uk.people.disability

(not quite so) Fat Sam wrote:
> Palindrome wrote:
>> (not quite so) Fat Sam wrote:
>>> Temprance wrote:
>>>> I don't wish to have to spend a lot of money which I have not enough
>>>> of so can anybody give me their thoughts on what is wrong with my
>>>> Mercury M48 Scooter. I've got a glitch where all of a sudden it
>>>> slows right down and then picks up again. A bit scary as I don't
>>>> want to be stuck somewhere. It did it today and about 40 seconds
>>>> before the power came back. According to my manual it should beep
>>>> at me when something is not working but so far it has remained
>>>> silent. temprance
>>> Sounds to me like it could be a problem with the rheostat that forms
>>> the throttle mechanism.
>>> It may be damaged, or it might need cleaning.
>>> Is there any regularity to it?
>>> Does it always happen when you hit the same speed?
>>>
>>> Or does it happen when you go over rough surfaces? That might just
>>> suggest somethng as simple as a loose connection or a cracked solder
>>> joint.
>> Wow, I haven't seen a speed controller with a rheostat for, oh, more
>> years than I can remember. IME, they all use pwm controllers, these
>> days. YMMV.
>>
>> If you mean "potentiometer", I dare say that the OP gave the throttle
>> a good old waggle when it stopped. So any poor contact point would
>> have been moved off, quite quickly.
>
> Errr. I may have my terminology mixed up...
> I mean a variable resistor type thingy...Is that a rheostat or a
> potentiometer?
>
>
They both are...

A rheostat is a variable *power* resistor which typically is used to
directly control the current going to the motor (or the motor field). eg
an old Singer sewing machine has one of these in the foot control. It
normally has just two terminals.

A potentiometer is a low power resistor with three terminals. Typically
a reference voltage is fed to one end of the resistor, ground is
connected to the other end and the third terminal is connected to a
wiper that can move along the resistor - giving an output voltage
somewhere between zero and the reference voltage, depending on position.
A radio volume control is an example. The output voltage is used to
control electronics which in turn changes the current going to the motor.

Before electronics arrived, rheostats were almost exclusively used to
vary the speed of small motors - now, the only place you may find one is
controlling the speed of the fan in your pc...


--
Sue