Do rectifiers get old?

Discussion in 'General Electronics Chat' started by xtal_01, May 1, 2016.

  1. xtal_01

    Thread Starter New Member

    May 1, 2016
    It is embarrassing to ask such a simple question but I am stuck.

    I have a very old simple 12 volt charger for a car battery that I was given. I was going to throw it away but once in a while it is nice to have a "junk" one to leave outside on the tractor overnight (if it gets destroyed, who cares).

    Anyway .. this one had no output. Took off the cover ... simple, the negative wire that was riveted onto an aluminum plate that the two rectifies are mounted to had broke. Actually, this must have happened a long time ago. Someone looks like they tried to repair it once before (tried to solder wire to the aluminum plate ... hmmmmm).

    So, put the wire on ...11.6 VDC out. I would have thought it should have had about 13 volts out to charge a car battery?

    I checked the AC voltage (center tapped transformer ... 13.35 VAC each side ... 26.8 VAC across both).

    If a rectifier drops say .7 volts I would think I would have an output of around 12.5 VDC. Still lower than I expect but probably O.K. to charge a batter?

    I have included the wiring diagram.

    These are very old rectifiers. They look like think plates about 2" square. The unit was probably built in the late 60's maybe early 70's.

    Should I be getting this much of a drop across the rectifiers?

    Can I just get a couple of diodes and put them in place?

    The charge says 6 amps on it.

    Thanks .... Mike

  2. takao21203

    AAC Fanatic!

    Apr 28, 2012
    Selenium rectifiers.
    This technology was abandoned, indeed they do age and some day, burn out with a nasty smell.

    Old chargers are not nice to batteries either especially deep discharged ones.
    They need to be charged with low current first.

    So a modern charger tests the battery with a load current, adapts charging current, checks how voltage increases. If it increases too fast, it scales back charging current.

    If the battery is just empty, not deep discharged, you can use any charger.
  3. DC_Kid

    Distinguished Member

    Feb 25, 2008
    the junctions inside the silicon can become damaged over time which can alter their specs.

    are you measuring when the leads are connected to battery? if so then it makes sense, but current is still flowing into the battery. do you have a clamp amp meter to verify?

    but, how are you measuring the voltage? with the diagram you show i would suggest an o-scope, or if you have a DVM then i would clamp a 220uF or greater cap across the outputs you are measuring.
  4. xtal_01

    Thread Starter New Member

    May 1, 2016
    I did not put this on a battery. I just plugged it in and used my digital volt meter on it ... so there is no load right now.

    Before I opened it up, it had no output. I fixed the broken wire and now have the 11.6 volts.

    I was just surprised it was so low ... especially with no load.

    Will putting a load on it make the voltage go up?

    I do realize this is a "junk" charger. I would never want to use it on a "good" battery and just leave it on. I very seldom need a charger but for that once every two years that I forget to turn the lights off on my 30 year old truck (junk but it runs) it would be nice to be able to start it without calling over a friend for a jump.

    So, should I go any further testing or since I know the transformer works, just replace the old rectifiers with diodes?

    Should I just put in a single diode or should I put in two bridge rectifiers or ?

    Thanks so very much !!!!!!!!!

  5. Lestraveled

    Well-Known Member

    May 19, 2014
    Excellent, I think you fixed it. How long will it last? No telling. Treat it nice and it might out live you. This is a dumb battery charger, meaning, you are responsible for turning it on or off.

    I would not throw it away. The 6 amp chargers you buy at the big stores are crap compared to your 1970's vintage charger. Keep it.

    Oh yea, don't worry that your DVM reads low. There is no cap on the charger output and your DVM is confused. Put a 10 to 1000uF cap on the output and you will read something like 19 volts.
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
    I've updated many old battery chargers like yours to use modern high current diodes. In most cases they had enough built-in voltage drop in the wiring and transformer to work just fine for regular use. If not then you can simply put two or three diodes in series to bring the at load charging voltage down to a reasonable level.

    For basic dumb battery charging if you can get a charger to level off at 15 or less volts at a 1 amp or less current output while connected to a good battery after setting and charging for several hours you have your voltage drops about right.

    I just rebuilt an old cheapo Schumacher to have digital readout for both volts and amps and ended up putting a full wave bridge unit in series with its output to bring it down to where it could sit on a lawn mower battery over night and not exceed 15 volts and more than one amp charge current which is not enough to even warm up a small battery like that if left on for days by accident.

    For the rebuild the cheap old rectifiers got replaced with dual 40 amp stud diodes on each side of the half bridge followed by a 35 volt 20,000 uf capacitor, so that the digital meters would have a clean stable voltage and current to work with.
    Down side was with the large capacitor the output voltage would sit at around 19 volts and only pull down to around 16 volts when loaded with a one amp draw.

    The cheap fix was to put the big full wave bridge in series to add two more ~.6 volt drops to get the output to be under 15 volts at a 1 amp draw.

