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Reostate restoration

Before I show you how I restored two reostats, first some theory. Not in the mood for theory? Then click on: Rheostat quick fix and read the practical restoration information there.

Difference between rheostat and potentiometer

A rheostat, like a potentiometer, is a variable resistor. But what is the difference between a rheostat and a potentiometer? The difference in the name, is in the application.

A rheostat is used as a series regulator to control the current in a circuit. Eem potentiometer is used as a voltage divider, to control a voltage or an analogue signal.

A rheostat is connected between voltage source and load as an adjustable series wall. To allow sufficient current through without the rheostat failing, most types are robustly constructed. You can recognise this type of rheostat by its robust construction where resistance wire or resistor tape is wound around a ceramic body.

With potentiometers connected as voltage dividers, the current flowing through the resistor is often small. Little power needs to be summed which is why most variable resistors used as potentiometers are small and compact.

Maximum current, maximum power

On a rheostat, either the maximum current or the maximum power is indicated. If only the maximum power is indicated, you can calculate the maximum current by dividing the power by the resistance and taking the square root of that.


It is important to note that the maximum current applies to the entire control range. The maximum power specified applies only to the maximum resistance of the rheostat.

This makes sense in itself, because the resistance wire or tape used, has a maximum current at which it gets hot but does not yet burn out. This maximum current is independent of the length of the wire; if the rheostat is set to a small resistance, a low power will be converted into heat at the maximum current; if the rheostat is set to a high resistance, much more power will be converted into heat.

Yet many types of rheostats specify the maximum power to be absorbed and not the maximum current. Manufacturers do this because the maximum power to be absorbed is proportional to the size of the rheostat. This applies not only to rheostats but also to standard resistors. Thus, like standard resistors, rheostats with the same dimensions can be designated with the same power. Useful for the catalogue.

To determine which rheostat you need for your circuit, you need to determine what is the minimum current you want to flow through your circuit when the rheostat resistor is at maximum setting and what is the maximum current that can flow through the circuit when the rheostat is at the minimum resistance.

With the minimum current value and the corresponding voltage drop across the rheostat, you can use Ohm's law to work out what the total resistance of the rheostat should be.

Then calculate the current when the rheostat is at zero Ohm. At zero Ohm, no power is drawn through the rheostat, but if the rheostat is rotated slightly so that the runner is on the first winding, then the series resistance is minuscule and at after this maximum current flows through this winding. For convenience, you can take the maximum current that calculates at zero Ohm series resistance as the current flowing through this winding.

With the maximum current and maximum resistance, you can use the formula `P=I^2xxR` to calculate the minimum power reostat you need. You then choose a rheostat from the supplier with the calculated maximum resistance and the calculated maximum power.

If the value you are looking for is not available, choose a higher resistance value closest to the known value. Using this value, again calculate the maximum power and choose this calculated power or a higher power that is closest to d calculated value.

With these values, you can order a rheostat that allows you to regulate your circuit the way you want without your maximum current that can flow overloading the rheostat.

Cause of failure

In practice, a rheostat is often connected between a voltage source and a load without extensive calculation.

At a high resistance setting of the rheostat, a low current is still flowing, but as soon as the rheostat resistor is regulated back, the current goes up, and this is often surprisingly fast with a series resistor, so that before you know it, you exceed the rheostat's maximum allowable current, after which the resistor wire or tape gets too hot and burns out.

The highest current flowing through the resistor wire or tape occurs at the position where the runner makes contact with the last winding of the resistor wire or tape. If a rheostat burns out, it is also often this last winding that is destroyed by too high a current. This happens faster than you think because the power generated in this last winding is proportional to the square of the current. If the current becomes twice too large, four times too much power is generated in the resistor wire and the last winding quickly says poof! and is burnt out…..

This is why it is important to calculate beforehand the maximum current that can flow through a where your a rheostat. If that is not possible, use an Amp meter in series with the rheostat and keep a close eye on it when turning down the rheostat resistor.

Rheostat quick fix

Enough theory now the practical work.

The two rheostats below were both overloaded at some point, which caused the last winding to burn out. In both cases, a previous owner provisionally fixed the problem. This was done by winding off the ceramic housing at the point where the last winding burned out, leaving enough length of wire available to reattach it to the connection terminal. The so-called 'bypass operation'.

