Davenset battery charger

[ppppenguin]

I've finallly taken a look at this lovely Davenset battery charger. ISTR I bought it at Harpenden years ago and it's been a static display item since then. The Vintage Wireless Museum has a similar or identical one in the vintage shop area. This used to be demonstrated regularly - the mercury vapour rectifier gave a lovely purple glow.

Mine has obviously been worked on. The transformer has been removed at some point. There are a lot of bits of choc block reconnecting the wires that were cut. The mains lead is some vile bit of rubber covered figure of 8 flex with a moulded 5A 2 pin plug. The cover of the transformer has been sprayed blue. It was alsmot certainly black crackle originally.

The transformer has multiple taps on the mains side. 3 at 20V apart at one end of the winding, 2 at 10V apart at the other. Probably 200V to 250V in 10V steps overall. The rating plate says 210V.

I've applied reduced mains via a variac with the bulbs and rect out. The transformer is working OK. The 2 tubular bulbs could be baretters, really not sure. One has an o/c filament, the other is mechanically poor. The rect looks a bit mechanically iffy but I have a NOS equivalent spare somewhere.

I've started to trace the circuit. So far just the primary and a bit of the secondary. If anyone has done this before it would save me the effort and I'd be very grateful.

Any info gratefully received.

After the building works and general revamp at home I hope to run this as a working exhibit. Possibly use it to charge the 12V SLA that  I use to run my Rhumkorff coil. I know it's not ideal for SLAs but a bit of limited gentle charging won't do any harm

[Murphyv310]

Dad had a Davenset charger from 1938, it did 2v 6v and 12v accumulators, I think it was binned in the early 70's. Be careful as its possible to boil batteries with them. I'd be very careful with SLA batteries with this type of charger, and don't tell anyone you use an old fashioned charger these days or you'll have your guts for garters.

[ppppenguin]

The output voltages are selectable as 15, 20, 25V. Presumably for strings of 2V cells at your local friendly cycle and wireless dealer.

I may not actually charge anything with it. The SLA on the Rhumkorff lasts for ages on a charge as I don't run it for long periods. provided the rect glows nicely when turned on I'll be happy. I understand that the rect wants to be well warmed up before switching on the output as it can suffer badly if current is drawn before the mercury is nicely vapourised.

[ppppenguin]

I've found a page from a catalogue which shows various Davenset chargers, including mine. I was given it as a photocopy in 2004, unfortuantely without any indication as to which catalogue it was from.

The model name "Garage Charger" tallies with the model name "GC" on the rating plate.

[ppppenguin]

I've tidied up and re-glued the base of the mechanically bad baretter. It's resistance is only a fraction of an ohm. Trying it on a variable PSU it seems to give some current stabilising action but not very much. The voltage across it remains near zero even with several amps flowing, to the point where I wonder if there's a near short circuit hidden in the base.

The other baretter has an o/c filament so it only for decorative purposes. If I'm runnig the unit I can just use 1 set of output terminals.

I also tightened up a number of loose connections and used Servisol 10 on the switches and rheostats. i've already run up the filament of the rectifier but haven't yet got an output from it. It's recommended to warm up the rectifier for a few minutes before attempting to draw current. The bulb of the rectifier gets pleasantly warm with just the filament but there's almost no glow from it.

[ppppenguin]

The rectifier is working, at least to some extent. I connected a dummy load (25R WW resistor) to the output terminals, warmed up the rectifier and turned on the output switch. I was rewarded by a modest blue glow in the rectifier and a very small voltage, less than a volt, at the output. So nowhere near 100% yet but at least it's heading in the right direction.

It's on the variac at the moment, I need to establish the correct operating voltage, probably by measuring the rectfier filament voltage. Then choosing the correct primary taps. As I wind up the variac the transformer starts to hum so for now I've left it at that point, just starting to hum. Another way might be to measure the primary current, wind up the voltage until it starts to rise steeply due to saturation and then back it off a bit.

I may need to make up some kind of stand to do any more work on it. Almost all the places I need to connect a meter are behind the panel but it won't sit safely except on its back. The rectifier and baretters stick out too far for it to sit on its front so either needs to be supported on its front spaced off the bench or on its side with the heavy transformer trying to pull it over.

[jimmc101]

I don't know about that photo, but I've here are some photos from a catalogue I have.
The prices shown are subject to some complicated discount structure so may not help dating the original picture.

               


It's great to see such a well preserved piece of of kit, good luck sorting it out.

Jim

[ppppenguin]

Jim, many thanks for all the information. It's interesting to note that the devices I've been calling "baretter" they call "resistance tube". Surely if they just wanted resistance they could have done it more simply than using a screw in tube.

The information also clarifies why the middle output is very different to the others.

I want to get to the stage where I can demonstrate a nice blue glow from the rectifier. I remember the one at the Vintage Wireless Museum as being quite impressive. That means I need at least one output working. If I want both outputs I can bypass the faulty baretter/resistance tube with a low value resistor hidden behind.

Need to solve how best to mount it on the wall. When I bought it, the previous owner included a wooden display stand which propped it at an angle. My first thought is mirror plates but a couple of pieces of C section aluminium ought to be both simple and secure. Will also allow it to be lifted off, rather than unscrewed.

[ppppenguin]

Hunting round the stores for some C section ali and there was none. But a couple of bits of DIN rail might work. THey seem to engage well and with the weight of the whole unit holding them engaged I don't think it can slip off very easily. Either by sideways slip or disengagement.

[ppppenguin]

I must get round to finishing the electrical side of this charger. At least to the point where I get a nice glow.

Can't find the spare mercury vapour rectifier despite a search. It will turn up when I'm not looking. At least the old one has some life in it.

Reckoned the DIN rail wasn't going to be secure enough so I bought some C section ali from Homebase.Photo shows the back panel of the charger and wall mount. The blue glow is a reflection from nearby painted surfaces. The 2 woodscrews at the ends of the wall mount stop the charger sliding out sideways.