    So far I have tested it on three different lawn mower batteries and one large group 31 truck battery and sure enough after sitting over night every one of them leveled off at 14.8 - 15 volts and .5 - 1 amps.

    A full charge semi hard float charging rate but still below the level of heating and or boiling them out! :cool:
    Last edited: May 1, 2016
  7. xtal_01

    Thread Starter New Member

    May 1, 2016
    Wow ... I just found this forum and you guys are awesome!

    This is exactly the information I was looking for.

    I am known to keep things running forever (even my weed trimmer is 31 years old).

    I am a machinist by trade. I am great with electrical wiring ... electronics is an another issue. I actually build electrical control panels as a side occupation. Things like motor starters, relay logic and even VFD's are simple ... don't ask me anything about component level devices. Basically I am an "end user".

    I am presently converting an old forklift into an elevator. The mechanics are straight forward. Hydraulics all made sense. The most complicated item is a flow control valve. It is controlled by a 4-20 mA signal. I am using a PLC to control everything (lots of interlocks for things like the doors and such). I have a lot of safety items built in like limit switches that shut down power if it over travels (contactors weld on, plc's fail, ...).

    Just FYI .... here is a link to a motor home I modified 7 or 8 years ago. My wife was paralyzed from the shoulders down in a diving accident when she was 12. She needs care every 4 hours and thus had never traveled. I bought an old RV, stripped it, cut a new door, built a lift, put in a hospital bed and an inside lift. Most of my projects are budget driven ... I paid $18K for the RV and they wanted $25K just to put in the outside lift!

    The elevator was the same way ... long story but our $15K quote (and we found a $10K grant so only $5K out of pocket) when we started building 3 years ago became almost $40K last year. I bought a working electric forklift for $850 ... I probably will have $3K in the project ... a lot better than $40K.

    Here is a link to the RV page on the web:

    And here is a link to a page my wife put up with pictures of our house project ... we lived for 3 years onsite in the RV (winter temps of -30 here in VT) to save rent money while building:

    I am super glad I found this forum! I am always coming up with some question on electronics!

    Thanks so much !!!!!!!


    PS ... here is a picture of a typical panel I build ...


  8. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    Thanks for sharing. You do really nice work. Hopefully, the people you build for appreciate it?

    I was asked once: "Why do the wires have to be neat"?

    In one system I built that spanned multiple rooms, I would only allow contact closure communications. So, if I had to activate a lamp in one room from another, the panel with the lamp supplied the power to the lamp. It had to go through a relay first.

    If I had to do it again, it might of been a PLC with monitored contacts.

    Having demarcation terminals made things a lot easier. Wires enter and then terminate and then cross-connects go to the controls. You don't have wires entering and then finding a place to go.

    In a "lab environment" I ran the instruments' cabling to a breakout box where I could separate analog in, analog out and digital.
    I built stuff so modular that it was never taken apart and the pieces re-used.
  9. xtal_01

    Thread Starter New Member

    May 1, 2016
    I think that my clients do. I don't have a lot of them. I wear a lot of hats to make a living. I am a machinist, do some welding, do some consulting. This week for example, one of the places I worked for 7 years ago sent me a couple of overlays to make for them. For some reason, their cad people were having problems designing them and on top of that the local company who printed them went out of business.

    Easy job ... a couple of hours work. ... love it, never turn down work.

    I don't make a lot on panels (varies from two a year to one a month) ... I think the last time I calculated out a job, I only paid myself $15 per hour ... but that is better than nothing per hour. I get repeat business because my clients like the work I do. The box is neat and clean. The front panel is customized with their name and logo (they all like this). I give them terminal blocks to tie into (a lot of companies I worked for would make the electrician tie into the starter). I make all field wiring 24 VDC (safe). I give them a manual in both hard copy and PDF format. On top of this I choose quality components. I save them a bit of money by using IEC rather than NEMA rated parts but if I need a 5 hp starter, I choose one rated at 7 1/2 (one size up). I use relays with a high rating (usually HP rated). Last year a client told me he was re-using a panel I built back in the 90's.

    Really, just a lot of "value added".

    As I said, my biggest downfall is in the application of component level items. I know how a transistor works but I would have no idea how to design a circuit from scratch. I know basically what a capacitor does but I don't know why it makes a difference in the voltage value in the circuit I first wrote about.

