Resistor - it's simple. Not good

This note will be devoted to such a simple thing as a variable wirewound resistor. But not such a simple resistor, like many, but SP5-35A.

At the moment when I thought that I already knew everything about variables and trimming resistors, information that was amazing to me surfaced.

I was going to make a classic scheme for coarse and fine adjustment of the output voltage in a linear power supply with two variable resistors, when a respected work colleague mentioned about Soviet resistors, which were of the two-in-one type.

First, flashbacks with double resistors on one shaft, type SP3-30G, popped into my head, then I remembered resistors that have two axes, internal and external, connected separately with different resistances, type SP3-30I, then I thought why Well, a double trimmer, such as SP5-4V1A, fit in:

But all this was not the same. My friend told me about a miracle resistor, where on one shaft there are two variable resistances, coarse and fine, and a cunning mechanism for their activation.

There was no reason not to believe this person, so I went to Google, but by the time I approached my workplace, I had already thoughtfully received a link to the device in question in the messenger. I really thought that I had seen and knew all the variables, but then I looked at the datasheet of a device that was completely new to me, but old to our society.

There are many different documentations on it, but the main points are as follows:

Resistors SP5-35A – for wall-mounted installation (1W)
Resistors SP5-35B – for printed circuit mounting (0.5W)

They are used for operation in circuits of direct, alternating and pulsating currents with voltages up to 250 V.

The design of the resistors is built according to a rough-precision scheme, has two resistive elements, while the moving systems are controlled from a single shaft. When adjusting the resistance, first the movable system of the fine resistive element is rotated from stop to stop, and then the movable system of the coarse resistive element is rotated.

Resistors SP5-35 belong to single-turn resistors with circular movement of the moving system.

Its diagram looks like this.

Main characteristics of the device:

– Range of nominal resistances
SP5-35A 68…22,000 Ohm
– Permissible resistance deviation from nominal +/-10%
– Number of moving cycles of the moving system 5,000
– Resource 25,000 hours
– Electrical resolution +/-0.01%

A separate graph even shows the dependence of power on temperature. Sometimes this information can be very useful.

Operating conditions for resistor SP5-35A:

Mechanical influences:
– Sinusoidal vibration:
– frequency range 1-600 Hz
– acceleration amplitude 100 m/s2
– repeated mechanical shocks (acceleration) 100 m/s2

Climatic influences:

– Operating temperature range from -60 to +70°С
– Relative air humidity (at temperature +35°C) 98%

After reading the documentation about the culprit of this note, I desperately wanted to understand what was inside. But I couldn’t find any information or photos on the Internet. But as a real engineer, I really wanted to look inside. After 20 minutes, I was already flipping through Avito in search of an experimental subject who would expand the circle of my knowledge.

Well, we’ve come to the part of the note for which many followed the link.

This wonderful resistor came to me. Looks pretty cool, especially compared to SP3xx

Indeed, when rotating the engine, you feel how it moves easily at first, but after a certain moment, the effort has to be increased. At the same time, it goes back easily again, and then the effort increases again. I really liked how it all felt.

By the way, the diagram depicted from the end of the resistor quite succinctly conveys the essence of everything that is happening.

I connected a multimeter and saw how it worked. Well, I didn’t see anything particularly predictable here.

Really low force results in small, precise adjustments. After this, resting and making greater effort to turn, this smooth area moves up or down throughout the entire adjustment range, depending on the direction of rotation.

Using a soldering gun and ingenuity, I removed the back cover.

The first dissection showed a rather primitive solution with signal transmission from a sliding contact during rotation. MGTF is a fairly flexible multi-core wire that allows you to twist the resistor a lot and for a long time, without fear that the contact will be lost. But I was a little disappointed. I really wanted to see something really unusual. Although, it is simplicity that is sometimes brilliant.

After unscrewing the nut, the aluminum cover is removed and the entire resistor is disassembled. There are no more fasteners. And this photo shows the whole essence of the device.

But when you twist it in your hands, it’s much easier to understand. But from the photo it’s more complicated.

This is what two wire drums look like, on the left – fine adjustment, on the right – you understand:

The essence of the work is this:

Easy movement occurs along a low resistance coil. As soon as its slider, the sliding contact, approaches the edge, the shaft clings to a special element and the movement, together with the low-resistance coil, is transferred to the sliding contact along the larger coil.

The smooth control drum has a resistance of 476 Ohms, and the total resistance of the resistor is 4.9 kOhms.

Am I talking confusingly? Of course, I’m not a designer. Let me show you in more detail:

I remove the coarse adjustment drum.

And I remove the fine tuning drum

Explosion – the diagram looks like this:

All sliding and moving elements are implemented not on bearings, but on fluoroplastic washers paired with brass ones. The main shaft on the output side of the resistor is also made simply through a bushing, but of ordinary plastic.

The principle of operation can be seen in the video below:

That's it. I hope someone was also interested in looking at the design of such a resistor. I love elegant design solutions. And this solution was also able to prove itself to be quite reliable. Now I definitely want to use it in some of my devices.

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PS: for those who especially liked this device, I took some beautiful photos under a microscope, which can tell in more detail about some of the features of the SP5-35.

If the contacts are more or less standard: