LM7805 unstable - Page 1
Forum section: electronics / beginners

author: bogdant
author_rank: Regular Contributor
timestamp: February 18, 2019, 05:59:17 am
content: For Vin=35V then Vout of Lm7805 is 9.5V! The Cin =0.1uF and Cout=0.1uF. This is a surprize for me, I would expect to work, even is on maxim specified voltage. Even more, for Vin>20V the stability of LM7805 dependes on load.Can you find this in datasheet ? What would be a solution in my case for Vin=35V?

author: ebastler
author_rank: Super Contributor
timestamp: February 18, 2019, 06:42:03 am
content: 35V is specified as the "absolute maximum rating", i.e. the LM7805 will not be damaged if you apply this voltage. But if you look at the "electrical characteristics" table, you will find that the output regulation propoerties are only specified for up to 25V input voltage.For such a large step down in voltage, I would prefer a buck converter which does not convert the excess power into heat. Off the cuff, I don't know one with 35V input voltage range, but I trust you can get them. Google for "step-down converter" or "buck converter". You can get the bare ICs, and also little piggyback modules which come complete with the required passive components. The latter are cheap on ebay, Aliexpress etc., and are convenient for a one-off or a small number of units.

author: Nerull
author_rank: Frequent Contributor
timestamp: February 18, 2019, 06:52:05 am
content: Just below "Absolute Maximum Ratings", it the TI datasheet states:QuoteThe 7805 is designed to survive 35V, not operate at 35V.Just below that are the "Recommended Operating Conditions", where Vin max is 25V.https://www.sparkfun.com/datasheets/Components/LM7805.pdf

author: bogdant
author_rank: Regular Contributor
timestamp: February 18, 2019, 08:33:02 am
content: thank you

author: wraper
author_rank: Unknown
timestamp: February 18, 2019, 08:34:19 am
content: In addition to already said, isn't it some counterfeit junk from ebay?

author: Zero999
author_rank: Super Contributor
timestamp: February 18, 2019, 08:35:10 am
content: Your LM7805 is bad. It shouldn't give that higher voltage, even with 35V in.

author: alex-sh
author_rank: Regular Contributor
timestamp: February 18, 2019, 09:03:56 am
content: Is your 7805 operating the same at, say, 24V? If yes, perhaps it is damaged?There are plenty of those buck converters on Ali or Amazon:https://www.amazon.co.uk/LM2596-Converter-3-0-40V-1-5-35V-Supply/dp/B01GJ0SC2Cthis is the one I have based on LM2596.35V is too high for LM7805. Either decrease voltage to below 25V or use the above buck converter.

author: StillTrying
author_rank: Super Contributor
timestamp: February 18, 2019, 09:21:14 am
content: You don't mention any mA load on the output, most regulators need at least some small load to stop the output rising.

author: wraper
author_rank: Unknown
timestamp: February 18, 2019, 09:39:31 am
content: Quote from: alex-sh on February 18, 2019, 09:03:56 amBased on fake LM2596 which does not even operate at the same frequency as original.

author: wraper
author_rank: Unknown
timestamp: February 18, 2019, 09:42:37 am
content: Quote from: StillTrying on February 18, 2019, 09:21:14 amActually they don't. Usually only adjustable regulators such as LM317 need minimum load. 7805 does not need it because a few mA quiescent current flows through it's GND pin.

author: JackJones
author_rank: Regular Contributor
timestamp: February 18, 2019, 10:05:51 am
content: Some of the 78xx datasheet do mention a minimum load current. For example here:https://www.st.com/resource/en/datasheet/l78.pdfQuoteOther datasheets don't, I guess that depends on the manufacturer. Better always to double check the datasheet from a particular manufacturer.Edit: Actually, I checked a bunch of other 7805 datasheets. While they don't explicitly mention minimum of 5mA, they all seem to use that 5mA as minimum in their electrical characteristics. So maybe it is a requirement for all of them.Here's a couple datasheets to check:http://ee-classes.usc.edu/ee459/library/datasheets/LM7805.pdfhttps://www.sparkfun.com/datasheets/Components/LM7805.pdfhttps://static1.squarespace.com/static/5416a926e4b09de8832655bc/t/54427078e4b03de3b67b89af/1413640312811/lm7805.pdfThey all have that output voltage parameter test conditions as "Io = 5 mA to 1 A"And you're absolutely correct about those fake LM2596s. I recently bought some of them from several different sellers, all of them fake. Most of them didn't work in the inverting configuration at all, and even those that did had only ~50kHz switching frequency. I wouldn't bother with ebay/ali for those, LCSC seems to have the for dirt cheap and the western sellers for next to dirt cheap.

