Battery Charger(?), 50V 20A dated 1975. Semi OT.

[The Artful Bodger]

I have been given this old unit labelled a 'Rectifier Unit', 50V 20A. The transformer is HUGE and there are a couple of smaller transformers of unknown type, and an SCR bridge plus a couple of mouldering circuit boards.

The big tranny is probably the most potentially useful component especially as it has 25V and 50V terminals.

Any thoughts? A 24V battery charger would be useful at our museum or I could use it with Fisher and Paykel washing machine motors in their stepper application.

I guess these terminals will be too high for simple rectifying battery charging? A buck circuit may be possible.

[OwenG]

It may have been a power supply for an electroplating bath?

[nickel-city-fab]

One of the guys at work got a junked charger for electric fork lifts. It has taps for 12v, 24,and 48v. Probably similar to what you have, rated for 40 amps. We're using it to charge 24v starting battery systems. The mechanics love it, it packs a punch and machines start right up.

Just simple rectifier circuits, nothing else in it.

[The Artful Bodger]

It may have been a power supply for an electroplating bath?

Maybe, but it has a name plate of STC, indicating it was most likely from a small telephone exchange. On the other hand it is in such a filthy state it may have served time electroplating.

[The Artful Bodger]

One of the guys at work got a junked charger for electric fork lifts. It has taps for 12v, 24,and 48v. Probably similar to what you have, rated for 40 amps. We're using it to charge 24v starting battery systems. The mechanics love it, it packs a punch and machines start right up.

Just simple rectifier circuits, nothing else in it.

Voltages may be a bit high but I will know more when I put 240V on the transformer. I am hoping there is also a 12V tap.

[mickeyf]

If memory serves, POTS ("Plain Old Telephone Service) Ran off 50 V batteries. Besides plating, it might server for electrolytic de-rusting. Photos of the innards, the circuit boards in particular, might offer more clues. You're certain those are SCRs, not plain rectifiers?

[The Artful Bodger]

If memory serves, POTS ("Plain Old Telephone Service) Ran off 50 V batteries. Besides plating, it might server for electrolytic de-rusting. Photos of the innards, the circuit boards in particular, might offer more clues. You're certain those are SCRs, not plain rectifiers?

I am not very concerned at the history of it but one of the terminals is labelled 'FLOAT' so no doubt it was charging batteries. It has suppression components on the mains input. The SCRs are clearly labelled as such.

[danlb]

That sound like something used in the telephone industry. The ones I worked at used 24 and 48volt battery strings. The phone equipment ran off the grid with the batteries providing filtering for the DC. The charging equipment had a float setting.

Dan

[The Artful Bodger]

I have reduced it to a mess on the bench, transformer, two chokes (same style and almost as big as the transformer), 5 SCRs, a couple of circuit boards, a low voltage supply for the circuit boards, 4 or 5 electrolytics and a healthy hoard of nuts and bolts, screws, terminal blocks and a small pile of dirt and rust.

[The Artful Bodger]

First tests with transformer......

Wired primary in series with 100W 230V incandescent and nothing dramatic happened when mains power was applied. The light dimly lit and took about 50V of the 230 while the secondary taps metered as up to 70+V (open circuit). Obviously the secondary taps are too high for a simple full wave rectifier circuit. Of course I could take a much lower voltage if I used half wave rectification but JT says that upsets the transformer.

So what to do?

Perhaps I can reactivate the original SCR rectifier network and use that to control the output voltage? But control the 230V primary or the lower voltage secondary?

Rather than messing with 50 year old SCRs would a regular commercially made lamp dimmer be appropriate?

Then there is the daft idea....... I have one or two 230/110 step down transformers and I could put one on the mains side and bring everything into line!

[The Artful Bodger]

Furthermore.............I expect those big chokes were to provide reactance and control any overcurrent situation. I am pretty sure they were wired in the secondary circuit but do I need that reactance in the secondary circuit to keep the transformer in a placid state?

[J Tiers]

They are used in a "choke input" rectifier, to smooth the current. Not necessarily needed by the transformer, although it was likely designed for the application. If it was a battery charger, they would provide a steady output current, and cut down on pulsing.

Yes, batteries like a steady current. So do transformers, since a battery connected to a voltage higher than it's voltage acts like a short circuit across the difference in voltage.... So a battery at 12 V connected to 18V supply, would draw current based on the 6V difference and whatever impedance is in the rest of the circuit.

The choke inductor provides that impedance and prevents excessive current. At the same time, it is storing up "volt-seconds" across it, (energy) which then is discharged when the difference in voltage is low, zero, or negative. That is what keeps the current relatively steady. (same thing is done in many SMPS).

If you are charging a big battery, maybe you want those, otherwise, they may be not needed in some other usage.

[The Artful Bodger]

Thanks Mr Tiers.
That makes sense. There was a lot of stray emf suppression stuff and everything was earthed like on a railroad so they were obviously seeking a pure DC if they could get it.


I have ordered a 5000w dimmer....

https://www.banggood.com/5000W-AC-220V-High-Power-Electronic-Regulator-SCR-Voltage-Regulator-Module-p-1022617.html

.....and I am thinking of putting the transformer back in the box along with some suitable connectors, sockets etc and a volt meter plus of course the dimmer.

[Mike Burch]

IIRC from my days as an NZPO tech 60 years ago, the step-by-step telephone exchanges all ran on banks of 48 volt batteries. This STC gadget could well have been one of the float chargers that helped to keep a small exchange going.

I think the demise of step-by-step was some years later than the advent of solid-state electronics, so SCRs in such a charger would probably not be an anachronism.

[The Artful Bodger]

Hi Mike, built like a battleship and no skimping on design. The circuit boards were made by STC in NZ but I did not see any NZPO marks.

Cheers
John ZL2AYQ