    Thanks so much again .... Mike
  10. crutschow


    Mar 14, 2008
    A battery charger has no filter at the output (since the battery acts as a filter) thus the voltage you read with nothing connected (the rectified sinewave) will be different then when a filter or battery is connected.
    If the charger indicates charging current when connected to a battery, then it's okay.
  11. MaxHeadRoom


    Jul 18, 2013
    @xtal_01 nice work with the lacing and wire duct and layout.
    Very professional, you don't mention your C o O, if N.A. I would suggest getting the copy of NFPA79 if you don't have, for reference.
    I use a local engraver for my custom panels, he offers reverse engraved lamacoid, cheap way to get multicolour font with totally smooth front surface.
    BTW, the old Selenium rectifiers smell like cats pee on failure.!:eek:
  12. xtal_01

    Thread Starter New Member

    May 1, 2016
    C o O ... Country of origin? Hope this is what you meant. I am in Vermont ... I would say rural Vermont but all of VT is rural. Our largest city is only 42,000 people.

    I actually got lucky. My last consulting job was with Westinghouse Nuclear in SC. We built the machines and maintained the factory that makes nuclear fuel. They hired me for a 6 month job ... I stayed for over 10 years, finally leaving to take a full time job (with them as a machinist) ... and then quite and moved to VT two years later when I met my wife here ... her job and health insurance is here so she had to stay ... I was born in Canada (just 20 minutes from Buffalo / Niagara Falls) so I am use to the cold and snow.

    All jobs were reviewed (before, during and after) by a group of senior engineers (mechanical, electrical and safety) using NFPA79 and a lot of other standards. There was no room for error when working with nuclear fuel. Safety was drilled into us. Plus I was always told to design equipment "idiot proof". Honestly, I think that is impossible ... I have seem people do some strange things that I would have never expected ... I could write a book on this. Every time someone does something "stupid", I try to incorporate the solution into my next project.

    When I started building panel (employed with other companies), they used either individual labels that (the kind that go around the switch) or engraved plastic ones (glued to the panel .. fell off ... or riveted). I figured out how to make these laminated front panels. What is nice is that I print the overlay first and then use it for my hole locations (just in case it is not printed exactly at 1:1). They have a peal and stick back (plus with the indicators holding them in place) and a laminated front layer .... they hold up very well.

    I have never seen a reverse engraved panel. So is the plastic multi layered (clear on the top and a color below) or ???

    How do you get the multi color? In a "normal" engraved tag you cut through the top colored layer into a lower deferentially colored layer. How does this work with reverse engraving? How do you get the multi color?

    I would be very interested to know!

    Thanks so very much ...... Mike
  13. MaxHeadRoom


    Jul 18, 2013
    The lamacoid is a basic selectable solid colour with a transparent front, the reverse engraving cuts through to the surface of transparent front, the legends and graphics are then sprayed from the rear with an selected colour aerosol etc.
    This is not a good example but one I did awhile ago, I did not photo all of them unfortunately.
    I just send him a cad DWG.
  14. xtal_01

    Thread Starter New Member

    May 1, 2016
    Very interesting idea. I do the same thing ... send a file to printer and pick up the finished printed panel. It is great you found someone who takes a DWG file. I have three printers who can make my overlays ... none will take a DWG. I "plot" them as a PDF and send them that file.

    Most of my panels are built on large NEMA 4 enclosures. It looks like you use your panel to actually mount the switches and indicators on? So I am guessing that you cut one big hole and then use your overlay as the actual front of the enclosure?

    So you seal your panel into the enclosure to maintain the NEMA 4 rating?

    If this is correct and you use the panel as the entire front, how strong is it? In my case, I still use the steel enclosure as the front . How thick is the plastic? Would it be strong and stiff enough if it were say 24" x 24"?

    Roughly, what can you I expect to pay for a panel like this?

    One of the places I use "prints" my panel then laminates it ... they charge about $50 for a 11 x 17

    The other uses a thermo print process and then laminates the front .... they are around $125 for an 11 x 17

    Both have a "peel and stick" back.

    I appreciate you showing me how you make your front panels!

  15. recklessrog


    May 23, 2013
    You've not lived until you have inhaled the smell of a burning Selenium rectifier lol :)
  16. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    Funny isn't an option, but it is. At work we got a tank of H2Se that we accepted and it was leaking. We had to send all of it through the scrubber. Took like a week. New procedures went in place - no acceptance until leak checked.
  17. MaxHeadRoom


    Jul 18, 2013
    I drill holes and cut-outs in the steel front and the overlay goes over this, so the panel integrity is intact.
    Depending on detail the cost is around $25.00 to $40.00
  18. xtal_01

    Thread Starter New Member

    May 1, 2016
    Wow ... awesome!

    I will have to give this a try on one of my panels. I will call around and see if anyone has 1) an engraver with reverse text 2) material with a clear front.

    Now I need to get my CNC mill up and running (another project waiting on the workshop ... mill is in great shape but it needs a new controller).

    At Westinghouse, we did all our own engraving of tags. We had a older CNC mill with a program just for text ... worked great ... and could use everything from plastic to stainless for the tags (every valve, inlet, outlet, electrical plug, .... everything, had a tag on it).

    I remember it did have a mirror function that would reverse everything ... including text if you were not careful.

    Thanks so very much for the info !!!!!!!!!!!