Battery Charger Puzzle

[Gridiron]

I have a small Lucas Major 12 volt 0.5 amp battery charger which consists of the usual transformer and bridge rectifier in a plug in wall wart type housing. In series with the mains input to the transformer is a small 12 volt 1.5 watt bulb, presumably to act as a fuse. I have been using the charger to charge a 2 volt battery via a large 50 ohm resistor so giving a current of around 200 mA, well within spec. What I am puzzled about is that occasionally the bulb will give a brief flash on switching on but mostly doesn't light at all.
I was thinking this effect is something to do with the magnetisation current of the transformer or where in the mains cycle switch on happens but would have thought any current surge would be too brief to light the bulb, can anyone give an explanation of this.

[The Philpott]

Not sure about the varying result (presumably down to the part of the sine wave where you switch on, as you theorised) but i would have thought the brief illumination is simply down to the low resistance of the filament until it warms up- then having warmed up it extinguishes...?

Now i've just realised that this theory sounds flawed..

Dave

[broadgage]

The brightness of the bulb will depend on two factors, firstly at what point in the mains cycle the appliance was last turned off, and secondly at what point in the mains cycle it was next turned on.
Both are in practice random.

[Techman]

I had one of these and I think it uses an 'auto' transformer rather than an isolating type. Mine went open circuit, unfortunately. They date from the early 60s.

[Gridiron]

I hadn't considered the switch off conditions would affect it, could be a contributory factor.
The charger does not appear to use an auto-transformer, surely this would not be considered safe practice by the 1960's.

[Radio Wrangler]

Chargers for lead acid batteries need to have some means of regulating the current. In really cheapo crude chargers it's often integrated into the transformer. Having a lot of flux leakage gives the necessary characteristic, though at some cost in efficiency. For a cheap low power thing, this was seen as a good compromise.

The bulb has a stong positive temperature coefficient of resistance. So at turn on (filament cold) its resistance is very low and it'll take a strong current to get it toflash, then as it warms, R goes up and the current falls.

This shows that the turn on magnetising current of the transformer is appreciable. And that's probably why the bulb is there not just to act as a cheap fuse.

Leakage flux current limiting in a transformer is a common ploy in classic battery chargers ant arc welding transformers. The current control moves a magnetic shunt using a screw and nut mechanism. The lleaked field makes the mains see an inductive load even when the rod has stuck to rthe work and the transformer is presented with a short circuit. A charger confronted with a fully discharged battery is in a similar situation.

David

[Techman]

Originally Posted by Gridiron

The charger does not appear to use an auto-transformer, surely this would not be considered safe practice by the 1960's.

It could be older than 60s. It was my fathers and I remember it from the early to mid 60s, but he could have bought it years earlier - the first ever wall wart!

I knew I still had it somewhere, so you've made me go outside into the workshop in the damp and dark to find it. There's only three connections to the transformer and there's continuity from the neutral plug pin through to the positive output crock clip, but the rest of the transformer is o/c - I think more investigation is required

I do remember dad using it years ago, but it was only when I inherited it that I found that it no longer worked. He accidentally shorted it out once back in the late 60s and had someone where he worked repair it, so perhaps it's been somehow modified. I'm not sure if the original instructions still exist anywhere among old paperwork, but I seem to remember that there was a warning to always connect the clips to the battery before plugging the unit in.

[kalee20]

Originally Posted by Techman

I seem to remember that there was a warning to always connect the clips to the battery before plugging the unit in.

Quite likely - and probably another warning to switch off power before disconnecting from the battery.

The reason for this, would be to avoid a possible spark at the terminal of the battery, which could ignite hydrogen gas emerging from battery.

[Techman]

Originally Posted by kalee20

and probably another warning to switch off power before disconnecting from the battery.

Yes, you're right, I remember that and it was not doing this and letting the croc clips touch that caused my dad to break it back in the 60s!

Looking inside, there's a fuse in the negative output line and I guess that this was all that needed replacing, as the rectifier with its four connections looks special to the unit and likely to be original.

It looks like two of the rectifier terminals are linked together and connected to one of the transformer terminals. One diode connection then goes to the earth wall wart pin where the positive croc clip connection is also taken from. The other diode connection goes via a small fuse to the negative croc clip connection - definitely an auto-transformer.

