Interesting variable resistance device

[ChristianFletcher]

I’m continuing with my deep dive into the Regentone Battery Eliminator and this 1920s device keep the surprises coming. I found the variable voltage control potentiometer to be open circuited according to a low voltage ohm meter. So it was reasonable to dismantle the control and inspect it. You can imagine my surprise when I found the control filled with graphite powder. It looks like the resistance is variable by compressing the powder. I have a vague memory such systems were used as voltage regulators in early aircraft but not sure. Has anyone seen anything like this before and can provide any additional information.

I assume this had some advantages over a conventional wire wound or carbon potentiometer? It didn’t measure on an ohm meter but it probably works with a few hundred volts across it?

Thanks regards Chris

[Cobaltblue]

One form of carbon pile regulator can be seen here.

https://www.brighthubengineering.com...pile-regulator

Cheers

Mike T

[ChristianFletcher]

I remember a number of my old cars and trucks using Dynamo generator had them. I think they connected back to the field winding

[ChristianFletcher]

Another interesting feature is the control's shaft is live to the body just to make it more interesting. Better not loose that knob!

[Bazz4CQJ]

It vaguely reminds me of my Pilot Resistograd which I acquired as a boy. For those of you not familiar with this wonderful device, the attached shows an advertisement for one in a 1930's copy of Wireless World. I did actually use it just a few weeks ago for some task. It takes quite a few turns to go from hi to lo and you can feel the load increase as you increase the compression.

B

[Ed_Dinning]

Hi Folks, also used at quite high powers to control the speed of early fork trucks/ electric vehicles.
They usually consisted of a stack of carbon/ graphite tiles the were compressed together to reduce the resistance value and increase the speed of series wound traction motors.

Ed

[ex seismic]

Also used in some 40s/50s military radios for voltage control. Carbon piles that is, not necessarily this particular device.

[Cobaltblue]

I have a couple of home made Radios from the early 1920's that use a much smaller long thin device as a "variable grid leak"

Picture grabbed off picklick
These also use carbon, of course carbon as a variable resistance was a well known technique being used in microphones.

Cheers

Mike T

[kalee20]

I've come across the carbon pile rheostat, as referred to by Ed Dinning, as a high-power load (for testing 5V 200A power supplies).

Let's not also forget that (once) extremely common, pressure-operated, variable carbon resistance device - the telephone microphone.

[ChristianFletcher]

Thanks for the information and the pictures. I suspect there could be some power dissipation in this device under normal load and especially in the event of an overload condition. I wonder if this was just cheaper than using a conventional wire wound rheostat or a high resistance value wire wound rheostat may not have been practical to manufacture . Interesting that I’m not measuring any resistance using a conventional ohm meter so I will see if it measures better using a few hundred volts. Better remember to put the knob back on!

I do also remember that the carbon pile type voltage regulators were used in aircraft late after the invention of semiconductor as they have very high reliability figures and aren’t easily damaged

[The Philpott]

..An interesting precursor to Quantum Tunnelling Composite!
Dave

I would have thought they cost less to make than a wire wound pot and have a considerably higher power rating for their size, especially when a pot is using but few turns of the wire. The downside being no reliable way of setting them using a pointer or some such.

[ChristianFletcher]

That’s a really interesting point about the pointer so impossible to calibrate them against a scale

[Ed_Dinning]

NO scale, hence the expression "tune for maximum smoke"

Merry Xmas Ed

[MotorBikeLes]

The one problem with stuff like this is if it gets hot, then its significant NTC characteristics take over. Runaway current flow can follow.
Of course in some situations an advantage.
Les.

[ChristianFletcher]

In between all the Christmas festivities I have been spending some time playing with the graphite rheostat. I Found that I could easily adjust voltage output to within a couple of voltage into a load. Worse case the device was dissipating just over two watts & 90V and just gets warm to the touch. Remember that this isn’t a voltage regulator and needs manual adjustment the output is still very consistent within a few volts. I did however find that the control knob does have to be wound tightly to achieve the required compression of the graphite to pass the desired current. In summary the device works very well in this application.

[trobbins]

If I recall correctly, one advantage of the large carbon block rheostats was that the number of inserted carbon blocks could be modified to adjust the range for a particular setup. Nowadays the rheostat frame and clamp could be used for a variety of applications like a bench drill clamp, or flower press, or miniature book press.

Although not exactly the same style as what I was used to, the link is pretty similar in style: https://www.montagar.com/~patj/ebjagabi12.htm

[Lucien Nunes]

Carbon pile rheostats were widely used in industrial applications as part of control loops e.g. for generator voltage regulation, because they are controlled by force rather than position. A solenoid can be arranged to compress the pile with minimal movement of parts, making it responsive. The lack of positional calibration is unimportant because it's in the loop.

I have about five different machines with carbon piles, including a fork lift where it controls the traction motor field, about 4A at 24V. It is used as a kind of operational amplifier with multiple inputs from the accelerator pedal and two solenoids, one for battery voltage and one for armature current. The pile is pre-loaded by a spring to minimum resistance (= maximum field for high torque but low rpm.) At 50% accelerator when all the armature resistance has been cut out of circuit, a cam starts to unload the pile to weaken the field and increase the speed. At the same time, the armature current solenoid assists the spring to strengthen the field (more torque available on demand, to make the shunt motor behave like a compound) but the battery voltage solenoid competes with the spring to unload the pile, making the torque less dependent on state of charge. The traction motor is built into the wheel hub; it is necessarily as compact as it reasonably can be and the 'intelligent' controller helps to get the most out of a small unit and protect it from driver abuse.

Pics and explanation here: http://www.electrokinetica.org/d1/6/index.php
Video here: https://www.youtube.com/watch?v=2M79FmIHScQ

But moving on, I have a device very similar to Chris's unit that was sold in the 1920s or so as a light dimmer or fan speed controller. It is in a ventilated decorative case that mounts on the wall like a large lightswitch and in my case it was a showroom sample so it is mounted on a pattress with a descriptive ivorine label. The resistive element consists of multiple nested cups of powder, which is not pure graphite but a mixture containing mica flakes. I found the patent for it which specifically refers to the insulative ingredient in the powder, the exact makeup of which presumably was chosen to suit the voltage and current.

I will dig out some pics of that. I have not attempted to operate it. I think some powder is missing.

[ChristianFletcher]

For those that want to take a closer look and see me doing something testing I have made a video. Skip forward 24 minutes in https://youtu.be/DvYxR51YYus

[Maarten]

I think they were used in sewing machines as a speed controller as well.