LM7805 climbs above 5 volts why - Page 1
Forum section: electronics / beginners

author: ascot
author_rank: Contributor
timestamp: July 21, 2020, 03:58:36 pm
content: just tried the 7805 for first time knowing it's a 5 volt regulator up to 20 volts it stays stable about 5.08 volts but any higher the voltage climbs data sheet says 25 volts max any ideas on why its climbs above 5 volts with input over 20 volts

author: KMoffett
author_rank: Regular Contributor
timestamp: July 21, 2020, 04:04:55 pm
content: Where did you buy it? May be an out of spec regulator that was dumped by the manufacturer and picked up by someone that acquires junk and sells it as new.

author: tooki
author_rank: Super Contributor
timestamp: July 21, 2020, 04:06:32 pm
content: Quote from: ascot on July 21, 2020, 03:58:36 pmHow much higher are we talking? I looked at a few datasheets and they specify an output voltage of 4.75-5.25V.

author: ascot
author_rank: Contributor
timestamp: July 21, 2020, 04:09:38 pm
content: 25 volts takes it to about 7 volts

author: TimFox
author_rank: Super Contributor
timestamp: July 21, 2020, 04:11:06 pm
content: Exactly which model of "7805" are you using, including prefixes, suffixes, manufacturer, and case size?

author: dietert1
author_rank: Super Contributor
timestamp: July 21, 2020, 04:15:24 pm
content: Yes, running a LM7805 with more than 20 V input voltage is bad design.Also you may need 10 uF capacitors close (< 5 cm) to the regulator in order to avoid oscillation. Put one on input and one on output.Regards, Dieter

author: wraper
author_rank: Unknown
timestamp: July 21, 2020, 04:32:51 pm
content: Likely it's simply a counterfeit. Unless datasheet of particular manufacturer shows lower maximum voltage.

author: ascot
author_rank: Contributor
timestamp: July 21, 2020, 04:33:42 pm
content: love to tell you all the package type but microscope went on the blink as a side note tried 1 from 2 different suppliers both read 6.68 volts at 25 volts input

author: wraper
author_rank: Unknown
timestamp: July 21, 2020, 04:35:36 pm
content: Quote from: dietert1 on July 21, 2020, 04:15:24 pm7805 is not an LDO and is fine with 0.1uf cap. Datasheet of 7805 from Ti even says it's stable without output cap.

author: TimFox
author_rank: Super Contributor
timestamp: July 21, 2020, 04:44:14 pm
content: The original data sheets for the 7805 required a minimuminputcapacitor of 330 nF (unless the device was extremely close to the filter capacitor), and I sometimes saw high-frequency oscillation (> 100 kHz) when that requirement was ignored. Perhaps this circuit is oscillating and was only measured with a voltmeter?

author: ascot
author_rank: Contributor
timestamp: July 21, 2020, 04:47:54 pm
content: there is 2 10uf capacitors on input , output could be its only rated 20 volts was from amazon after all

author: MarkF
author_rank: Super Contributor
timestamp: July 21, 2020, 04:49:56 pm
content: You also need to provide a 5mA minimum load current.

author: TimFox
author_rank: Super Contributor
timestamp: July 21, 2020, 04:53:56 pm
content: Yes, the output voltage is only specified for Iout > 5 mA.

author: ascot
author_rank: Contributor
timestamp: July 21, 2020, 05:01:15 pm
content: thanks to all who replied especially the load reply was doing it without a load put an led in now registers 5.08 volts at 25 volts side, note go above rated amps and itsmokesnice to know can find answers to questions

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 21, 2020, 05:08:17 pm
content: Have you assumed anything about the connectivity tab/heatsink? Might be worth checking... i've certainly got that totally wrong in the past!

author: wraper
author_rank: Unknown
timestamp: July 21, 2020, 05:09:19 pm
content: Quote from: MarkF on July 21, 2020, 04:49:56 pmThat is not true at all. For keeping output voltage you need the load only for adjustable regulators like LM317 and sort of. 7805 bleeds output current internally through quite low resistance internal feedback resistive divider. Datasheet may specify output voltage at certain current range but it does not mean that below this current output voltage will raise to any significant extent.

