OLD PSU I like the case, Restore or Makes something new? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 12, 2015, 07:11:58 pm
content: So Ive got a ham friend who gave me tons of stuff he doesnt use..... on of those things is a fixed 13.8 power supply which he said put out dirty power this thing is way before my time so im kinda hoping someone can tell me what exactly the deal with it is. To me it looks like a huge transformer with a rectifier and huge filter cap. The thing is the transformer is pretty large for a 13.8v 1.7 amp supply output... and the bottom windings go to some kind of hi voltage cap that you would find in a microwave..... Im not sure if this is for ESD or what, the power cord was in poor condition but the earth main was bolted to the case..... This thing also doesnt have adjustable current and there is some huge power resistor in there not sure why. I need to solder a decent cord to it before I even turn it on.Im really just trying to figure out why the transformer is so damn big and wether or not this thing is worth restoring or should I just clean up the case which I like and build my own adjustable PSU in it? I mean 1.7 amps isnt a whole lot when this thing should be acting like a cars power output

author: SeanB
author_rank: Super Contributor
timestamp: September 12, 2015, 07:51:41 pm
content: That is a transformer with a saturable core, designed to give a regulated AC output voltage with a varying input voltage or load. Designed to charge a 12V lead acid battery which would be permanently connected in float charge, so it will keep it at 13.8V irrespective of the AC input varying and will charge at around 10A ( the 1.7A is the input current, the secondary current will be a lot more, depending on the diode bridge they put in but 10A is around right for these units) if the battery is low or you connect a flat battery. Big transformer, so low losses in the core and generous copper used so it runs cool.Just note that it have degraded insulation if it has been stored in damp conditions as the interwinding insulation is paper. It may smell a little on power up. Please replace trhe power cord where it is damaged in any case.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 12, 2015, 08:15:34 pm
content: It got damp because I discharged caps and hosed it down with alcohol. I plan on replacing the power cord ASAP the looking at the output on scope I was told it dirty... (meaning it made his radio sound muddy) and then I will slowly replace and needed parts I have some other caps that size and HV caps from microwaves. If they dont match up as far as unit size (there all physically the same) ill just order something. All wiring will be replaced too!So basically your telling me I will get a constant 13.8v at huge loads with no sag? If I intent end to power complex loads should I stick a filter network in front of the linear regulator or should this put out very smooth DC when working properly?Seems like a good score resorting the case, will be the hardest part if I want it to look nice

author: TerraHertz
author_rank: Super Contributor
timestamp: September 13, 2015, 02:05:29 pm
content: As SeanB says, it's a battery charger. The output is probably just unfiltered rectified transformer output. As for 'no sag', actually no. If the design is any good it will sag under load but in a way that results in a reasonable charging current curve for the battery. If the battery is a bit low this unit's output voltage will be the same as the battery, but it will deliver a lot of current. As the battery voltage rises to 13.8, the current from this supply will taper off.Makes this supply quite unsuitable for use as a general purpose DC supply. Even with a big lead acid battery in circuit you'd still get ripple due to the battery internal impedance.Otoh, if you have a battery you want to keep charged, it's perfect.

author: edpalmer42
author_rank: Super Contributor
timestamp: September 13, 2015, 04:35:00 pm
content: Another name for this type of transformer is a ferroresonant transformer. As Sean said, its main feature is the ability to produce a constant output voltage as the input voltage varies. Like any transformer, the output voltage will drop as the load increases, but a ferroresonant will automatically limit its output current so it's almost immune to overloads. Most ferroresonant transformers have an output waveform that's approximately square rather than sinusoidal. Add a rectifier to this and you've got an almost perfect battery charger.Watch out for that capacitor and the winding that it's attached to. Depending on the design, you can see sine waves of many hundreds of volts across that capacitor! By the way, since it's an old supply, that capacitor is probably full of PCBs. It's safe as long as it's contained, but if it leaks it's a problem.Ed

author: Macbeth
author_rank: Super Contributor
timestamp: September 13, 2015, 05:47:26 pm
content: Quote from: edpalmer42 on September 13, 2015, 04:35:00 pmAhh, good old fashioned point to point wiring, where the PCB is in the capacitor!

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2015, 06:02:07 pm
content: I have some very nice film capacitors which are made in the mid 1950's. there is no mention of PCB on the label, though of course the oil is PCB based. The wiring in the building is all PCB laden though, the lovely green goo that drips off it into the conduit boxes is kind of a give away, along with the ratings being way conservative compared to the modern one. 4mm for a 20A circuit ( 2.5mm now) and 2.5mm for lighting ( 1.5mm now) plus the steel conduit is the earth.I pull through an earth wire now whenever I replace a section of wiring in a conduit as a matter of course. Just have to attach to conduit at both ends. Some of the wiring has been abandoned as it will not pull out, the conduit has rusted through, so now cut off inside the distribution board flush with the end of the conduit and a new cable surface mounted in place of it.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 26, 2015, 02:49:51 am
content: Ok so I touched this thing up a bit, did some epoxy and paint work along with testing and replacing a lot of wires. It is pretty nosiey it puts out about 120mv of ripple.Im still not sure exactly how this things works, nor could find much info on it, even at Ritz Electronics. I just see alot of them for sale on e-bay. It says it is magnetically regulated, Im guessing that is due to the special transformer? What is the deal with the power resistor? It is 13.8ohms and its a 13.8v supply. The resistors wattage is not labeled, and it is wired in parallel with the output. I would guess this is there to taper off the current? The problem is the thing gets so hot with no load the solder joints melt..... its taking the full load of the psu when nothing else is connected. I had planned to add some filtering on the ouput and use it mostly to power things like RC battery chargers that are meant to be ran from 12v tec, maybe charge a car battery if needed. So if I take the sandbrick resistor will it pump to much current in to a lead acid thats fully charged? If I decide to leave it in, wont it limit the amount of current on the output, the higher voltage of the load is? Im trying to decide whether to put some cooling on the resistor and leave it, or whether to just remove it totally from the circuit. I just want to make sure I am right about the function the resistor serves, maybe all add a switch that either changes the configuration from the power resistor, or to a better filter depending on what I am using it for.

author: Duane Degn
author_rank: Newbie
timestamp: October 23, 2015, 03:00:48 am
content: Quote from: edpalmer42 on September 13, 2015, 04:35:00 pmI'm not sure if it's clear to some of the younger readings what Ed meant by PCBs. PCB in this case is a chemical which used to be commonly used as a dielectric. You can read about it on Wikipedia"Polychlorinated biphenyl."