# Relacing a smoked diode



## Totalwrecker (Feb 26, 2009)

What's the best/easiest way to replace a diode on a board?

Briefly I had a smoking tender!







and then no go! I found upon close inspection that there are 2 burned spots on the traces and a bad diode between them. 

I feel more confident slodering new leads than trying to unsolder the old... so I'm thinking a mechanical fix to remove it.

Either clipping the leads and drilling new holes for the replacement and soldering from behind,
or
clipping the leads close to the diode and slodering the new one to those clipped leads.

Any thoughts?

Thanks in advance,

your friend,
John


----------



## Greg Elmassian (Jan 3, 2008)

John, is it a leaded part or a surface mount? Is it mounted in a through hole situation? A picture would be great. 

Barring that, if you use a axial leaded part (leads out each end in a line) you can use some of the leads to lie on top of the circuit traces, and then do a long solder joint, i.e. the leads of the diode will "Replace" the burned up traces. 

Caution: I would strongly suggest determining (and eliminating) the cause of the overcurrent situation that killed your diode first! 

Regards, Greg


----------



## Totalwrecker (Feb 26, 2009)

Hi Greg, 
It's a surface mount and I'm not sure the traces are burned through other than being darker where the leads passed through the board and were soldered. Thanks for the repair tip. 
I figured when I drilled out the old leads I'd be able to see if there was more damage and could enlarge a layer for solder to bridge a gap, but using the leads horizontally makes sense too. 

My camera isn't very good at close ups or I would have. 

I was lazy when I caused the blue demon's release... using a DPST toggle for lights and sound and an un-insulated jumper, stuffing the wires back in caused the short. That has been eliminated. 
Had worked fine open on the bench, but The Kids were coming and I got in a rush.... 

John


----------



## Dan Pierce (Jan 2, 2008)

If the board has internal layers, drilling is not an option. 

A surface mounted diode can be replaced with an axial lead diode providing there is room for bending the leads. 

Also, why did the diode go bad, possibly another component shorted and took pout the diode???


----------



## TonyWalsham (Jan 2, 2008)

Where is the diode? 
Is it on a socket pcb inside a loco? 
Is it a -9 or SD-45?


----------



## Totalwrecker (Feb 26, 2009)

Hi Tony and Dan, 

Regarding the diode, leads come out the ends and are bent 90 degrees to go through the board.... I don't know which kind that is. 

It's on a R/C board, (not yours) I installed in the tender of a C-16. Aristocraft. 

Finances are tight and the $72 repair quote keeps the item at home. Upon inspection only the diode appears harmed. It's tucked in between larger devices. 

The board only has two layers, the lead through the board is soldered to a trace, looks good but heat tarnished and the other lead goes to a surface trace that an axial lead could be bent to lay on 'n soldered. 

I feel confident enough to solder in the replacement, , but need to know the best way to remove the bad. 

The cause has been eliminated. 

Thanks, 
John


----------



## TonyWalsham (Jan 2, 2008)

If it has leads it won't be surface mounted. 
The holes are probably plated through from one side to the other which will make removal much more difficult. 
What I do in that situation is to cut the legs off each end of the component. Then hold the pcb upside down and heat the underside of the pcb pad just sufficiently so that the reaming pieces of legs drop out. Don't get the pads too hot as they will likely lift off. Although if plated through that is unlikely to happen. 
You will probably need a solder sucker to remove any excess solder from the holes before soldering a new component.


----------



## Totalwrecker (Feb 26, 2009)

Thanks Tony, 
That's what I needed to know. 
Do you ever need to nudge them or will gravity alone do the trick? 

John


----------



## Greg Elmassian (Jan 3, 2008)

Cut the diode in two, and unsolder one lead at a time... too hard to have the solder molten on both leads at once. You should be able to keep heat on with one hand, and pull the diode lead out with needle nose with the other. 

Your radio shack will have some "solder wick", which will "wick up solder" if you put it near the solder and heat everything, but try the above method first, excessive heat will start removing circuit traces. 

It's almost impossible to get all the solder out so that something just drops out. 

Regards, Greg


----------



## TonyWalsham (Jan 2, 2008)

Granted the cut legs do not always just drop out with the application of heat to melt the solder. 
That does depend on how big the hole is and how sticky the surface tension of the solder is. 
I find a gentle rap downwards will often do the trick. 

Just be sure not to apply too much heat for too long. 
De lamination of the copper traces from the pcb material *WILL* occur.


----------



## George Schreyer (Jan 16, 2009)

Diodes rarely fail on their own, usually some external cause killed it. You need to figure out what that cause is BEFORE you install a new part or you'll probably smoke the new one too. 

