The danger of HT batteries

[emeritus]

Capacitors can take some time to fully discharge due to the phenomenon of dielectric strain. Something I read in an old book from when the hydraulic analogy was often used to explain electricity, was to consider a capacitor as a wide chamber in a run of narrow pipe, blocked by a rubber membrane. If pressure is applied to one end of the pipe by briefly opening a first tap, the rubber would stretch, the energy stored in the rubber being analogous to the charge stored in a capacitor (dielectric strain). If another tap is briefly opened to release the pressure, then the membrane will push water back out until the rubber is no longer in tension. Because the stretched rubber does not immediately return to its original state, after a while tension will build up as the rubber recovers, and if the tap is opened again, more water will be forced out. Thus if you only briefly short out a charged capacitor that exhibits very low leakage, you will find that voltage will re-appear across it as the dielectric strain releases.

When I was a schoolboy I used to use the tongue test to check my PP3s. At the time I had a flash gun that took a small (hearing aid?) 22 1/2 V battery with the contacts at opposite ends. I did once check it by putting it between my lips. Never again!

[Nanozeugma]

Originally Posted by Jon_G4MDC

Once upon a time the 90V battery of my HAC one valver did get wired to the side hinges of my school desk.

The light fingered bully who had been making my life hell for months squawked quite a bit when he encountered it. He blubbed to the teacher and I got a roasting for it.

Catharsis. Love it
When a bench engineer back in the 70's, the classic trick was to use a two ounce tobacco tin to catch the unwary, as they were much used for storing bits and bobs. Carefully insulate the lid from the body with black vinyl tape, then solder one of those bi-coloured Dubilier non polarised capacitors (much used as decouplers in sets at the time) between the two halves. Charge the cap and leave lying on the bench... Kangaroo impersonations followed.

[Beardyman]

Like most I've had a number of jolts from equipment but the one that sticks for the sheer level of pain at the time & for around 3-4 days afterwards was from a 24VDC to 110V 3-phase 400Hz inverter, Ye Gods! Yes, the Hertz hurt!

This one https://www.vintage-radio.net/forum/...ad.php?t=66030 of mine is particularly lethal.

[CambridgeWorks]

Symon, It was said with tongue in cheek! Alan now has more respect for the humble 90v battery.
Where I worked, I was often aware of the hazards on our tx gear, max was 265W am at vhf airband.
No, not valvd pa, just 28v dc at various parts around the chassis and pcb.
Amazing how quck I reacted when my forearm lightly touched the chassis edge when my fingers were on a 28v supply line.
Another event. After replacing a part on the pcb, we always cleaned flux from the area with IPA. A small paintbrush with an aluminium clamp around the bristles was used, dipped in a pot of OIPA.
Said clamp once shorted across the 28v rail and the IPA caught fire.
I had forgotten to turn off the supply following test of replacement part!
Rob

[Chris55000]

Hi!

Practical Wireless, Radio Constructor and a few of the books of the 1950s/early 1960s always cautioned about using headphones directly connected in series with valve–anode circuits, even in battery receivers!

I have to admit that I've never had an actual shock from a high–tension dry battery, I have.come across supplies over 50–60V in the most innocuous looking pcbs I've had to service in the course of my employment, and I know I would certainly get a nasty belt from any sort of dry–battery H.T. supply!

It's not only simple established custom that valve anode supplies are always referred to as "high–tension", you'll feel the reason why as well if you accidently contact it!

Chris Williams

[TonyDuell]

Originally Posted by Chris55000

Practical Wireless, Radio Constructor and a few of the books of the 1950s/early 1960s always cautioned about using headphones directly connected in series with valve–anode circuits, even in battery receivers!

That's odd...

I remember a beginner's course in Practical Wireless in the 1950s. You got to build ever-more complicated receivers, on a metal chassis. One of the first was a single valve set with headphones either in the anode circuit or maybe capacitively coupled to them. What worried me was that while the valve heater was powered from a mains transformer, the HT was obtained by half-wave rectifying the mains. The chassis (remember this was experimental, not in a case) was connected to mains neutral (I hope!).

Now perhaps I am over-cautious, but the idea of headphones connected to the anode of a valve in what is effectively an AC/DC set worries me. If the insulation breaks down the device could be better described as an electric chair.

[broadgage]

I agree, IMHO, headphones should never be connected directly to any form of mains powered live chassis set.
And would be better not directly connected to anything mains powered.

[Ed_Dinning]

Hi Gents, I remember that design well. Better known as the "Beginners Destructional Course"


Ed