author: shaunakde
author_rank: Contributor
timestamp: July 21, 2020, 05:18:18 pm
content: I was about to ask about the load. I had something similar happen with a negative 12-volt regulator: LM7912. It behaved pretty badly before I added a small load. You can see this happen on video to SDG Electronics around the 18 min mark.See:https://youtu.be/ML-lmuHoh-0?t=1107Since watching this video, I have just always added LEDs on the output.

author: wraper
author_rank: Unknown
timestamp: July 21, 2020, 05:27:08 pm
content: I just grabbed LM7805 from National semi and it kept 5V output all the way up to 60V input voltage my PSU could supply. No input/output capacitors and load attached.

author: wraper
author_rank: Unknown
timestamp: July 21, 2020, 05:32:05 pm
content: Quote from: shaunakde on July 21, 2020, 05:18:18 pmMost likely it was a counterfeit trash as well.

author: bdunham7
author_rank: Super Contributor
timestamp: July 21, 2020, 08:00:27 pm
content: Quote from: ascot on July 21, 2020, 04:09:38 pmCan you measure the input supply current? I just tried this with an ST L7805CV part from a scrapped HP power supply (so I'm pretty sure it isn't fake) and it maintained 5.03V +/- 10mV from 6.2 to 25V input, but it had a no-load current draw of about 5-7mA.

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2020, 10:20:30 pm
content: You could add a resistor in series with the LM7805.

author: dietert1
author_rank: Super Contributor
timestamp: July 22, 2020, 05:26:44 am
content: Quote from: wraper on July 21, 2020, 05:27:08 pmIf someone designs a circuit based on tests like that or on datasheet limits, there will be a bad surprise later on. I can walk on one foot for a while, but the conclusion to buy one shoe only is certainly bad. The reason for not using a LM7805 with more than 20 V input is heat.Only recently i fixed a Keithley 213 quad DAC (a rebranded IOTECH DAC488) that has four 5 V regulators running from 31 V input, all without heat sinks, at 55 mA each. That can't work. One of the regulators was burnt, although in theory they have built-in over-temperature protection. All capacitors were dry from heat, so the other three regulators were oscillating. What a mess.Regards, Dieter

author: james_s
author_rank: Super Contributor
timestamp: July 22, 2020, 06:19:26 am
content: What on earth were they thinking feeding the regulators with 31V?

author: Zero999
author_rank: Super Contributor
timestamp: July 22, 2020, 07:32:52 am
content: Quote from: dietert1 on July 22, 2020, 05:26:44 amI don't think he's advocating using the LM7805 at 60V, just that based on his test, a high input voltage is unlikely to be the cause of a higher than normal output voltage.The maxium recommended input voltage for the LM7805 is 25V, so the Keithley is using it out of specification. The power dissipation issue can be fixed with a series resistor.https://www.sparkfun.com/datasheets/Components/LM7805.pdf

author: tooki
author_rank: Super Contributor
timestamp: July 22, 2020, 08:59:40 am
content: Quote from: Zero999 on July 22, 2020, 07:32:52 amNo, it’s not out of spec.Somedatasheets recommend 25V, but many do not, leaving only the maximum input voltage of 35V or 36V, which 31V is comfortably below. What the designers at IOTECH clearly didn’t look at were the graphs explaining how much current one can draw at a given voltage differential...

author: dietert1
author_rank: Super Contributor
timestamp: July 22, 2020, 09:28:54 am
content: Quote from: james_s on July 22, 2020, 06:19:26 amThey wanted +/- 15 V for the analog circuitry. So they put 2x 18 Vac PCB transformers to leave some headroom for rectifiers, voltage regulators and mains voltage variation. With light load the transformers output 2x 22 V. Later mains voltage increased from 220 to 230 V and there you are. Since there was no space to have another set of transformers, they just took the same 31 Vdc for the 5V regulators for logic (DACs, shift registers and opto isolators). Instead of LM7805 they used LM340T-5.0 with a datasheet upper limit of 35 V. Now consider another +20 % mains voltage margin.I solved the heat problem by inserting those very small Polulu switchers as down-converters to 8.5 V. The instrument also got a simple mains pre-regulator that dumps most of the excess power into two mosfets mounted on the enclosure frame, and instead of 31 V there is a constant +/- 22 V for the +/- 15 V regulators.I also added schottky diodes over all +/- 15 V and +5 V supplies. Without such diodes JFET opamps may get killed after turning off power.Something similar happened, when i looked into the HP3456A in-guard power supply. Building a good power supply with integrated voltage regulators may appear simple, but it needs some care.Regards, Dieter

author: exe
author_rank: Unknown
timestamp: July 22, 2020, 09:58:46 am
content: Quote from: dietert1 on July 22, 2020, 09:28:54 amI've never seen this idea before, do you have a schematic for this?