If there are four diodes mounted closely together, the bad diode is probably part of a bridge rectifier. Other diodes in that same bridge may be blown too. 


Your part is a thru hole part. Use Greg's method. Cut the package in half. Grab each half with a pair on needle nose pliers and melt the solder on that side and simply pull it out. Do the same for the other side. The solderwick is useful for getting the remaining solder out of the hole to allow another lead to be pushed through the hole.


----------



## Totalwrecker (Feb 26, 2009)

First a big Thank You for all the help so far 

The short that caused this mess was resolved. It was an off board DPST switch for lights and sound. Bare jumper got moved and shorted when wires were stuffed back into tender shell. 

Before I cut the diode in half I need it's replacement... I got Dumbstruck again when I opened Allelectronics.com looking at diodes... there are a ton of different kinds! 

Bad diode; .2" long x.1" diameter, black body silver stripe on one end. Some letters or numbers printed on it. 

The R/C board has 2 rectifiers; the larger for inpit and the smaller for lights (I think). The bad diode is attached to the center leg of the smaller rectifier (black end lead). I think this the lighting circuit. If so then I think I'd need one rated a 5-6 v???? 

Any suggestions? I could take the board into a Radio Shack, but then I'd be relying on the sales person.... not sure they all know this stuff anymore. 

Is solderwick the same as fluxed stranded wire? That's what I've used in the past to wick up excess solder.... 

Thanks again for sharing your knowledge. 

John


----------



## Del Tapparo (Jan 4, 2008)

If at all possible, you need to read those numbers and letters on the diode. That is the part number. It is most likely a 1N4001 or something similar, which is a general purpose diode. But it depends on the circuit the diode is used in. If you don't use the same exact part number, and you have no knowledge of the circuit, then you are altering the circuit design. And that may not be successful.


If you don't have any experience in replacing parts on printed circuit boards, I would strongly suggest you at least practice on an old junk board first. It is really easy to remove traces and pads from boards as stated above. I use solder-wick for my repairs. And it works about 90% of the time, but it isn't necessarily easy to use.


The other problem is that after you replace the diode, there may be other failed parts that aren't so obvious.

If you send me your board and return postage, I'll replace the part for you. But no guarantees that it will work, since I don't even know whose board we are talking about here.


----------



## George Schreyer (Jan 16, 2009)

It's gotta be a member of the 1N4000 family. This is about a plain a part as there is. Any member of that family will work, 1N4000-1N4007. 

Basically, any diode that looks the same will work. This doesn't work with most electronic parts, but in this case, it's a good bet.


----------



## Greg Elmassian (Jan 3, 2008)

Be sure to remember the direction the end with the stripe was at. (good sentence construction!) 

Picture would still be great. 

Regards, Greg


----------



## RimfireJim (Mar 25, 2009)

Posted By Totalwrecker on 21 Aug 2010 10:41 AM 
My camera isn't very good at close ups or I would have. 


Try this trick (I use it frequently): Set your camera on the highest resolution possible. Stabilize the camera by resting it on something so that you are not holding it (tripod preferred, but not necessary). Set the distance at something well within the capability of the camera to focus; you'll have a lot of other things in the field of view that you don't want, but we'll fix that later. Use lighting other than the built in flash to eliminate reflected glare. Using the self-timer on the camera, take the picture. On your computer, open the picture with any image-editing software, e.g.,Microsoft Office Picture Manager, GIMP (free), your camera software, etc. Crop the image down to the area of interest. Save it. You now have a small file-size, in-focus image of only the area of interest.

Here is a low-res version of a 6 megapixel image of the controller board on a hard drive, taken from about 2 feet (original file size 3.6MB). The damaged part is the square chip on the left, between the 4-contact connector and the white Kimwipe:









Here is the cropped image of the damaged chip, 100KB file:


----------



## Totalwrecker (Feb 26, 2009)

Took me loupe and optivisors outside and in bright sunlight saw IN40.... and AH below that then the curvature hid the rest. 

So maybe I should cut one lead and try to pull the whole diode from a single hole... to get the full number. 

Greg that's goodish Anglish okie dokie, I understand that direction is important! 

Will try Jim's cropping suggestion for at least a referrance photo. 

Thanks to a bunch of wonderful guys! 

John


----------



## Greg Elmassian (Jan 3, 2008)

Just get a 1n4001 from the radio shack, the 1n400x series are all 1 amp diodes, just the voltage varies. I believe the 1n4001 is 50v, and we know you aren't above that. 

You should be good to go. Good detective work! 

Regards, Greg


----------



## Totalwrecker (Feb 26, 2009)

Thanks and you're making me blush! ha ha 
I think I'm in the 30v max range.. 