Edit: The lamp is in the neutral connection to the transformer.

[Gridiron]

I have checked mine and can find no continuity between the mains input pins and the output connections. It's actually rated at 1.25 amps when I looked, not 0.5 amps! I did accidentally short the outputs together some time ago which blew the bulb, I managed to get another from eBay, somebody selling Morris Minor spares, if I remember correctly. (Box marked LLB280 12V 1.5W E5).

[Techman]

Originally Posted by Gridiron

I have checked mine and can find no continuity between the mains input pins and the output connections. It's actually rated at 1.25 amps when I looked, not 0.5 amps!

Yes, it is 1.25 amps and just to confirm we're talking about the exact same charger I've linked to a previous thread on mine below. When I say continuity, I meant that it won't be zero ohms, but will be whatever the resistance of the lamp and diode in the rectifier is, and of course is dependent on which way round the meter is connected (black lead to neutral pin if using an AVO or red lead if using a standard DMM), as it passes through part of the bridge rectifier, which I don't think is actually connected as a bridge. The mains transformer only has three connections to it and is one continuous winding with a low voltage tap at one end. Continuity is good at the low voltage end, but the long part of the winding is unfortunately o/c on mine. There's a small 2 amp fuse inside the unit, not accessible to the user. Try that resistance check again with yours and use an 'ohms' range, observing polarity.
https://www.vintage-radio.net/forum/...d.php?p=777602

[Techman]

Carrying on from the above, I had used a proper AVO to do the resistance test. I've just tried doing the same test with a DMM and struggled to get a reading, due to it not being able to properly forward bias the diode in the rectifier, so best to use an AVO for the test. I did manage to eventually get a reading with the diode test function - probably due to a bad connection with just using probes rather than croc clips with the AVO, but obviously nothing on the resistance range. The reading is around 50 ohms or slightly over, observing correct polarity. There should be close to zero ohms from the positive charging clip to the earth pin on the charger.

All I can say if you don't get these readings is that your charger is newer and Lucas must have been forced to redesign it using an isolating transformer for safety reasons.

[Gridiron]

I have done as you suggested, Techman, using an Avo 8 and the reading between the mains inputs and both outputs is infinity even on ohms X 100, as in your charger the positive output is connected to the earth pin so the charger must have been redesigned with an isolating transformer.
I still can't believe that, postwar, a reputable company such as Lucas would have put a charger with an output directly connected to the mains on the market, even if it was the neutral, sounds like the kind of thing that was done in the 1920's!

[Techman]

Originally Posted by Techman

All I can say if you don't get these readings is that your charger is newer and Lucas must have been forced to redesign it using an isolating transformer for safety reasons.

Well, as it turns out that's not all I can say!

The mystery has been solved...well most of it.

It had been 'got-at' by a person or persons unknown!

The charger is now fully working.

Here's the story...

Firstly, if you hadn't started this thread I would never have bothered to reinvestigate mine. Secondly, if you hadn't replied with that final test using an AVO, thus absolutely proving that your charger was electrically very different to mine, then I probably wouldn't have decided to dig deeper into the wiring of my charger.

I had started drawing out the circuit on the back of an envelope, but was struggling to see how the thing had ever managed to work, even in auto-transformer configuration. I had checked all the diode junctions in the bridge rectifier block and they all checked out perfectly. I remember that my father had blown this charger up on more than one occasion and it had also been dropped at some point in time and the case broken and glued back together, but have no memory of who did the case repair and was surprised to see this when I 'found' it again a few years ago. My father retired in 1971, so when it was repaired where he worked it would have been pre this date - strangely, I even remember the name of the chap that repaired it on the first occasion. This chap I do know was technically minded and seem to remember being told that he'd had to replace the rectifier, but to me the rectifier looks like an original fitment, so I'm now unsure about this.

I think it's down to what happened to it on the second time it got blown up, but perhaps much more likely when it got dropped and smashed, which possibly pulled off internal connections, particularly from the transformer.

I decided to have a very close look at the transformer, as it obviously has had some damage. The corner of one of its bobbin cheeks is broken off and missing. Also, the dark grey card wrapping covering the outer of the windings had obviously been cut into and removed at some point and there was a bit of yellow tape that had lost its 'sticky' with age over that part of it, which may or may not have been original to the unit. Two of the connections, the low voltage ones, would have been riveted terminals on this card wrapping, but were ripped out of what remained of their fixing holes and are just floating. The only other connection to the transformer was a wire that went in through a slot in the bobbin cheek and disappeared into the windings, thus making it appear to be an auto-transformer.