author: wraper
author_rank: Unknown
timestamp: February 18, 2019, 11:10:19 am
content: Quote from: JackJones on February 18, 2019, 10:05:51 amSome might say minimum load but any of them will work without a load just fine. It's only that some spec might not be met. Unless op says where he bought it, I expect it to be counterfeit.

author: CJay
author_rank: Super Contributor
timestamp: February 18, 2019, 11:17:13 am
content: Quote from: wraper on February 18, 2019, 11:10:19 amI've always found them to be on the mark for rated voltage output too when unloaded but...Must, must must check capacitor requirements for regulators, there's a batch I have which give really odd output voltages if you don't have a suitable capacitor on the output, add the capacitor and it behaves perfectly.I found out the hard way and now the box has a large handwritten comment with capacitor specs.

author: ebastler
author_rank: Super Contributor
timestamp: February 18, 2019, 11:39:02 am
content: Quote from: wraper on February 18, 2019, 09:39:31 amHmm... How do you know that? Can't recognize anything obvious that seems suspicious (beyond the low price, of course) in the Amazon description?

author: bogdant
author_rank: Regular Contributor
timestamp: February 18, 2019, 12:03:08 pm
content: Quote from: StillTrying on February 18, 2019, 09:21:14 amif you add a load 500 the voltage drops from Vout= 9V-> 5V, but does not regulate.

author: bogdant
author_rank: Regular Contributor
timestamp: February 18, 2019, 12:05:21 pm
content: Quote from: alex-sh on February 18, 2019, 09:03:56 amIf Vin <20V the LM7805 works Ok. I did try another sample, but it is the same. There are from ST.

author: ebastler
author_rank: Super Contributor
timestamp: February 18, 2019, 12:15:01 pm
content: Quote from: bogdant on February 18, 2019, 12:05:21 pmBut where did you buy them? Can you be sure they are genuine?

author: bd139
author_rank: Super Contributor
timestamp: February 18, 2019, 12:24:57 pm
content: None of the voltage regulators work properly above about 20V if you want 5V out. If you need to drop more, add a preregulator.I found this out the hard way as well dropping 28V to 5V DC for logic in an RF PA driver. Currently waiting for a board spin. I only needed 50mA and didn't want RF noise so I went with a simple MOSFET/zener/resistor preregulator to drop it down to 18Vish. Because the current requirements are relatively low I don't need to heatsink either then as the load is spread evenly across both MOSFET and 7805.Edit: also if you value the circuit it's driving, stick a 5W 5.6V zener across the regulator output. That will smoke the prereg / regulator not the load!

author: wraper
author_rank: Unknown
timestamp: February 18, 2019, 12:39:06 pm
content: Quote from: bogdant on February 18, 2019, 12:05:21 pmIf it's from ebay/ali, 95+% of them are counterfeit. Post a picture.

author: bogdant
author_rank: Regular Contributor
timestamp: February 18, 2019, 01:20:16 pm
content: Quote from: ebastler on February 18, 2019, 12:15:01 pmThey are from tme.eu. I have no idee if there are contrafit.

author: ebastler
author_rank: Super Contributor
timestamp: February 18, 2019, 01:34:14 pm
content: TME is a „proper“ distributor; so the regulators should be genuine. I think the tendency on this forum to conclude „counterfeit!“ is a bit too pronounced...

author: bd139
author_rank: Super Contributor
timestamp: February 18, 2019, 02:18:47 pm
content: Yes. Even the counterfeit ones tend to work properly as well.It's that huge dropout voltage I reckon. All the designs are marginal above 20V. Says so on every vendors' datasheets, including Linear who product exceptional stuff.

