500K Potentiometer - Page 1
Forum section: electronics / repair

author: vidarr
author_rank: Frequent Contributor
timestamp: September 22, 2019, 06:43:19 pm
content: I found this old potentiometer and tested it and it works a little, but very scratchy. Before I open this up and waste time, can this be used on a 120V AC circuit? It is marked 500K LOG 1. Is this maybe an audio only potentiometer?It is pretty old and I do not know what the pins and wires on the back are for, but it seems maybe they are jumpers for voltage??? Next to one pin is marked "1/250" and the other says "2/125". The first marking I am positive, the second mark I am 90% sure that is what it says. Those two pins are "jumped" together.The two other pins are marked "0" and "4" and are jumped together with wire also. These jumper wires are definitely added by someone outside the manufacturing process. BTW, the shaft is aluminum.What do you guys think? Audio only? Or, 120 volt AC OK?Thank You

author: tooki
author_rank: Super Contributor
timestamp: September 22, 2019, 07:04:19 pm
content: You’ve hidden the pins on the side, but most likely, there are 3 pins on the side for the potentiometer, and the pins on the back are a switch. I’d guess the switch is OK for mains AC (1A at 230V or 2A at 120V), but the potentiometer itself probably is not.

author: vidarr
author_rank: Frequent Contributor
timestamp: September 22, 2019, 07:12:42 pm
content: Yes, the front has three pins -- it looks just like any other typical pot from that view. Only the back looks different (to me anyway) with those four pins and jumper wires.I don't understand what you mean the switch looks OK for 120 VAC, but the pot not OK for 120 VAC. If I hook this up to a 120VAC dimmer circuit using a TRIAC/DIAC, etc, will this potentiometer be OK, safe, used properly? Assuming I use the proper rated components to go with this 500K pot.edit: If it is OK to use in this application, then I will open it up and attempt repairing/cleaning it.Thanks

author: Nusa
author_rank: Super Contributor
timestamp: September 22, 2019, 10:18:47 pm
content: It's probably an old volume control power switch combo from an old radio or the like. If the switch action is at one extreme of the turning range, that would tend to confirm that. The switch is to turn on the AC power and the pot is to control the volume. The two functions may be mechanically connected, but they are NOT electrically connected. Treat them as separate components with separate ratings.So no, it's unlikely to be suitable for controlling your TRIAC circuit.

author: vidarr
author_rank: Frequent Contributor
timestamp: September 22, 2019, 11:13:41 pm
content: If you mean like there is a click, then smooth rotation of the shaft, then there is no "switch" action. It turns simply like a volume knob -- smooth rotation for a full turn. That's it.Thanks.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 23, 2019, 12:52:06 am
content: No! Don't use it for a Mains dimmer!!If you look at the pots used for a dimmer, they have plastic shafts to insulate the user from the "big bities".An ordinary old tube era 500k volume control does not have this feature, as it is intended to be used with low level audio signals.These sort of pots often had a switch which was operated by the same mechanism as the volume control, but not electrically connected to it.(If you look at such a pot, the actual potentiometer connections come out of the side of the device, whereas those of the switch protrude from the rear.Most radios were switched on by turning the volume control from minimum, when you would hear a click,(If the switch mechanism has failed, you may not hear the click), then after the tubes had warmed up, you could advance the volume control to your required sound level.More rarely, the switch was operated by pushing the shaft in, or rarer still, by pulling it out.These switches generally look cruddy, but they were safe & quite reliable, even when used at 240v.Not the pots, though!! I must reiterate that the guts of these were quite separate from the switches & not insulated for Mains voltages.

author: vidarr
author_rank: Frequent Contributor
timestamp: September 23, 2019, 01:42:45 pm
content: Quote from: vk6zgo on September 23, 2019, 12:52:06 amIf I take it apart to clean it, then put a plastic shaft in would that be OK? I have another potentiometer that I know 100% is made for 120/240VAC. That potentiometer got burnt, but the shaft and housing is still intact. I just need the "resistor strip" -- I don't know what it is called.Thanks

author: rsjsouza
author_rank: Super Contributor
timestamp: September 23, 2019, 03:15:03 pm
content: As others have said, the potentiometer is a standard one for audio with either switch at one side or it acts on the switch with a push/pull. If you turn the potentiometer counterclockwise (na direção contrária aos ponteiros do relógio) you will end up reaching a point of resistance that, if pushed further, will "click". That or you can push or pull the shaft to see if it "clicks".Although its value is Ok for a TRIAC dimmer (although 100kΩ is more common on circuits around), its distribution of resistance is logarithmic (the big "LOG" letters on its side). The practical effect is that you will have a very high variation of resistance very close to the end - your lamp will get very dim for most of the rotation of the potentiometer and the brightness will increase quickly at about 1/4th near the end of the rotation.http://www.learnabout-electronics.org/Resistors/resistors_09a.phpI partially disagree with vk6zgo's assessment of danger here; IME heaps of TRIAC and SCR controls used similar Constanta potentiometers over the years (I myself used many of these on my projects) - even in ungrounded installations so common in Brasil, where the danger was much higher. When grounded systems started to become more common, I used to ground the housing of the potentiometer and used a proper fuse on the live wire - if there was any fault to ground through the housing, the fuse would blow up. In addition, when possible I would also mechanically enclose the metal thread and mounting nut of the potentiometer from the user. In your particular case, the potentiometer seems quite worn and I really would be cautious to use it for this circuit.Despite this I can't say I completely disregard his concern, especially on a public forum where folks may get nuggets of information and try to apply without the proper training and/or information.

author: vidarr
author_rank: Frequent Contributor
timestamp: September 23, 2019, 04:02:23 pm
content: Yes, Constanta is the one. I put a wrench on the shaft and with some good force it did switch/click. Probably it did not turn for a long time and must have had a huge knob on it to be able to turn it. I am going to clean it, but save for another project that is audio.The LOG effect won't work for this application because the dimmer is a temperature control for a boiler pot.Thanks for all the input guys!

author: tooki
author_rank: Super Contributor
timestamp: September 24, 2019, 09:38:17 am
content: Quote from: vidarr on September 23, 2019, 01:42:45 pmGiven the amount of force the switch needs, a plastic shaft might not have worked anyway.Either way, I guess I don't understand why one would be so intent on pressing a part into service for an application it may not be suited for, given that new potentiometers are so inexpensive.FYI, I looked at a random selection of Bourns potentiometer spec sheets, and the voltage ratings are all over the place -- some models only rated to 30V, others to 1500V, some don't say at all. The outer appearance doesn't say anything about the rating. So without knowing exactly what model you have here, so you can look up its specs, it's literally impossible to know what its ratings are, and so I'd only use it for low voltage stuff.As for logarithmic pots for lamp dimmers: given how human vision works, that actually soundsbetterthan a linear pot!But yeah, for a boiler, that almost certainly makes no sense at all. However, for a boiler, you do not need a triac dimmer, you can also use very slow PWM, so you could do it with low voltage control, turning a relay off and on. You could do this with an arduino, or with a 555 timer and a comparator (more linear than just using the 555 alone).