Linear voltage regulator - Output higher than input?! - Page 1
Forum section: electronics / projects

author: xZero
author_rank: Contributor
timestamp: December 27, 2015, 01:53:17 am
content: In my project I used LM338T to drop 24VDC input to 7VDC (Something more suitable for Arduino's buil-in regulator). While experimenting with different resistors for regulator I found something very odd: Some values between ADJ and ground may cause output voltage to be high as 40VDC while input is only 24VDC!!! How is that possible?Can that be due to power source? It's an Christmas tree LED lights power adapter with 24VDC (Measurements shows 28VDC)I cannot assure that regulator is LM338T as I suspect it is LM317 with LM338T label. (Cheap eBay trash)Based on my test on this specific regulator it provided only 1.5A of current - MAX! SO probably fake LM338T but it still doesn't explain higher output voltage! (than input)I use an UNI-T UT-61E brand new DMM which works great.

author: Skimask
author_rank: Super Contributor
timestamp: December 27, 2015, 01:58:44 am
content: And is this little "experiment" with the output from the regulator open circuited?

author: xZero
author_rank: Contributor
timestamp: December 27, 2015, 02:05:07 am
content: Uhm.... Yes... ...so that matters?

author: langwadt
author_rank: Super Contributor
timestamp: December 27, 2015, 02:12:24 am
content: Quote from: xZero on December 27, 2015, 01:53:17 amyou measured 40V isn't real, the regulator is probably oscillating, what capacitor on the output?

author: fivefish
author_rank: Frequent Contributor
timestamp: December 27, 2015, 02:16:42 am
content: Scope it. My bet it's oscillating/spiking.

author: Skimask
author_rank: Super Contributor
timestamp: December 27, 2015, 02:21:19 am
content: Quote from: xZero on December 27, 2015, 02:05:07 amYes.RTFDS applies.Minimum load requirements, capacitive loading, and a hundred different other variables.

author: xZero
author_rank: Contributor
timestamp: December 27, 2015, 08:24:53 am
content: I would scope it but I'm a hobbyist without an scope - i can't afford it.On input there is 0.01 µF capacitor and on output is 1 µF capacitor.Between output and ADJ is an 270? resistor. Between ADJ and ground I was changing many values. Current config is 2x820? in parallel - was getting hot if just one of 410? is used.Regulator is running very warm (nearly hot) without heat-sink and load, but I guess it's normal as 270? resistor is actual load in series with 410?.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2015, 09:18:33 am
content: Sonds like oscillating. The high frequency (e.g. 100s kHz) signal can make the meter to give errant readings.Normally the regulator should not run hot without load. The resistor from output to the Adj. Pins sets the current flowing there. With 270 Ohms this is 4-5 mA so about the minimum specified load current. So only expect something like 100 mW power dissipation for the regulator. If it oscillates current consumption can get higher. Thus it might be good idea to measure the current from the supply.

author: Deckert
author_rank: Regular Contributor
timestamp: December 27, 2015, 10:47:54 am
content: Quote from: xZero on December 27, 2015, 08:24:53 amWhoa ... the resistor between ADJ and GND is getting hot? Sounds like the regulator might be faulty, since the ADJ-to-GND current should be extremely low. Assuming it is an LM317, the datasheet specs Iadjto be 50 microamps, typical.--deckert

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2015, 10:55:40 am
content: The adjust pin givea an extra 50 µA or so. Between output and ADJ there are 1.2x V in working mode. So the 270 Ohms resistor would get something like 4.5 mA and should not get hot.It is still possible that the regulator is just oscillating.The LM317 needs a good decoupling at the input side - e.g. 100 nF to 1 µF low ESR cap (non electrolytic) close (e.g. less than 20² mm tolal loop) to the chip. The output cap is not that critical but something like 100 nF is a verygood idea. Just a cap in the 100 pF-1nF range can lead to oscillation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2015, 12:27:09 pm
content: Is your multimeter too cheap, too?... I mean: do you have one of those awful $3 yellow boxy ones?If so, then they're often worse than useless, because they outright lie under so many trivial conditions. Instruments are your eyes and ears, don't cheap out on them!20V is quite a lot of voltage to drop with a linear regulator, even with a large heatsink. Don't expect more than 50mA without a heatsink, or up to 500mA with a full sized heatsink.Tim

author: xZero
author_rank: Contributor
timestamp: December 27, 2015, 06:21:18 pm
content: DMM I use is UNI-T UT-61E. It's not super expensive one like Fluke, but it does it's job very well.If my regulator is faulty, then all regulators I have are faulty as I tried 3 of them. They come from same source though - All labeled as LM338 but fails to deliver more than 1.5A.What I noticed is that output voltage is unstable - it jump 300mV +- . Maybe the power supply is faulty - I don't think manufacturer was concerned about output voltage quality¸so circuitry may be pretty basic, it's supply for bunch of LED-s so I don't expect it to be of high quality either. I didn't seen circuitry as it's hermetically sealed.

author: Audioguru
author_rank: Super Contributor
timestamp: December 27, 2015, 09:54:26 pm
content: The problem is mentioned in the first post: The regulators are "cheap ebay trash". Buy Texas Instruments name-brand parts from a reputable distributor like Digikey.

author: xZero
author_rank: Contributor
timestamp: December 30, 2015, 04:40:04 am
content: Yes, and for price of one genuine LM338T I can buy 5 of those ebay trash. Well, maybe the worth choice after all.

author: SharpEars
author_rank: Unknown
timestamp: December 30, 2015, 03:18:20 pm
content: Quote from: Audioguru on December 27, 2015, 09:54:26 pm+1.Buy shit, it has odd behavior, complain here, answer you get is above ^^.

author: tggzzz
author_rank: Super Contributor
timestamp: December 30, 2015, 03:25:36 pm
content: What inductors do you have in the circuit? Inductors+capacitors=>resonance, with or without a voltage regulator. A voltage regulator, particularly a fake one, could easily supply sufficient power to cause oscillation if it doesn't like the load.Hint: a piece of wire is an inductor.

author: Fungus
author_rank: Super Contributor
timestamp: December 30, 2015, 03:41:40 pm
content: Measurement error. Your multimeter can't handle the weird signal coming from the regulator.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 30, 2015, 03:53:02 pm
content: Operation without a DC load is unpredictable/undefined. (Not to mention NOT a "Real World" condition.)ANY of the components (resistor, regulator, etc.) getting warm with NO LOAD is NOT normal. Something is dramatically wrong here. I agree with the others that you probably have oscillation from an unstable circuit and/or dodgy (or faulty) components. You COULD try measuring the output with the AC voltage range on your meter. Of course, you should read zero volts AC.

author: TimFox
author_rank: Super Contributor
timestamp: December 30, 2015, 06:00:56 pm
content: See page 7 of this TI datasheet for recommended capacitors.http://www.ti.com/lit/ds/symlink/lm338.pdfI am old enough to remember when the 7805, etc. regulators were introduced and learned the importance of a good input capacitor immediately. Without a good capacitor there, the 7805 or 7815 would oscillate typically at 100 kHz or above.