With this, the rheostat works again, but exhibits another problem. Because a winding is missing, the runner no longer makes contact when it turns over the last missing winding. In this position, there is no contact and the current in the circuit suddenly drops off If the runner is turned a little further, the current suddenly rises again. For most setups, this is unpredictable and undesirable behaviour. Since both rheostats are otherwise in good condition, I decided to fix this shortcoming.

Restoration rheostat 30 Ohm 3.5 Ampere

Rheostat after restoration

I got this rheostat from my father. He used it to set the charging current of model batteries.

Rheostat 30 Ohm 3.5 Ampere

The maximum resistance of 30 Ohm, which is a useful value for testing power supplies I can safely load up to a maximum of 3.5 Ampere with this rheostat. The inscription 12/64 suggests that this specimen was made in week or month 12 of the year 64. To stay on the safe side I assume it is month 12, so made in this classic was made in December 1964.

The maximum power: `Pmax = I^2xxR = 3.5A^2xx30Ω = 367.5 W `

Reostat Albert van der Perk

According to the cabinet label, this rheostat was built by Albert van de Perk from Rotterdam. This rheostat may have originally been used as a current regulator for the field winding of a DC motor. This rheostat originally has no connection sockets but was permanently wired. As I want to use it for test set-ups, I fitted the box with three 4-mm plug sockets during the restoration.

Reostat broken winding

To work properly on the rheostat, I removed it from the cabinet and unscrewed it from the lid. Here you can clearly see that the last winding had been removed by a previous owner to reattach it to the end terminal. As a result, the runner no longer made contact with the resistor tape at this point. I solved this by disassembling the end terminal and bending a piece of latoon copper around the ceramic body and gluing it in place, Then I reassembled the terminal neatly. Be careful not to over-tighten the screw and nut when assembling so as not to damage the ceramic body.

Three holes for three plug sockets

Three holes for three plug sockets

Reostat wiring with thick wire

Reostat rewired

Wiring with sufficiently thick wire and crimp terminals

Reostat with Variac knob

The original knob died sometime in the distant past. At a radio market, I came across a nice large variac knob, which turned out to fit well on the shaft. With this large knob, the rheostat is easy to set accurately again.

Rheostat Albert van der Perk final result

The result of the restoration, a well-functioning rheostat that is easy to connect with standard most cables.

Restoration rheostat 200 Ohm 100 Watt

Rheostat 200 Ohm 100 Watt

I bought this rheostat a long time ago during my internship at Philips Huizen. During the lunch break, volunteers manned the “Usable Remaining Materials Department”, which was better known as “The Scrapyard”. All kinds of items left over from departments, which would otherwise end up in the dustbin, were sold to staff for a soft price. Trainees were also welcome and I could often be found at the ABR. I still have many parts and other stuff like this rheostat I bought there.

This rheostat also has the well-known defect, a burnt-out last winding that was provisionally repaired by the previous owner. This defect is probably also the reason it moved to the ABR and thus found a new home with me.

Scale distribution rheostat.jpg

The paper label is labelled 200 Ohm.

Rheostat maximum current 650 mA

The metal ring of the inscription max 650 mA.

Reostat illogical connection

The housing is fitted with plug sockets, but they are illogically connected so that the left terminal is connected to the right terminal of the rheostat.

Rheostat dismantled

First dismantling

Reostat 100 watt bypass

After disassembly, it is clearly visible that on rheostat the 'bypass operation' has been performed. The last winding was removed from the ceramic body, after which the resistor wire was long enough to provisionally attach to the terminal

Reostat 100 watts disassembled.

The values 200 Ohm 100 Watts and the formula: Power divided by resistance and from that the square root, yields a maximum current of 707 mA. So the maximum indicated current on the metal ring seems to be on the conservative side.

The maximum current: `Imax = sqrt((100W)/(200Ω)) = 0.707A -= 707mA`

Reostat 200 Ohm measurement

When measured up, the resistance turns out to be higher than indicated, 218 Ohm. This value yields a maximum current of 677 mA. So I am that I am not the first to have measured this rheostat. ;-)

Inspection reveals that the terminal protrudes slightly above the resistor wire, after I rectified this and connected the resistor wire properly to the terminal, the runner turns out nicely, from what is now the last winding. without electrical interruption to the terminal. No latoon copper yet.

Reostat 100 watts connected

After some puzzling, it turns out it is possible to mount the rheostat so that I can connect the terminal to the plug sockets with short wiring.

Two restored rheostats

The result of my restoration work, two functioning rheostats

en/projects/reostat_restoration.txt · Last modified: 2023/04/10 11:36 by bart

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