author: tsman
author_rank: Frequent Contributor
timestamp: February 18, 2019, 02:44:56 pm
content: Quote from: ebastler on February 18, 2019, 11:39:02 amThe low price nearly guarantees that it'll have a counterfeit LM2596 chip. It has been tested by multiple people online so just search for "LM2596 counterfeit". If you put a scope on the output then you'll see the switching frequency is far lower than the 150KHz for a real LM2596.The modules do work but you shouldn't draw anything approaching the advertised maximum current and you must provide good cooling. Whatever chip they're actually using doesn't have thermal overload protection and will fail with a input to output short.

author: mariush
author_rank: Super Contributor
timestamp: February 18, 2019, 03:04:49 pm
content: Some datasheets don't say a minimum load is required, but you basically satisfy that condition through the current used to set the output voltage.In case of an adjustable regulator like LM317, datasheets recommend a 120 ohm or 240 ohm resistor and a higher value resistor to set the output voltage ... using the simplified formula Vout = Reference Voltage (1.25v typ.) x ( 1 + R2/R1) where R1 is usually 120 ohm or 240 ohm.So you get a few mA of load just by setting the output voltage.Often I see people just connecting a red led with a 1-2k resistor in series on the output, so you basically get 3-5mA of current consumed by the led which also functions as a convenient "power on" indicator.If you know the average current consumed by your circuit, you could put a resistor before the regulator to drop some voltage ... for example, if your circuit will use maximum 100mA, then use V = I x R ... so if you use a 150 ohm resistor, you have a voltage drop of V = 0.1A x 150 = 15v and the power dissipated on the resistor is P = IxIxR = 0.1x0.1x150 = 1.5w , so you'll probably have to use a 3w or 5w resistor.

author: Jwillis
author_rank: Super Contributor
timestamp: February 19, 2019, 05:21:18 am
content: Sometimes data sheets can be a little unclear .This is the one I use and I have never destroyed a 78xx/79xx with this information.It clearly states the maximum input voltage for each rated regulator on page 3 .https://www.sparkfun.com/datasheets/Components/LM7805.pdf. I find the Texas Instrument Data sheets to be the clearest to understand.

author: bogdant
author_rank: Regular Contributor
timestamp: February 19, 2019, 09:20:08 am
content: Do you think, replacing the LM7805 with a LM317 would be an option? I am thinking 3k//1k on the adjust pin and a load of 1kohm rezistor, 1uF to input and 0.1uF to output capacitor for stabilization.The Vin=35V and the target is to have Vout=5V. As consummers I have a micro + LCD, should be no more then 50mA.

author: bd139
author_rank: Super Contributor
timestamp: February 19, 2019, 09:31:01 am
content: LM317 will do the job. Buy a TI branded one, make sure it has a 10mA minimum load current either from the load or the voltage setting network and make sure it is adequately heatsinked. It'll still kick out over a watt there. A small clip on will probably be enough heat sink for that.

author: mariush
author_rank: Super Contributor
timestamp: February 19, 2019, 11:10:19 am
content: You should also look intoAP7381orLM2931.LM2931 is available on* Farnell :https://ro.farnell.com/search/prl/results?st=lm2931&sort=P_PRICE* RS-Components:https://ro.rsdelivers.com/productlist/search?query=lm2931&tag=&family=9492#PI-41|PS-99|viewtype-2|sortBy-Price|sortOrder-ascAP7381 is available on* RS-Components :https://ro.rsdelivers.com/productlist/search?query=ap7381&tag=&family=9492#PI-1|PS-21|viewtype-2|sortBy-Price|sortOrder-ascNote that the cheapest on RS-Components require buying certain quantities.Digikey has a larger selection but being in US would be more expensive to ship :https://www.digikey.com/products/en?keywords=ap7381

author: bd139
author_rank: Super Contributor
timestamp: February 19, 2019, 11:24:12 am
content: LM2931 discounted because max input voltage is too low (26V)AP7381 outside SOA ... user needs 1.6W dissipation.High dropout voltages are a special problem space.