On looking closely at the slots in the bobbin cheeks I declared that I could see a layer of insulation between what looked like two separate windings. So I decided to remove all the outer wrapping from the transformer windings and under a strong light and with my strongest reading glasses I started to have a poke around and found the smallest remains of the end of a broken off wire end of a primary winding. I threaded a piece of insulated wire through a slot in the bobbin and managed to solder onto this wire end, having first checked that there was continuity, or rather resistance through to the other wire that disappeared into the windings. I then held the connection in place with a small blob of mixed up Araldite, as there'll be no second chances with this transformer if it ever comes off again.

I can't remember how I originally found out that the charger wasn't working. Whether I plugged it in connected to a battery or just put a meter on the output and got nothing, or perhaps I did an initial check across the live and neutral pins and got an o/c reading, therefore realising that it would be pointless plugging it in, certainly the internal fuse and the bulb were still intact. I was certainly a bit surprised at the time that it wasn't capable of working, as I'd no memory of it ever being said that it had ultimately failed, but we are talking about a lifetime ago!

So it looks to me like it probably got dropped onto a hard concrete floor in the garage at some time decades ago and got smashed. Some wires got pulled off, particularly the one that got pulled out of the transformer. Without any evidence of where this wire had come from it got soldered back onto another terminal. Also, one of the two orange wires which are the AC input to the bridge rectifier must have got ripped off and ended up being soldered together, seeing as they were the same colour, and the connection where this wire should have been soldered to was where the wire from the bulb that had been ripped out of the transformer was soldered to.

It looks like we can now rest assured that Lucas didn't make an unsafe charger and that all their chargers use an isolated step-down transformer. It had made me think that we all know what a radio with a live chassis is, well with this charger when connected to a battery still on a car we'd have then had a car with a live chassis! The positive DC connection being grounded would have been the sensible thing at the time with all vehicles being positive ground to body.

I've only quickly tried the charger on a small lead acid gel battery, as that was all I had to hand in the house and it did the same as yours with a brief bright flash of the lamp when first plugged in and it then quickly dimming down to near nothing - I'll try it on a proper car battery, monitoring current and voltage when I go out into to the outside workshop later. On the small gel battery I got something like 800ma dropping to around 400ma or so, but didn't leave it too long on the 'very' small battery.

[Roger Ramjet]

I once had a cheapo £9.99 trickle charger which stopped charging. It was just a double wound transformer outputting to a bridge rectifier. There was a charge LED as well, which I presume was wired across a load resistor.

After checking with a DMM it seemed the secondary AC output voltage was a tad lower than I would have expected thus resulting in a lower rectified DC which was unable to overcome the emf of the battery.

Knowing that the (off load) output voltage is directly proportional to the incoming AC voltage I put this down to the fact that my domestic mains voltage had been lowered from 240 - 250 down to [Europe harmonised] 230.

Rog

[Techman]

Originally Posted by Roger Ramjet

I put this down to the fact that my domestic mains voltage had been lowered from 240 - 250 down to [Europe harmonised] 230.

Seeing your post I just measured my mains voltage and it's 238 volts at the moment.

I went out to the workshop and fetched in a proper 12 volt car battery to try out on the charger. It's actually a nearly new battery and was only recently charged, so on connecting it up and plugging in I got a current of around 1 amp and a terminal voltage of around 4.1 volts. It wasn't long before the current had dropped to around 700 ma and the terminal voltage had risen to 15.7 volts, so at this point I disconnected it. The lamp started off at a reasonable brightness and then dimmed down quite a bit. I'll take this battery back out to the workshop in a bit and fetch another older one in to try out - it was just that the first battery was nice and clean to bring into the house, not that the older one is particularly mucky!

[Roger Ramjet]

Originally Posted by Techman

Seeing your post I just measured my mains voltage and it's 238 volts at the moment.

