Rheostat definition

[Ted Kendall]

A curiosity I hadn't come across before.

I'm fettling a Heathkit ID-18, a design which has been "greened" to death, but I'm leaving aside some of the more extreme tweaks and concentrating on the original Williamson article. Now this is an early example, and has an ordinary dual pot for the fine frequency adjustment. Williamson says that the control should go open circuit at the minimum end of the track, and that this has been addressed by Heathkit in "current production". He then goes on to say that "what they should have fitted here is a rheostat", a usage I've never come across before. I've never thought it a distinguishing characteristic of a rheostat that it goes open circuit at one end of its track. To my mind, a rheostat is simply a variable resistor rather than a potential divider.

Williamson was no fool, to put it mildly, and worked for the Post Ofice, as far as I know, so he should know what he's talkiing about, but I don't follow this at all.

That issue aside, where on earth can I get a 1M dual variable resistance which goes o/c at its minimum setting, short of cracking the existing component open and doing violence to it?

[paulsherwin]

I agree with your understanding of the term. A rheostat is a variable resistor with two connections and a potentiometer is a variable resistor with three connections. No standard part goes open circuit at one end of the track, though obviously such a component could be made.

That said, there may be some specialist additional meaning of 'rheostat' of which I'm unaware.

[Dave Moll]

You could always use a pot. with an on/off switch and connect the two parts in series.

[G6Tanuki]

I've always considered 'rheostat' to be an archaic term for a variable resistor [2-terminal device] and 'Potentiometer' to be a 3-terminal device.

[Equally, I don't ascribe any particular higher-level-of-sanctity to Williamson - he was just producing designs-and-stuff-that-sold]

[M0FYA Andy]

I would also never consider that a rheostat should go open circuit at the end of its track, unless specifically designed to do so.
However I have come across such devices used for example in series with a lamp as a dimmer, without an independent switch.
Andy

[John M0GLN]

F J Camm doesn't have a lot to say about it in his 'Practical Wireless Encyclopaedia' other than,

"Rheostat, a variable resistance connected in series to vary the amount of current flowing in a circuit. It differs from a potentiometer which is connected in parallel with the supply."

which would be the same as that in the posts by Paul and G6Tanuki, one has 2 and the other 3 terminals.

John

[ColinTheAmpMan1]

Horowitz and Hill don't even mention rheostats and neither does Roy C. Norris in "Radio, Television and Electrical Repairs" (Odhams, 1964). Could it be that a potentiometer adjusts a variable voltage, but perhaps a rheostat adjusts a variable current? I'm not sure that this solves the OP's quandary, but surely the idea that a variable resistor of any sort goes open-circuit at the minimum end of the track is counter-intuitive; shouldn't it go completely short-circuit? Or am I being completely dim?
Colin.

[Ed_Dinning]

Hi Gents, going back a long way, some filament rheostats would go o/c at the max resistance end to act as an off switch.

Ed

[russell_w_b]

The 1943 Technical Glossary for BBC Engineers defines a rheostat as 'a variable resistor, especially one designed for use in a circuit carrying a comparatively heavy current'.

[paulsherwin]

Originally Posted by Ed_Dinning

Hi Gents, going back a long way, some filament rheostats would go o/c at the max resistance end to act as an off switch.

Ed

Old sewing machine rheostat foot controllers go open circuit when the pedal is released, but I always assumed that that was a specific design for the application and nothing to do with rheostats generally.

[kalee20]

Originally Posted by M0FYA Andy

However I have come across such devices used for example in series with a lamp as a dimmer, without an independent switch.

I have too, wire wound slider dimmers for a couple of kilowatts worth of hall lighting. At one extreme the wiper reduces resistance to zero, thus maximum brightness. Approaching the other extreme, the resistance is such that the lamps barely glow and the last inch of movement the slider hops off the resistance winding, with a crafty spring-loaded quick-break contact to minimise arcing.

The fan speed control in my Land-Rover is another such control, though this is rotary and rated at a few watts dissipation only. Here the wiper just leaves the track, with no special features (12V isn't enough to worry about!) thus the fan is 'off' (fully anti-clockwise). Fully clockwise and the resistance is zero.

[Ted Kendall]

Thanks, people - at least I haven't committed a gross error! Some more thoughts...

I got the type number wrong - it's actually an IG-18, the audio signal generator. The Williamson in question is Reg of that ilk rather than D.T.N, better known for transistor designs than valve, and the modifications were described in The Audio Amateur. Most of the modifications are pretty straightforward and sensible, like buffering the output meter, but some have subsequently been found to be unreliable. Whether Heath actually supplied such an esoteric component as he describes I take leave to doubt. There are other ways of doing this, as has been pointed out - the only snag being whether a 1M dual linear with a switch is available off-the-shelf, and whether said switch has a low enough contact resistance for small signal switching.

