DC POWER SUPPLY - Page 1
Forum section: electronics / beginners

author: WIRENUT
author_rank: Contributor
timestamp: December 11, 2013, 01:32:52 am
content: I built a DC Power Supply for my bench, I needed it quick so I ended up buying a PCB from Velleman manufacturer. Instead of using the 4.7k trimpot that was given to me to adjust the DC output voltage, I simply wired a 4.7k POT to the PCB so I can put a knob on it.If you change the resistance of the POT, you are adjusting the DC output voltage.My PROBLEM is that output voltage of the circuit changes drastically with the smallest turn of the POT. It literally jumps from 5.0V to 24.0V with not so much as a 1/8 of a turn. Is there a certain type of POT out there that changes its resistance in very small increments?

author: abbotsmike
author_rank: Contributor
timestamp: December 11, 2013, 01:50:45 am
content: Without a bit more info to go on, you're not using a linear pot where you should be using a log pot or vice versa are you?

author: minime72706
author_rank: Frequent Contributor
timestamp: December 11, 2013, 03:11:04 am
content: Either your potentiometer is damaged or you ought to get a multi-turn potentiometer. Given the mention of random jumping, I would guess that it's damaged.Where did this potentiometer come from and how sure are you that it's linear?

author: GreyWoolfe
author_rank: Unknown
timestamp: December 11, 2013, 03:34:12 am
content: One of the upgrades I did to my Elenco XP-720K power supply kit was replace the standard 1 turn voltage adjustment pots with fake Bourne 10 turn pots from China. Works a treat to echo Dave's words. The first DPM I installed is a 3 digit but I have a 4 digit display with adjustable decimal point to give me to the hundredths of a volt. The pots were cheap, also. Here is something suitable for your needs:http://www.ebay.com/itm/1pc-10-Turn-Dial-Pointer-Potentiometer-4-7k-Ohm-WXD2-53-/250915329437?pt=LH_DefaultDomain_0&hash=item3a6bb8159dTom, NW0LF

author: Andy Watson
author_rank: Super Contributor
timestamp: December 11, 2013, 04:18:07 pm
content: My money is on the pot wiper and end terminal being swapped. Though, a little more detail would be useful.

author: WIRENUT
author_rank: Contributor
timestamp: December 11, 2013, 08:45:24 pm
content: This is the complete circuit that I am using. It is very simple and uses the LM317 voltage regulator. I apologize for how crud this schematic is, my software didn’t have certain components in its library. However the schematic that I have for this circuit is identical to what I drew up. The terminals on the the LM317 are not labeled on their schematic.I am not sure exactly what is the difference between a Linear and Log POT but based on what I know, I am using a linear POT because ohming the center lead on the POT and one of the ends, the resistance increases or decreases when turning the knob.I metered the resistance measurements from the POT, it definitely doesn’t seem to be broken, it doesn’t “skip” from one resistance to another. But it is difficult to turn it slow enough to step through each value of resistance.Do you think a log POT will work better? I saw online that it changes its resistance slower with respect to (WRT) its position.

author: AG6QR
author_rank: Frequent Contributor
timestamp: December 11, 2013, 09:16:47 pm
content: Quote from: WIRENUT on December 11, 2013, 08:45:24 pmA log pot won't work better. The difference between a linear pot and a log pot is that a linear pot changes its resistance as a linear function of the wiper position, while a log pot changes its resistance as a logarithmic function of the wiper position. That's slower over part of the wiper's range, but faster over another part.Set up your ohmmeter to measure the resistance of the pot again. Turn the pot in equal increments, maybe 30 degrees at a time. Does the resistance steadily increase, an equal amount for each small turn of the wiper? Or do some 30 degree segments change resistance more than others? The first type is linear, the second type is (probably) logarithmic.Another way of doing the same experiment is to divide the total resistance by ten. Then mark the position of the pointer as you see your meter indicate one tenth, two tenths, three tenths, four tenths, etc. of the total resistance. Are the marks equally spaced out? Then the pot is linear.A ten-turn linear pot is ideal for this type of application. A conventional linear pot is much cheaper, and should be generally adequate, but a bit hard to adjust precisely.

author: Shock
author_rank: Super Contributor
timestamp: December 11, 2013, 09:21:56 pm
content: This shows you how you can tell the difference in tapers. As you can see log/audio taper is quite different from linear.So you can compare the one supplied which is probably log/audio taper. It's most noticeable at 1/3 through the movement.I would try the trim pot in the circuit and be sure it's behaving as expected as well as checking if it's log/audio.Then decide if the movement is too quick and replace it with an appropriate multi turn pot.

