This again has capacitors on either side of the regulator to ensure a smooth DC output. Here we can see a real version which is connected to a twelve volt AC supply and we see it has an output of around five V DC.

If you are looking for more details, kindly visit China Fast Recovery Diode Manufacturer.