Bush Dac 90 Capacitive Dropper

[pwdrive]

With the diode you can fuse the input and crowbar the output with another diode to chassis.

Lawrence.

[Aerodyne]

Presumably with a zener or zeners? Surely a diode across the supply output would blow the fuse immediately.
Tony

[pwdrive]

The cathode of the crowbar diode connects to the cathode (output) of the dropper diode, the anode of the crowbar diode goes to chassis so normally it doesn't conduct, if the dropper diode goes short the crowbar diode will conduct (-ve half cycles) and blow the fuse.

Lawrence.

[pwdrive]

Here's a link, I've never tried a diode crowbar thus described.

http://members.iinet.net.au/~cool386/dro...opper.html

Lawrence.

[Aerodyne]

Right, silly me - the diode is in reverse mode.
The link you gave suggests that loads of British TVs of the 50s and 60s were fitted with diode wattless dropper systems. Is this true? If so I wasn't aware of it. I personally only know of the Thorn sets mentioned in my earlier post.
I recall only once using a zener as a crowbar in conjunction with a 1.4V filament supply, part of a power supply for battery portables. That was fused of course, but the preceding regulator has never failed to this point in time, so just how safe the valves would be in the event of failure is hard to judge, except to say that I trust it won't give me the chance to find out.
Tony

[pwdrive]

Yes, I remember the 1400 chassis (swing wing) I think that had a diode dropper, possibly a bush chassis as well? can't remember for sure.
One of the HMV "transportables" used a capacitive dropper, can't remember the chassis type off hand but probably a 9xx series.
I serviced hundreds if not thousands of BRC/Thorn tellys but all forgotten now, last time I was in the trade was around '86 mainly doing Hitachi, Sony and Panasonic with a few ITT's, anyone remember ITT's digital chassis from the '80's?

Lawrence.

[tiv48]

We bought some itt digi TV's when vision hire were selling them off cheap my god I hated them looked nice but that was there only redeeming feature
Rob T

[pwdrive]

Yes, nothing nice about them at all, just an early exercise in DSP and factory setup time maybe...Not had the back off a telly since.

Lawrence.

[Refugee]

Lawrence has got it close to my proposed circuit for a little Marconi radio that was once owned by my great aunt.
The HT diode in my case is a valve but the rest of the circuit is as I will be testing it.
The HT load uses the positive half cycle of mains and then the negative half cycle operates the heaters.
Should the heater diode fail short circuit the protection diode conducts and blows the plug fuse.
If one fails you would be best off to replace them as a pair.
Use the higher powered 1N5406 type diodes due to the inrush from the cold heaters during switch on.

[Aerodyne]

That looks to be the answer here, Refugee. Typically straightforward, with just the two ceramic supply resistors, neither of which should need to be physically large - just a matter of doing the sums or better still and definitely easier, using the spreadsheet. Entirely endorse the choice of diode, too, having used them myself in power supplies.
Also like you Lawrence I haven't done much with TVs since I closed my repair business in the mid-80s so I guess you could say I'm out of touch; but I still don't recall a widespread use of wattless droppers.
Tony