author: Ian.M
author_rank: Super Contributor
timestamp: February 19, 2019, 11:37:30 am
content: A LM317 *NEEDS* that 10mA minimum load current to guarantee regulation. If the real load current can drop to zero, as the regulator acts to maintain Out 1.25V above Adj, the upper resistor of its potential divider should always be under 125 ohms to guarantee 10mA minimum load current for the regulator. Use 120R and calculate the lower resistor for the desired voltage (96 ohms per volt at Adj).References to a 5mA minimum load current and a 240 ohm upper resistor only apply to the LM117 which is on the same National Semiconductor datasheet as the LM317.If there's any chance of spikes or ringing on the 35V supply, get a LM317HV which can withstand up to 60V input to output differental.

author: StillTrying
author_rank: Super Contributor
timestamp: February 19, 2019, 11:39:57 am
content: Quote from: bogdant on February 19, 2019, 09:20:08 amMax. 50mA @ 30V is 1.5W, you couldjust aboutget away with a 15V 2W zenner in series with the 35V, to share the max. dissipation to about 750mW each, in practice a little heat sinking for the reg.Quote from: Jwillis on February 19, 2019, 05:21:18 amIt's clear they go to great lengths not to mention what happens below 5mA or above 25V.

author: bd139
author_rank: Super Contributor
timestamp: February 19, 2019, 11:53:06 am
content: Quote from: StillTrying on February 19, 2019, 11:39:57 amThat's always something my first EE mentor said to me. Look out for what the datasheet doesn't say!One reason LT got a good reputation is they were honest where stuff didn't work.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 19, 2019, 12:41:32 pm
content: Aren't some brands of 7805 rated for Vin 40V? I up'd the 7805's heatsink on my ADS200 (fighting back 33v at ~200ma)AFAIK, it was running close to 80c (20c below its thermal protect) with the standard heat sink shown on the right..

author: ebastler
author_rank: Super Contributor
timestamp: February 19, 2019, 12:53:57 pm
content: Quote from: Cliff Matthews on February 19, 2019, 12:41:32 pmOnly if you interpret the term "7805" broadly, to my knowledge.Murata has (had?) switching regulators with 78xx pinout which can work up to 40V input voltage, with reduced current capability:https://power.murata.com/data/power/dms-78xxsr.pdf

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2019, 01:26:40 pm
content: The LM78xx or LM317 can be cascoded with an external transistor, to work at higher voltages. At higher currents, Tr1 should be a Darlington pair and R1 can be higher.

author: Jwillis
author_rank: Super Contributor
timestamp: February 19, 2019, 06:28:54 pm
content: Regardless whether you use a 78xx or a lm317 you still need to remember that Power in Watts = (Volts in - Volts out) X I . The LM317 won't run any cooler than a 78XX so adequate heat sinking is still required.

author: Km
author_rank: Newbie
timestamp: February 21, 2019, 10:55:13 am
content: you use a 100 oham resistor half watt input side of 7805 its drops small voltages its cheap solutioni use it many time

author: jackthomson41
author_rank: Regular Contributor
timestamp: February 21, 2019, 12:56:22 pm
content: Yeah its better to use heat sink with LM series as they get hot and burnt out, happened to me a lot. Moreover, use7805instead of LM317 as there's no need to design the small basic circuit but it will give fixed voltage of 5V.LM317gives variable output.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 21, 2019, 05:34:47 pm
content: Note that it's difficult to completely eliminate input voltage peaks and ringing during powerup, or sudden load changes. This ringing originates from cable stray inductance oscillating with the input capacitance of your circuit, and can go up to 2x the supply, i.e., 70V in this case! This happens easily if you have a big ceramic capacitor on the input.With electrolytic (high ESR) bulk capacitance in place, this peaking is easily damped, but even then, when running any part at the "absolute maximum ratings", it means that even a very slight input voltage peak may permanently damage the part the first time you apply power; or at any point in time later. You have zero margin for such inrush voltage peak!

author: bogdant
author_rank: Regular Contributor
timestamp: February 22, 2019, 10:39:16 am
content: Quote from: Siwastaja on February 21, 2019, 05:34:47 pmI think it is clear I need to do same redesigne. It is clear the LM7805 will not work at 35V inputSent from my VTR-L29 using Tapatalk