author: Zero999
author_rank: Super Contributor
timestamp: July 22, 2020, 10:07:33 am
content: Quote from: tooki on July 22, 2020, 08:59:40 amAssuming you're right, I've no desire to spend the time looking at all of the data sheets, a voltage drop of 26V, will only dissipate 1.43W, at a current of only 55mA, which should be fine for the TO-220 package, even without a heatsink.

author: magic
author_rank: Super Contributor
timestamp: July 22, 2020, 11:12:29 am
content: Quote from: wraper on July 21, 2020, 05:09:19 pmBoth output voltage and load regulation were only specified for Io≥5mA by National. The 78L05 was specified for Io≥1mA.This implies precisely nothing about operation with zero load. The parts may work nevertheless, or they may work in 99.9% of cases, or they may work below 50°C, or whatever.

author: wraper
author_rank: Unknown
timestamp: July 22, 2020, 03:53:12 pm
content: Quote from: dietert1 on July 22, 2020, 09:28:54 amLM7805 and LM340T-5.0 have exactly the same ratings.

author: dietert1
author_rank: Super Contributor
timestamp: July 22, 2020, 06:08:49 pm
content: Quote from: exe on July 22, 2020, 09:58:46 amSee appended sketch. Of course you need to add some protective parts, like zeners for the gate voltage and an over-voltage protection on the input. But it is simple and works well. For the HP 3456A i made something better: A regulator for the transformer magnetic flux that separates in-guard from out-guard consumption.Regards, Dieter

author: SteveyG
author_rank: Unknown
timestamp: July 29, 2020, 09:43:56 pm
content: Quote from: wraper on July 21, 2020, 05:32:05 pmShouldn't have been counterfeit - It was from Farnell which is one of their specified distributors. I've since had confirmed during a sales visit from some of the UK reps that indeed they may require minimum loading for regulation anyway.

author: tkamiya
author_rank: Super Contributor
timestamp: September 19, 2020, 11:20:28 pm
content: There is no longer THE 7805.... You can buy all sorts of LM7805 from Amazon or eBay and every one of them behave differently. Some has max input of 20V, some 25V, and so on. I don't trust them to agree with datasheet either. They may not be a "counterfeit" but definitely not a first rate manufacturing. Besides.... one should not design circuit so close to the max limit anyway. Heat generated is so much that energy waist and heatsinking requirement will cost you more than what you save for a cheap component.By the way.... on similar applications, I've used DC/DC converter module purchased from Amazon. Applied 12V in, 5V out. Put a ferrite bead on output. OK... less spikes. Put two... not much difference. Put THREE, it went into full swing oscillations. 37V output at max current. Fried my components crisp! Be careful with these....

author: exe
author_rank: Unknown
timestamp: September 20, 2020, 08:22:37 am
content: Quote from: tkamiya on September 19, 2020, 11:20:28 pmNot sure how much this article is relevant, but I'll leave it here. It's about designing second-order lc-filters for power supplies. It explains some stability issues:http://ridleyengineering.com/images/phocadownload/1%20second%20stage%20filter%20design.pdf

author: dietert1
author_rank: Super Contributor
timestamp: September 20, 2020, 10:09:57 am
content: For DC/DC down converter i can recommend Pololu modules. Never had any difficulties with them, on the contrary, they helped to solve several problems. For example i was able to pull the switcher from inside an Arroyo precision temperature controller and replace it with a Pololu 9V module that is about 1x1" and rated up to 7 A with proper input. I am using it as a secondary switcher with less RF interference on a nearby 24-bit ADC and supply some 18 to 20 V from a nice linear power supply (external).Regards, Dieter