I went out to the workshop and fetched in a proper 12 volt car battery to try out on the charger. It's actually a nearly new battery and was only recently charged, so on connecting it up and plugging in I got a current of around 1 amp and a terminal voltage of around 4.1 volts. It wasn't long before the current had dropped to around 700 ma and the terminal voltage had risen to 15.7 volts, so at this point I disconnected it. The lamp started off at a reasonable brightness and then dimmed down quite a bit. I'll take this battery back out to the workshop in a bit and fetch another older one in to try out - it was just that the first battery was nice and clean to bring into the house, not that the older one is particularly mucky!

Hi Techman,

It would seem your charger is fine so I would not worry too much. Mine was not charging at all due to low output voltage(even off load). Alas I now longer have it so cannot test further. Might have a stab at my mains voltage again though.

Rog

[G6Tanuki]

Originally Posted by Techman

It wasn't long before the current had dropped to around 700 ma and the terminal voltage had risen to 15.7 volts, so at this point I disconnected it. The lamp started off at a reasonable brightness and then dimmed down quite a bit. I'll take this battery back out to the workshop in a bit and fetch another older one in to try out - it was just that the first battery was nice and clean to bring into the house, not that the older one is particularly mucky!

A battery charger that is still pushing-out 700mA when the terminal voltage has risen to 15.7V seems to me to be a recipe for driving a battery to destruction!

A cutoff at 14.5V maximum [with maybe a smidgin each way - let's say +/- 0.1V - to account for temperature changes] would be more what I would be wanting.

Don't charge lead-acid batteries until they vent or start gassing/bubbling!

[Techman]

Originally Posted by G6Tanuki

A cutoff at 14.5V maximum [with maybe a smidgin each way - let's say +/- 0.1V - to account for temperature changes] would be more what I would be wanting.

Yes, I agree. I always like to aim for 14.4 volts maximum if I can. This subject regarding charging lead acid batteries has been discussed before on here.

I'm now on charging my third battery with this charger today. I swapped the first and newest battery for another reasonably good one, but one that was 8 years old. It had also recently been charged, so wasn't going to require a lot. This one started out with exactly 1.25 amps, the rating of the charger, but dropped down to similar values to the first battery.

I then took this battery back out to the workshop and brought in a much older battery, one that came with a car some 15 odd years ago and didn't look new then, so could well be around 20 years old - it has removable filler plugs to top up the cells with distilled water. This battery is obviously not now at its best, although it does hold a good 12 volt terminal voltage, but you probably wouldn't want to rely on it passing a high starting current anymore. This battery had been charged fairly recently, but due to its age would be expected to have lost some of its charge from internal leakage and it has been left for lengths of time between charges in the past, so it's doing well considering. This battery initially took just over an amp, but settled down at about 800ma with a terminal voltage of around 15 volts. I measured the mains again and it's now 241 volts, so I got a Variac to plug the charger into and set the terminal voltage of the battery to around 14.4 volts, this gave a charge current of around 400ma. I noted that the output from the Variac for these readings was 208 volts and the input series lamp is only just glowing at that.

So perhaps the thinking was different regarding lead acid battery charging back in the mid 60s. Perhaps the rating of the lamp could be altered to limit things a bit more. I think this is a charger that you'd use to charge a fairly discharged battery on a car, with it on the end of an extension lead under the bonnet, but you'd need to not go and forget that it was on and remember to keep an eye on the terminal voltage.

One thing that I did notice with proper car batteries was that there's no initial bright flash of the indicator/series input lamp when plugging the charger in, this just being confined to the gel batteries, of which it was tested on three different ones, but they were fairly old ones that were former backup batteries removed from equipment after routine replacement, probably a decade ago, so won't be at their best.

These batteries get used for such as the radio static mobile event the other week and also used in the vintage 60s caravan, plus sometimes on the odd car. One thing that's for certain is that I'll have plenty of battery backup power when the lights go out!

[MotorBikeLes]

A caution for anybody repairing a very old charger.They were originally equipped with metal rectifiers which have a fairly high voltage drop. Most people will simply replace that with a modern silicon type, not realising that the charging voltage will rise somewhat, and overcharging is a likely result.
I have updated quite a few such chargers, and I always add an additional dropping resistor. In fact I usually go further, adding a three position switch allowing a range of charging currents.
A wet battery will withstand some over current, but try the same with a gel battery and you will finish up with a badly buckle-celled dead battery.
Les.