Diode bridge rectifiers - How to know what diodes and capacitor to use

Thread Starter

alchemizt

Joined Mar 23, 2021
24
I bought a 30V voltage regulator kit (see photo attached) and it comes with a 220V to 30V 2A transformer. It comes with 4 iN4007 diodes and a 2200uF 35V capacitor as the rectifier part of the circuit. What if I was to use a different type of diode instead of iN4007?

And for the capacitor, I also have a 2200uF 25V capacitor, what if I used that instead? Does capactor needs to have at least 30V (the output voltage of the transformer) to be able to smooth the voltage properly? And what if I used a different capacitance like 700uF?
 

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ThePanMan

Joined Mar 13, 2020
357
Well, there's a whole lot to consider. First, 30VAC rectified and filtered will become 42.4VDC minus the Vf (forward voltage) of the diodes. In a bridge rectifier there are always two diodes conducting, so the standard 0.6Vf per diode would be a voltage drop of about 1.2V total drop. That will reduce the DC voltage to approximately 42.2 volts DC. Never use a capacitor who's voltage is not at least 1 1/2 times higher than the expected voltage. In the case of 42 volts, I'd use a capacitor of at least 63.3 volts. Well, you can probably get away with a cap rated at 50 volts, that's higher than the expected loaded voltage. But that's a loaded voltage. It's hard to guess what that voltage could be when not loaded. So I'd opt for a higher voltage cap.

The 1N4007 is a bit over rated, but it won't harm anything. Won't waste any power or cause any concerns with your project. As for the capacitors you are asking about using; DON'T use the lower voltage cap - it'll likely explode. No, you won't blow the roof off the house, but it will be a rather exciting moment when it goes bang. Using a 700µF cap - yeah, you can do that but you won't have the filtering capacity you will with the higher (2200µF) cap.

On the output side, after the regulator, using a 30V cap will be close to the max. To be honest, I just don't like the design they gave you. How do you know it's 30V output? From where did you get it? It's possible you got something someone misrepresented. Perhaps they don't fully understand AC (RMS) to DC (Filtered). I wouldn't have designed a regulator like that. Off hand, without looking at the data sheet for the LM317, I'm not even sure it can handle 42 volts as an input. It might be above what it's capable of handling. I'm going to look up the 317 and see. I'll post what I find in a few minutes.
 

ThePanMan

Joined Mar 13, 2020
357
Unless your LM317 is a LM317HV - it won't handle more than 40 volts input. That is yet another nail in the coffin of this board. HERE is the data sheet for a Texas Instruments LM317 which includes the LM317HV. It'll show you what its input voltage is likely to be - provided it's a legitimate 317. Stuff from China is always in question as far as authenticity. Too often I've seen things come with ratings that are unrealistic.

Also keep in mind that if you regulate that high voltage down to a much lower voltage then the regulator has to waste the extra voltage as heat. There's a good chance you will need a heatsink for the regulator as well. The 5K pot makes me think you can adjust this way down to some very low voltage. Caution is strongly advised. And don't get your hopes up that you might get lucky with this kit. I think it's a stinker of a kit.

Good luck.
 

Veracohr

Joined Jan 3, 2011
764
The capacitor value depends on how much ripple the subsequent circuits can accept, and the diodes need to be able to handle the surge current that depends on the capacitor value, in addition to the max load rms current and transformer secondary voltage (peak).

Why do you want to use different components?
 

crutschow

Joined Mar 14, 2008
29,520
Oh, and the data sheet says it's rated for 1.5 amps.
That's too high.
The DC output current for a full-wave rectifier/filter should be no more that about 60% of the transformer RMS current rating due to the high peak RMS current the rectifier takes.
The DC current out should thus be no more that 1.2A.
 

Thread Starter

alchemizt

Joined Mar 23, 2021
24
So if I was to replace the parts that wont work with parts that will, what should I replace in this circuit?

The 2200uF 35V capacitor with a 2200uF 60V capacitor. The LM317 with a LM317HV?
Unless your LM317 is a LM317HV - it won't handle more than 40 volts input. That is yet another nail in the coffin of this board. HERE is the data sheet for a Texas Instruments LM317 which includes the LM317HV. It'll show you what its input voltage is likely to be - provided it's a legitimate 317. Stuff from China is always in question as far as authenticity. Too often I've seen things come with ratings that are unrealistic.

Also keep in mind that if you regulate that high voltage down to a much lower voltage then the regulator has to waste the extra voltage as heat. There's a good chance you will need a heatsink for the regulator as well. The 5K pot makes me think you can adjust this way down to some very low voltage. Caution is strongly advised. And don't get your hopes up that you might get lucky with this kit. I think it's a stinker of a kit.

Good luck.
So I should get a LM317HV, a 60V 2200uF capacitor and a heat sink. Thanks for the guidance, these are exactly the things Im wanting to learn.
 

BobTPH

Joined Jun 5, 2013
4,773
A 24V transformer would be more suitable. Then a 35V cap would at least be within specs, though I would use a higher rated one for reliability.

Bob
 

MisterBill2

Joined Jan 23, 2018
11,621
In voltage but not for current.
The are only rated for 1A average.
I just saw this thread and certainly the 1N4007 is a poor choice for a 2 amp supply because it is a 1 amp diode. And likewise the capacitor certainly does need to have a higher voltage rating.
So it is quite clear that the kit sellers are either incompetent or dishonest, or both.
 

Thread Starter

alchemizt

Joined Mar 23, 2021
24
So I put together a voltage regulator circuit kit (see pictures attached) that I bought. When I plug it in and test the output with a multimeter, it reads 0V so something has gone wrong. How can I begin to troubleshoot and diagnose where the issue is?

So far Ive only tested the input wires and they read 30V as expected. Can I attach my multimeter to a part of the circuit to test only the diode bridge rectifier? Then I can test the rectifier with the capacitor, and step by step test everything until I find what part of the circuit is broken? Or is there a better way to do this?

To test the rectifier would I put the multimeter probes on the leads of the big capacitor?
 

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bertus

Joined Apr 5, 2008
21,861
Hello,

A lot of connections are not soldered or not soldered properly.
To make an electrical connection the points need to be soldered properly.

the following page will give you some tips on through hole soldering:
https://www.mikroe.com/blog/through-hole-soldering
Don´t let you be put off by the number of solderspots.
I once made a board with 30 chips on it.

These are more decent videos on soldering:



Bertus
 

dl324

Joined Mar 30, 2015
14,323
How can I begin to troubleshoot and diagnose where the issue is?
The usual ways. You can start where the circuit isn't functioning and work back until you find something that is, or you can start at the beginning and work your way forward until you find something that isn't working.

Your problem is the many leads that aren't soldered and, likely, bad solder joints.

When soldering, you apply the iron tip to the component lead and pad to heat them. Then you apply solder at a point away from the tip. You allow solder to flow until it forms an adequate joint, remove the solder wire, then the tip, and allow the joint to cool without subjecting it to any vibration that cause the joint to be cold.

A small blob of solder on the tip and/or flux would facilitate heat transfer.
 

bertus

Joined Apr 5, 2008
21,861
Hello,

Also there are many long wires all over the PCB that can create shorts.
When a point is soldered properly, cut off the wire to avoid the shorts.

Bertus
 

Audioguru again

Joined Oct 21, 2019
4,536
EEK and Yikes.
No wire clippers? You had very poor vision while soldering? Run out of solder while soldering?
Of course there is no link to the kit's manufacturer and no schematic. I also could not find the kit on Google.
 
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