I have a potentiometer that looks like this:


I don't want to solder wire onto the leads. Instead, I want to do something like this:


What is the standard size of the hole that I need to put the screw in?

  • \$\begingroup\$ What is the actual size of the potentiometer? Mind you the two images are about same size, but in reality the lower potentiometer is quite a bit larger than the top one. \$\endgroup\$ – jippie Jun 30 '12 at 18:08
  • \$\begingroup\$ These are the only specs I have available: jameco.com/webapp/wcs/stores/servlet/… \$\endgroup\$ – blake305 Jun 30 '12 at 18:37
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    \$\begingroup\$ These are not meant for screwing. Your selected device has a body of about 1 inch. The bolt alone on the other one is maybe 4mm or more, making the other device about 4 inch body. The soldering eyes on your device will probably break from the body once you try to bolt wires to it rather than soldering to it. \$\endgroup\$ – jippie Jun 30 '12 at 18:47
  • \$\begingroup\$ It seems like this question has become "How can I correctly solder this potentiometer" - there are many guides and you just may need heavier equipment and/or practice. \$\endgroup\$ – shuckc Jul 1 '12 at 20:44

What's your rationale for not wanting to solder to the terminals of the pot in Fig.1 ?

They are designed to be soldered to wires. The best practice is to hook the wire through the little hole, and then solder it. (Special MIL-spec bonus if you can prevent solder from wicking up into the stranded wire.) For strain relief, add heat shrink such that it covers the terminal and some length of wire.

Update. In response to Blake's comment.

I can give you a couple of tips on soldering.

  • Solder should melt on the pin (that included wires) rather than on the soldering iron. That means that the pin should to be heated up sufficiently.

  • Flux helps remove oxidation. If you use no-clean flux, you don't need to clean it.

  • Leaded solder has a lower melting temperature, so it's easier to use than lead-free.

May be, I'll be able to find a good tutorial video and post a link in another comment. Also, there ought to be a good "soldering tutorial" thread or wiki on this board too.

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  • \$\begingroup\$ Pretty much every time I solder, the solder doesn't want to stick to the wire (if it's a silver color) or the thing I'm soldering the wire to (my last failed attempt was a DC power connector from radioshack). I could try soldering, but do you have a suggestion on how I can make the soldering "work"? \$\endgroup\$ – blake305 Jun 30 '12 at 22:41

The leads on the relatively small potentiometer you linked to in your comment are solder tags, not bolt flanges. The orientation of the device in relation to the tags means even if you could find sufficiently small bolts (M1.6 or smaller) tightening them would be mechanically difficult. Straitening the tags is likely to shear them.

If you cannot heat the device for soldering, say due to lack of tools, or need something removable you might be able to use the very smallest "spade" terminal connectors used in some push-fit (eg. automobile) applications. You would however need a crimp tool to fit the terminal ends to your cable, and frankly it will be both mechanically and electronically inferior to soldering.

enter image description here

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  • \$\begingroup\$ This is the idea I had in mind. I just need to know what size screws to buy... \$\endgroup\$ – blake305 Jun 30 '12 at 20:10
  • \$\begingroup\$ @Blake - you misread the answer given. Do not use screws. Solder wires to the lugs. In worst case crimp spade terminals, but it is not a good idea. \$\endgroup\$ – Rory Alsop Jun 30 '12 at 21:33
  • \$\begingroup\$ Female spade terminals, correct? \$\endgroup\$ – blake305 Jun 30 '12 at 22:38
  • \$\begingroup\$ I believe the answer is yes. You will probably end up distorting them pretty heavily. \$\endgroup\$ – Alex Hirzel Jul 1 '12 at 17:53

This type of connections dates back from the pre-PCB era, frankly I thought they were extinct. They're not screw holes, they're solder lugs, in which a wire was soldered to connect to other components before PCBs were used.

I suggest you try soldering again. If you say your solder doesn't stick to the wire you're probably doing it wrong. That's not a reproach, it's something you often don't learn in college, and I've seen many engineers doing it wrong too.
Your solder contains a flux which cleans the surface of the objects to solder. If you apply the solder to your iron that flux evaporates, and your solder will not make good contact with the lug.
So put the wire through the lug, and heat both for some seconds with the soldering iron. Then apply the solder to lug + wire. The solder should flow smoothly over the solder lug's surface. Don't move the wire until the solder cools down.

