Lissen Battery eliminator

[unrealdave]

Hi – I bought this battery eliminator as it looks good and kind of worked – but only gave 20 volts ht. It only give HT – three different values linked by resistance wire that was open, so only the first one gave a reading.

I think the paper and wax cap’s are dud and so put the unit in the greenhouse for a day then drew them out easily with a corkscrew.
Only the anode and the filament are connected underneath (what I know as the grid on the socket isn’t connected to anything)

When I put a meter across the anode and earth I got a reading of 120 volts dc (but the anode was negative). I thought the anode only passed positive cycles?

Anyway I stuck an 8 / 16 cap in with the neg to the anode, put some 10k resisters across the other ht outputs and connected it up and then read 240 volts.

Any thoughts?

It don't sound right!?

[crackle]

When I put a meter across the anode and earth I got a reading of 120 volts dc (but the anode was negative). I thought the anode only passed positive cycles?

Same here Dave, that seems to only work in a silicon diode9the band is the + end), but in a valve radio the 2 anodes are connected to the AC from the transformer and the cathode is the + HT supply.

Hopefully someone will give us a simple explanation, or point out where I am wrong

Is that a Bakelite case?

Mike

[unrealdave]

Same here Dave, that seems to only work in a silicon diode9the band is the + end), but in a valve radio the 2 anodes are connected to the AC from the transformer and the cathode is the + HT supply.

Hopefully someone will give us a simple explanation, or point out where I am wrong

Is that a Bakelite case?

Mike

A simple explanation would be much appreciated

Yup - its Bakelite and looks great.

[Terrykc]

Perhaps the the fact that the ring on the cathode of a solid state diode tends to be be marked + on the schematic is what is causing the confusion?

Here are two diode rectifiers - one is thermionic, the other solid state:

Diodes.png (3.67 KiB) Viewed 1917 times

Both have AC fed to the anode and the rectified DC appearing at the cathode feeds the smoothing capacitor.

Of course, current flows from cathode (negative) to anode (positive) - not the other way round, so the arrowhead shape of a solid state device is particularly confusing!

I haven't shown the load - which is effectively a resistor - but if you consider the load as being the same as the cathode bias resistor or emitter resistor of an npn transistor of a conventional amplification stage, the cathode or emitter will still be positive with respect to the chassis, it is just not as high as we expect from a rectifier stage where we expect most of the voltage drop to be in the anode/collector circuit. In a transformer fed rectifier, the DC resistance of the transformer windings is usually sufficient for surge limiting although in a direct mains fed design we expect a surge limiting resistance and possible voltage droppers to get the correct output voltage.

Dave, the reason that there is nothing connected to the 'grid pin' is that diodes don't have grids!

[unrealdave]

Dave, the reason that there is nothing connected to the 'grid pin' is that diodes don't have grids!

Yup - I have got that but the rest of it I would really like to get my head around. Unfortunately I have some business I have to take care of over the next 2 days so will take this up again with you once it's sorted if that's ok terry?

But...

why is the anode always connected to HT +ve in my battery sets?

Is it correct then to connect the -ve of the electrolytic to the anode line and the +ve to the line I consider the earth?

Why would I get 240v when its all connected up but 120v with no smoothing?

[Terrykc]

... why is the anode always connected to HT +ve in my battery sets?

When heated, the filament or cathode has a special coating that emits electrons, which are negatively charged particles. As with magnetism, like fields repel and unlike fields attract thus, when a positive charge is applied to the anode, the electrons are attracted to it and a current flows. If we introduce a grid between filament/cathode and the anode, a negative voltage can be applied to control the electron flow through the valve, the more negative it is, the more it repels the electrons and less current flows.

...Is it correct then to connect the -ve of the electrolytic to the anode line and the +ve to the line I consider the earth?

Only if you like big bangs and the prospect of picking out innumerable strands of tinfoil from inside your radio! In a word, no! The positive connection of an electrolytic must always be positive with respect to the negative terminal or at the same potential but never reverse polarised.

...Why would I get 240v when its all connected up but 120v with no smoothing?

Rectifier.png (2.68 KiB) Viewed 1900 times

As the AC input rises, the diode conducts and the smoothing charges up to the peak positive voltage. When the input drops, the diode stops conducting and the charge on the smoothing capacitor keeps the voltage constant.

If you remove the smoothing, the diode output will rise up to the peak of the input and then drop down to zero again, where it remains for the negative half cycle because the diode no longer conducts. I assume you have a full wave rectifier, the other half of which repeats the action during the other half cycle. Your meter tries to follow this but effectively only sees an input for roughly half of the time, so it averages this out and reads 50% of the peak voltage.

[Herald1360]

Of course there's nothing to prevent you from doing this, which could have some bearing on negative at the anode......

[Terrykc]

Of course there's nothing to prevent you from doing this, which could have some bearing on negative at the anode......

Yes, but this is a battery eliminator, remember, so -240V (or even -120V) is a heck of a lot of grid bias ...!

[Terrykc]

... I thought the anode only passed positive cycles?

I just took in the precise wording of Dave's original post and it might be helpful to clarify it.

As written, Dave's comment is not right - it would be better to say that the anode only passed current on the positive cycles.

This is because the current flow is always from negative to positive, not the other way round. I'm sure we all think in terms of current flowing flow HT to ground at some time or another and, most of the time, it doesn't matter. Occasionally, though, it is very important to get it right!

I assume that the convention of positive and negative were decided long before current was found to be caused by electron flow and too late to change the convention.

Incidentally, on the subject of terminology, here's the origin for the P and N in solid state diodes and transistors ...

http://us8.campaign-archive1.com/?u=4ec ... abe57c8595

[Herald1360]

If the half wave valve rectifier is floating on the secondary of a transformer, it doesn't matter whether it's connected with anode to TX giving +dc from the cathode and -dc from the other end of the secondary or cathode to the TX giving -dc at the anode and +dc from the other end of the secondary. Ground can be connected to the -dc point in either case.

Only when a full wave twin anode common cathode valve rectifier is used do the anodes have to be connected to the TX.

Although it would be technically feasible I don't think there ever has been a twin cathode common anode valve rectifier nor a complete bridge in a bottle.

[Terrykc]

If the half wave valve rectifier is floating on the secondary of a transformer, it doesn't matter whether it's connected with anode to TX giving +dc from the cathode and -dc from the other end of the secondary or cathode to the TX giving -dc at the anode and +dc from the other end of the secondary. Ground can be connected to the -dc point in either case ...

True but having the valve at the earthy end would be very unusual, if not unique!

It would certainly confuse the uninitiated - and quite a few of the more experienced - particularly if they were trying to trace the circuitry out without a circuit diagram!

[unrealdave]

I’ve found a diagram very similar to mine and now happy with what’s going on

http://www.radiomuseum.org/r/lissen_pop ... limin.html

[Terrykc]

... having the valve at the earthy end would be very unusual, if not unique!

Well, well, well, who would have thought it?

Usually the filament of the diode would have been fed by a completely isolated secondary, thus allowing the conventional layout to work but, in this case, Lissen obviously decided to save a few pennies by using a single tapped winding - although the filament section would have to be a heavier gauge, anyway ...

[Herald1360]

... having the valve at the earthy end would be very unusual, if not unique!

Well, well, well, who would have thought it?