Selenium rectifier upgrade query

[The Philpott]

I have a (healthy) selenium rectifier within an old lead-acid type Heayberd battery charger. Should i ever need to replace the component with a Silicon rectifier, what diodes would be suitable...and would i need to account for the different characteristics of the modern diodes?

The transformer has taps for 12,6, or 2 volts, and tends to charge a 12 volt car battery at 1.25A which then drops to 1A near termination of charge.

Thankyou in anticipation of any advice.

Dave

[MotorBikeLes]

I have done many of these over the years. I just fit a 10A or more bridge rectifier, then add about three resistors(0R5, 1R0 and 2R5) with a three position switch giving me a choice of charging rates. The output voltage WILL be increased, and without at least the lowest of those Rs, you are likely to burn out the transformer if you try to charge a big battery.
Put the Rs in the 0v end of the transformer, so they are effective on all ranges.
Les.

[Nuvistor]

Or one of these, keep it selenium.
https://www.gdrectifiers.co.uk/produ...um_rectifiers/

[MeerKat]

If you only intend to charge 12 volt batteries then another way round this is to remove a few secondary turns if you can get at them easy.

Replace the rectifier with a silicon one, 10 amp minimum, check the cooling is adequate as well.

Take off a few turns at a time, check the charge rate to a fully charged battery, you will be aiming for the 1 amp you had before. Only take a few turns at a time, you don't want to remove to many. Be careful you don't damage the transformer and be sure you are taking off secondary turns.

Once that is right check the charge rate to a flat battery, you may need to insert some series resistance if the charge rate is excessive.

One advantage of doing it this way is you keep the heat dissipated to a minimum.

Worked fine for me.

[Cobaltblue]

I have just just had to replace the metal oxide (copper) on an early 1930's EKCO Battery radio HT eliminator. the forward voltage drop had become too high the 120V under load just 50V.

I learned this little trick although I think pretty well known in vintage restoration circles these days from an old independent repairman in the late 60's. (Mr Westlake of Liskeard, Cornwall).

I wanted to retain the original look so I removed one copper disc and replaced it with an srbp washer (insulator).

I then soldered a IN4007 with a series resistor of 470 Ohms WW resistor underneath the now OC rectifier.

HT was restored.

Cheers

Mike T

[The Philpott]

Thanks all, i will note this. Fortunately it probably has very few hours on it, and when 'scoped on purchase the trace was immaculate.. but forearmed now in case the magic smoke does escape one day. The transformer is so under-run and well mounted that there's no noise (heat from the dropper, but the case is well ventilated)
Dave

[JonSnell]

Selenium rectifiers have a similar voltage drop to silicon diodes, per section.
Each section has a PIV of 20volts.
If your battery charger has single sections then 1N5401 diodes will be fine as a direct replacement. The problem comes with high voltage types.
An HT of say 300 volts will have at least 15 sections on each leg, that means a 15 volt drop. A 1N4007 has a 0.6 volt drop so there will be 14 volts too much HT and the higher the voltage rating, the greater the loss, hence series resistors for replacements.
As selenium rectifiers get older, they go higher resistance and therefore less efficient with more heat and the spiral continues until the magic smoke that stinks very badly is produced.

[Graham G3ZVT]

Coincidentally Big Clive has just done selenium rectifiers.
https://youtu.be/OOA1NaoKV6I
He pronounces the element sel-en-ium, perhaps that's a Scots thing, I always say sel-ee-nium

[Radio Wrangler]

Sel-ee-nium round here.

David

[Oldcodger]

I use to remember Selienium as "spelly ionun" rectifiers in my GPO days. Once smelt, never was a burnt out smelly renium rectifier forgotten. As found in 51 type bottom of the rack power units.

[Peter.N.]

Those in the catalogue look like the last ones that appeared in the '50s prior to the introduction of the silicon diodes with a thread on them for higher voltage applications. BY100?

Peter

[Herald1360]

Not BY100, they were in a "top hat" ish metal package.

https://www.silicon-ark.co.uk/mullar...ion-diode-do-1

Maybe something like the D04 12F120 for stud mount?