author: mariush
author_rank: Super Contributor
timestamp: December 11, 2013, 09:23:38 pm
content: The pot may "break" connection when you twist it, thereby leaving the adjust pin floating, like the pot is not there... so you get the 24v because you give the regulator a bit more than that at the input.You're not going to have 35v at the output - based on your schematic, you'll have 25.2v AC at the input .. that's 25.2*1.414 = 35.6v DC peak (let's round to 35.5v), then you lose voltage on two diodes at a time (in the rectifier) so you're left with 35.5v - 2 x 0.75v-1v .. let's say 35.5-1.5v = 33v.Then the LM317 has a voltage drop of up to 2v up to 1A and above that, it could go up to 2.5v, so at best you're looking at 33v - 2v drop = 31v DC at the output.I would also reduce that 10uF capacitor (C3) to a 1uF or a 2.2-4.7uF ceramic capacitor (100v rated) which should make the adjustment more instant I thinkI would raise the C2 to about 10uF but that's just me... 1uF would work just as well.If you plan to use it up to 25v and more, you should follow the advice in the datasheet (http://www.ti.com/lit/ds/symlink/lm117.pdf, page 11 - datasheet is common for lm117 and lm317) and add a couple of protection diodes (1n4002-1n4007, 1n4001 is only rated up to 50v and 35v, too close for comfort)Don't forget about a proper heatsink for that linear regulator... the dissipated power is (Vin-Vout) x current ... so if you think you're going to give it 35v and get 2-3v out at 1a you're kidding yourself, because you would probably need a cpu cooler to keep the regulator working.Do the math on page 12 on that datasheet and figure out the minimum heatsink you're comfortable with (or use an online calculator :http://www.ef-uk.net/data/heatsinking.htm- the page also has the formulas and explains how to figure out proper heatsink at the bottom).

author: minime72706
author_rank: Frequent Contributor
timestamp: December 11, 2013, 10:12:53 pm
content: In the very least, you can get a 10-turn potentiometer (or better, like 15) for fairly cheap on ebay. I have one in front of me.

author: WIRENUT
author_rank: Contributor
timestamp: December 14, 2013, 09:01:01 pm
content: Wow, I apologize guys, I must have made some crazy mistake. The POT is bad. At the point when the voltage jumps, is the same point when the resistance jumps. Thanks for the help. I probably would not have double checked.At least now I know the difference between a Log POT and a Linear POT. Thanks againProblem Solved!

author: minime72706
author_rank: Frequent Contributor
timestamp: December 14, 2013, 10:05:31 pm
content: Quote from: WIRENUT on December 14, 2013, 09:01:01 pmTOLD YA!Give kind regards to the cheap bastards at Velleman.

author: WIRENUT
author_rank: Contributor
timestamp: December 15, 2013, 01:09:17 pm
content: Yeah I know tell me about it. Can you tell me exactly what a muliti turn POT is. And what a 10 position POT means? Is it still liner POT, only it just has 10 positions?

author: mariush
author_rank: Super Contributor
timestamp: December 15, 2013, 03:40:46 pm
content: multi turn pot is a potentiometer where you need to rotate it several times to get from 0 to its maximum value. A complete turn of a potentiometer is 360 degrees. A lot of potentiometers normally only turn about 270 degrees.10 position pot doesn't mean anything as far as I know - he probably meant 10 turn potentiometer.You can see here a lot of trimmer potentiometers, and you can filter by the "turns" column to get potentiometers that have several turns - the most common are in the range 10-12 turns:http://uk.farnell.com/trimmer-potentiometersThe larger potentiometers are also here, and the most common ones are with 10 turns :http://uk.farnell.com/rotary-potentiometers

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: December 15, 2013, 04:09:07 pm
content: Would a dual-unit pot work here? i.e. A 2 pot unit with an outer ring and inner stem.Use one for gross control, the other for the fine adjustment.such as this item here ($2 plus change)http://www.mouser.com/ProductDetail/ALPS/RK09712200HA/?qs=sGAEpiMZZMtC25l1F4XBUza7emsVrz946TZ8WgjdyZU%3d

author: mariush
author_rank: Super Contributor
timestamp: December 15, 2013, 04:26:18 pm
content: The one you linked to is a simple potentiometer for audio applications .. basically two potentiometers in one, locked to give you the same value as you turn the knob. It's also just 300 degrees, so only a turn to move between 0 and 10 kohm, and it has around ±20% resistance tolerance.It's not a rough resistance control + fine resistance control , they're not independent.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: December 15, 2013, 04:53:08 pm
content: Quote from: mariush on December 15, 2013, 04:26:18 pmCool! Didn't know. Do they make dual independent pots?

author: Unknown
author_rank: Unknown
timestamp: December 15, 2013, 07:28:53 pm
content: Quote from: JohnnyGringo on December 15, 2013, 04:09:07 pmSure, this is a common technique in analog power supplies. The more expensive brands would often offer a 10-turn pot as optional upgrade. The issue with the coarse/fine arrangement is that it's more fiddly (first try turning the fine, turn the coarse if you run out of the fine adjust range, then switch back to the fine for the final adjustment). The advantage over 10-turn pots is that it's much easier to quickly go from say 20 V to 0 V without a lot of knob turning.Quote from: JohnnyGringo on December 15, 2013, 04:09:07 pmThat's a (logarithmic) audio volume control with two tracking wipers on the same axis. Not something generally used for variable power supplies.