John


----------



## Greg Elmassian (Jan 3, 2008)

I have an optivisor, use it for my Z scale trains, but every so often need it for similar "detective" work. 

Regards, Greg


----------



## Totalwrecker (Feb 26, 2009)

Conclusion; 
I'm going to pay Del do the repairs. 

Thank you everybody for my education. 
Electronics is the one facet of this hobby that continues to baffle me! 

Optivisors (5x) have become my reading glasses, but it was the 10x power loupe that enabled me to see the tiny letters and numbers! 

Z? I have the Amtrak train, also got a steam loco I used for an Nn3 conversion.... talk about tiny parts! 

Thanks again, 
John


----------



## Torby (Jan 2, 2008)

hehe

Any more I use my 30x magnifier to read part numbers.

(photo a couple times life size)


----------



## rmcintir (Apr 24, 2009)

Posted By Totalwrecker on 25 Aug 2010 07:05 AM 

Optivisors (5x) have become my reading glasses, but it was the 10x power loupe that enabled me to see the tiny letters and numbers! 

Thanks again, 
John 
It's tough getting older, at least I can still see motion. I can still see the little orange flying targets at the trap and skeet range and hit almost all of them! (sometimes all of them on a good day)


I use a large magnifying glass on a goose neck attached to my work bench. That's on top of the prescription reading glasses! I also have a very bright light which makes all the difference in the world. 


Thank God I can still find the beer in the fridge.


----------



## Dan Pierce (Jan 2, 2008)

Due to the 1N4001 having a peak of 50 volts reverse and 35 rms, I would never use this in large scale. The 1n4002 is 100 peak and 70 rms which is a much better choice as are the 1n4003-7, (The higher the last digit, ths higher the voltage ratings.) 

You can use the 1N4001 for clamping 5 and 12 volt relays back emf. 

PS, my rule for capacitors, 35WVDC or better, not 25WVDC.


----------



## George Schreyer (Jan 16, 2009)

with even 24 volts on the track, that's all you can ever see. Sounds like 50 volts is derated enough. It would meet spacecraft standards in the MIL specs. 

I use 1N4001's all the time, never blown one.


----------



## Dan Pierce (Jan 2, 2008)

George, I look at the 24 volts stored on a cap and then the polarity switches to the opposite direction and then you have 48 volts reverse bias on the diode. 

This is too close to the 50 volt spec for me. 
Using the 1N4002 is just a better safety factor. 

Same for 25 volt capacitors, for me 35 volts in my minimum.


----------



## Greg Elmassian (Jan 3, 2008)

??? 

If you have 24 volts stored and a cap, and you reverse polarity, the cap explodes! 

I don't see where you are getting 48 volts at all. Please explain. 

That said, higher voltage diodes are fairly cheap. 

Do NOT equate the voltage on capacitors, they are a completely different animal, and yes you don't run a 25 volt capacitor that is filtering AC to 24 volts like Aristo did... the next typical increment from 25 volts in electrolytic caps is usually 35 volts. The capacitor works very differently from a semiconductor device. Please don't lump the characteristics of all devices into one pot, people will get the wrong information. I'll be answering diode voltage questions for years. 

Regards, Greg


----------



## Dan Pierce (Jan 2, 2008)

When installing a diode on 18 volts AC and feeding a capacitor, the cap charges to 18volts minus the doide drop times 1.414. Now the voltage reverses and the diode is between -18(23 volts peak) AC and plus 23 volts DC giving more than 40 volts. Now change the AC factor to 24 volts and you have exceeded the 50 volt rating of the diode!!!! 

So, if I help all sorts of club members, I try to keep all caps at 35 WVDC and diodes at 100peak reverse voltage and I am unable to choose a value too low for any application. 

Just my way of doing electronics.


----------



## Totalwrecker (Feb 26, 2009)

Interesting...caps and AC... but in R/C Battery will I ever encounter such stuff? Batteries are my caps..no? And AC would just fry everything no? 

John


----------



## Totalwrecker (Feb 26, 2009)

Conclusion: 
While I trust my soldering skills and probably could have lifted out the bad diode and put in a new one...I would be no farther ahead! 

The easiest way to replace a diode is; Have somebody else do it! 

I sent it to Del and he tested the board and found I had fried the 5v circuitry, the list of failed parts was too long! 
Del is a pleasure to work with and he made me a great deal on a replacement board. 

Before I install it though, I'm thinking a complete loco tear down and wiring check is in order. it is an Aristo afterall! lol 

I want to thank all who replied, I did learn a bunch, mostly it's not as simple as it looks! 

John


----------