[Phil G4SPZ]

As youngsters, many of us will have unwittingly used rheostats which also break the circuit at one end of their travel when playing with the speed controllers in our Tri-ang or Hornby model railways or Scalextric or Minic slot-car sets.

The traction motor speed controllers in early trams and trolley buses were also rheostats with integral 'off' switches, as were the foot pedal-operated speed controllers fitted to simple electric vehicles like milk floats. In the NHS we maintained a fleet of electric 'tugs' where the controller comprised a stack of carbon discs that were mechanically squeezed harder together by the foot pedal to go faster, and when fully decompressed actually broke the circuit. Some special-purpose rheostats in Avo equipment incorporate a break of contact with the track at one end of travel, the Avo Model 7/40 "Q" pot being one example but there may be others. Maybe Williamson had these older examples in mind.

I suspect that a rheostat needs to be specifically designed to have an integral "off" or infinite resistance function. If using a modern potentiometer wired as a rheostat, then I'd agree that wiring the track in series with an integral on-off switch is the way to go, if (as you say) a stereo 1 Meg linear DPST switched pot can be found...

[Diabolical Artificer]

Here's the schematic, can't see anything that'd call for a one off pot. The IG-18 does seem to have been popular for tweaking, I presume to try and get the THD down to make it better for testing AF devices.

Edit, found this - variable frequency control- see 2nd att, which mentions modifying a pot.

Andy.

[Amraduk]

Hello Andy,

I can see no pots/rheostats in that circuit that need to go open circuit at either end of their travel, to do so would alter their operation adversely. What does the article say about modifying a pot and precisely which pot is it?

Regards,

Dave.

[Ted Kendall]

At the risk of thread drift, he main areas of modification are:

Buffering the meter circuit from the generator, linearising and damping the meter characteristic.

Increasing open loop gain in the amplifer.

Improving smoothing in the power supply.

Enhancing the accuracy of frequency control.

It is the last of these which needs this silly component. Frequency is set by three switches, with a continuous trim control - this is only shown in outline in the main diagram. The trim control sits across the resistance selected by the other three controls, so when at minimum ideally should be open circuit rather than the 1M of the full track. Williamson claims that, in unfavourable settings of the other controls, the frequency error can be over 10%, so on that basis it's worth fixing.

Williamson also claims that Heath agreed to supply a control which behaved in this way once he had drawn the matter to their attention. The example on which I'm working is an early one, with the un-sinked pass transistor in the power supply, and has a standard dual pot. Whether Heath went to the expense of making a custom pot just to keep a pundit happy I frankly don't know. I confess I'm drifting towards the pot plus switch solution.

[M0FYA Andy]

Originally Posted by kalee20

The fan speed control in my Land-Rover is another such control, though this is rotary and rated at a few watts dissipation only. Here the wiper just leaves the track, with no special features (12V isn't enough to worry about!) thus the fan is 'off' (fully anti-clockwise). Fully clockwise and the resistance is zero.

The examples I'm thinking of operate in exactly that way, used in many places in WW2 aircraft such as the Lancaster. The Navigator's Anglepoise Lamp, used so he can see his maps, is fitted with one. The gunners, working in freezing conditions, had heated gloves controlled by such a rheostat.

I don't believe the definition of 'rheostat' includes an open-circuit at one end of the track, but a great number of rheostats also provide this feature because of their circuit function. So it might be easy for someone to have wrongly thought this was necessary for the device to be called a rheostat?

Andy

[russell_w_b]

Originally Posted by M0FYA Andy

I don't believe the definition of 'rheostat' includes an open-circuit at one end of the track, but a great number of rheostats also provide this feature because of their circuit function. So it might be easy for someone to have wrongly thought this was necessary for the device to be called a rheostat?

I think the 'off-position-open-circuit' is a red herring and is just the design. It's easier (and likely cheaper) to build a variable resistance where the slider comes off the end than to put stops on it.

The Marconi BD272 / B6122 HF transmitters were full of rheostats (relatively high current applications) and the circuitry was most definitely designed not to go open-cct.

[M0FYA Andy]

The rheostats I'm thinking of still have a mechanical stop after the slider has come off the end.............

[russell_w_b]

I guess 'Rheo' means flow (water, current) and 'stat' means stationary: This appears contradictory, but makes sense if you consider the rheostat as designed to restrict a flow of current.