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  • \$\begingroup\$ If the solder doesn't stick, put the potmeter away for just a second and tin the end of the wire first. If the tin doesn't stick, then the problem is with the wire. \$\endgroup\$ – jippie Jul 1 '12 at 13:55
  • \$\begingroup\$ @jippie - and tinning a wire is a piece of cake if you can dip the end in liquid flux first. \$\endgroup\$ – stevenvh Jul 1 '12 at 14:03
  • \$\begingroup\$ Not sure about the proper name of the wire, but is flux the solution for winding wire (with the thin email coating) too? \$\endgroup\$ – jippie Jul 1 '12 at 14:19
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    \$\begingroup\$ @jippie - I don't think so. Bare copper dipped in flux seems to capillarily attract the solder in a fraction of a second, especially braided wire. I think you'll need to heat it for a longer time to burn away the enamel. In a comment to this answer they said the enamel coating can sometimes be removed with a soldering iron, but in my experience this doesn't apply to transformer wire. \$\endgroup\$ – stevenvh Jul 1 '12 at 14:32

What you have is a standard panel mount potentiometer. The holes in the leads are meant to solder wires to, not as screw holes. The big shaft in the middle is meant to go thru the chassis of some device so that the shaft sticks out where the user can turn it but the electronics remain inside the chassis. Since where pots need to be mounted on a chassis for ergonomic reasons and where PC boards are available inside the device can be rather different places, these pots are designed to have wires soldered to them. The other ends of the wires would then typically go to solder holes on a PCB, or maybe a connector if it is envisioned that the unit will often need to be dissassembled for whatever reason.

What are you trying to accomplish by using mounting screws in the lead holes? Attaching a machine screw, eyelet, and nut is going to be more complicated and take longer than just soldering the wire to the hole as intended.

Here is yet another option altogether:

This was for a bench test unit to prove out a concept. A open PCB was fine for this, and dangling pots would have been a hassle. There are panel mount pots just like the one you show. It is easier to just stock one thing for lab use. I have a Eagle footprint for a panel mount pot mounted on a PCB. I arranged the pads to be where the pot leads would end up when bent flat to the board. I made them big enough to account for a little slop and to allow for a lot of solder to flow around. The pot is held mechanically to the PCB by its normal mounting screw. I also left a hole in the PCB for the orientation tab to stick thru, so there is little mechanical stress on the electrical connections.

Here is a view of the top side:

You can see the orientation tabs just to the right of each knob. The knobs themselves were separate accessories I added for convenience.

I have done this a few times and it worked out fine in all cases.

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Skip to end for short-cut solutions

If you used silver coloured conductor wire, there is a good change your experience was naturally poor because the wire was either Aluminum or the component used a welded Stainless steel lead. But items such as leaded resistors are tin plated steel are also silver in color. Pb-free solder requires a slightly higher temperature and is not as easy to flow as obsolete 50/50 tin-lead solder. Make sure it is resin core flux to reduce surface tension and allow easy flow. Then pre-heat the wire at the joint until solder melts on the wire near the tip. You can quickly melt some solder to start the heat transfer then move the solder to the other side and let the heat suck in the solder till it fills the wires. If the gap to the insulation is short an even better solder joint sucks its way under the wire insulation via the wire so that it becomes stiffer and makes a better "strain relief" to the solder joint. This explanation may seem like over-kill for this application, but practice on simple wire joints and understanding of how flux and solder flows towards the heat helps in more difficult joints. It does not take blobs of solder, rather just heat and solder to flow between the strands to envelope the joint all around. If you are soldering to say a Ground pad on a PCB with a ground plane you need a lot more heat and that would be too much for a tiny pad, so awareness of heatsink demand for heat when soldering helps understand how much you need to apply.

In any case, soldering requires a clean tip so there are methods to "tin" your tip or remove dry oxides and the surface needs to be rust free as any oxide is essentially rust. Thus a suitable abrasive such as fine grit for the terminals or even a file to expose the bare metal on a terminal to be soldered will improve "resistance" to liquid solder flow significantly. a.k.a. surface tension.

WHen I started as an EE in aerospace, the NASA trained technicians I worked with and their foreman, the teacher used a soldering manual from NASA that was 4" thick, so there are many levels of skill to be acquired for critical requirements, but when you are done, the wire ought to be able to pass the standard "five pound pull test". If you notice the strands becoming loose or weak around the joint, try again or add a few mm of wire solder to prevent the strands from breaking from twists or pulls. The solder will appear bright and shiny when done and not dull from excessive oxidation ( too long), motion during cooling ( crystals forming ) or too cool ( not allowed to reach full liquidus stage) and thus called a "cold solder joint"

But if you really want to use your existing crimped wire, I think you need a 4-40 x 3/8"L screw ,a 4-40 star washer and 4-40 nut to dig thru the surface oxide that will accumulate. A metric equiv. size will work too. ( My guess is this was your thinking to re-use existing crimped wire, which you may also cut & strip 1/4" = 6~10mm then solder)

By the way the new pot you have shown will not replace the old one shown as it has signicantly lower wire resistance and higher current (power) handling capability, so make sure the new pot power rating is sufficient and has a similar resistance. (Take it to shop and ask)

Pots come in both Cermet ( ceramic metal) and Wire Wound (WW) for low R, high current types, and also linear and logarithmic sensitivity usually for audio.

HTH's Tony

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    \$\begingroup\$ Please remove the signature, they are not allowed. \$\endgroup\$ – Kortuk Jul 1 '12 at 19:28

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