Single shot 555 timer getting hot!!  - Page 1
Forum section: electronics / beginners

author: Unordung
author_rank: Contributor
timestamp: July 28, 2018, 03:51:35 am
content: Hello guys,I have built a single shot circuit which will drive a relay for a specific period once power is supplied to the circuit.Everything is working as expected until the system pulls the output low after the period.The current jumps from 40mA to 104mA and the 555 begins to get hot! Quite hot after a small period of time.Things I have done:I was using an LM7812 I changed this to a 7809 to reduce the voltage on the 555 vcc which helped but only a little.I placed a 1k resistor to reduce the current into the 555 this helped a ton and the 555 no longer gets hot.The Mosfet isn't the one in the schematic (I only have the student version of Multisim and it does not have the actual part) i'm using the IRF3205.My question is:Why is the 555 sinking current when the output is pulled low?What can I do (more professionally) to rectify it?***EditSo my reasoning behind putting the 1k resistor between the vcc rail and vcc was to limit the current through the 555. I'm thinking the 555 is shorting vcc to ground when the output is low. This did work but I'm not sure if it's,A) The "correct" thing to do.B) A long term solution.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 28, 2018, 04:43:23 am
content: Clearly there is something wrong somewhere. The R3 looks suspicious to me, but I don't think that's the cause of the problem.Are you using an actual LM555 or some other letter 555? (You probably aren't using a LM555CM, are you?)At any rate you should be able to run without the voltage regulator, directly from the 12 volt battery, if you've got the circuit right. It certainly should be drawing much less when the output is off.Where did you get the monostable multivibrator circuit you are using?

author: Unordung
author_rank: Contributor
timestamp: July 28, 2018, 05:40:32 am
content: Quote from: alsetalokin4017 on July 28, 2018, 04:43:23 amR3 is a pull down for the Mosfet gate, at the 12v vcc it would pull at maximum 1.2mA so I'm fairly confident that isn't the issue.Quote from: alsetalokin4017 on July 28, 2018, 04:43:23 amIm using the NE555p.Quote from: alsetalokin4017 on July 28, 2018, 04:43:23 amThis is for use in an automotive environment, i'm using the voltage regulator in conjunction with the 30v Zener as crude transient and noise protection. The NE555p has an absolute maximum VCC of 18v. Since automotive electrical systems can have transients in the 60v + range it's just a way to ensure system stability.Quote from: alsetalokin4017 on July 28, 2018, 04:43:23 amThis is the result of some help I received on this forum a while back.

author: sourcecharge
author_rank: Regular Contributor
timestamp: July 28, 2018, 06:08:03 am
content: Could you use the typical monostable circuit?https://www.allaboutcircuits.com/tools/555-timer-monostable-circuit/Edit:Never mind, I see the problem"The 555 Timer Applications Sourcebook with experiments"-Howard M. Berlinpg 20This has what you are looking for.

author: Unordung
author_rank: Contributor
timestamp: July 28, 2018, 06:32:44 am
content: You don't happen to know where one could find this book by any chance?Google has simply given me options to buy from Amazon or a bunch of sites which look like a sure fire way to catch an STD.

author: sourcecharge
author_rank: Regular Contributor
timestamp: July 28, 2018, 06:40:12 am
content: Quote from: Unordung on July 28, 2018, 06:32:44 amIDK, mine was handed down to me.The only difference that I could see is that your reset was on a RC network instead of tied directly to Vcc, and pin 5 did not have that cap.I will spice this on B2 and get back to you.BTW, that bread board looks kinda over complicated with connections that are doubling back that hide themselves very easily.I would also check your bread board.

author: sourcecharge
author_rank: Regular Contributor
timestamp: July 28, 2018, 06:57:00 am
content: Quote from: sourcecharge on July 28, 2018, 06:40:12 amB2 says it should work either way.It must be a bad connection or part.

author: Unordung
author_rank: Contributor
timestamp: July 28, 2018, 07:07:20 am
content: Thanking you very kindly for your time, I will go back over everything. The BB is very cheap and nasty from eBay and many of the connections seem "ok" but not reliable.I will build it up on protoboard and hopefully, that will solve it.Thanks again for your help!

author: sourcecharge
author_rank: Regular Contributor
timestamp: July 28, 2018, 07:12:34 am
content: Quote from: Unordung on July 28, 2018, 07:07:20 amI'm starting to possibly remember something about this circuit..The cap on pin 5 may actually be causing this..Just to make sure, take off the cap on pin 5 and connect reset to Vcc.

author: mikerj
author_rank: Super Contributor
timestamp: July 28, 2018, 11:07:54 am
content: It's quite possible the MOSFET is oscillating due to the all the stray inductance in your breadboard layout. You could try adding a resistor between the 555 output and the MOSFET gate, something of the order of 100 ohms or so would be fine as you aren't switching anything fast.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 28, 2018, 01:05:30 pm
content: Might be a couple of things. First, there appear to be many variations of the ‘555’ and although they should functionally work the same, the circuitry inside is different and can cause many different types of problems. A lot of 555 chips may have what is known as a totem pole output where there are 2 transistors, stacked one on top of the other, between the + and – supply with the output from their common junction. This configuration allows the 555 to either source or sink current. There is an extremely small moment of time when switching that both transistors could be on, producing a short and having a large current flowing through the transistors. The way around this is to wire a, say 22 ohm, bypassed resistor in series with the + for the 555 to limit the current to a safe value.The second problem is somewhat related to the first and relates to the NE555 you are using. Here is an answer to a question I found on line.“You've discovered one of the best kept secrets of the 555. That is, they aren't all the same. In fact, I know which one you have. It's the NE555 from Texas Instrument (silver top, right?), because that's what RadioShack stocks even though the catalog says LM555. The difference is that the LM555 is a bipolar chip and the NE555 is a CMOS chip. And I bet that pin 2 of this circuit is waving in the air — picking up static electricity. When the first CMOS 555s showed up on my workbench, I had the same problem until somebody told me they were CMOS, then the light went on. When working with CMOS technology, every pin has to go somewhere, used or not. What you need to do is tie pin 2 high through a 10k resistor, that's pretty close to its bipolar equivalent, and the circuit will work.”

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2018, 01:25:56 pm
content: Perhaps the 555 or MOSFET are bad?The circuit is more complex than it needs to be.How much current does the relay's coil need? Can it be powered directly off the 555 timer?The relay's coil will need a back-EMF suppression diode in reverse parallel with it.Try this circuit. The 555 can be powered from a voltage regulator, if required and the relay switched using a transistor, if the 555 can't source enough current.

author: StillTrying
author_rank: Super Contributor
timestamp: July 28, 2018, 03:32:46 pm
content: There's no decoupling very near the 555, so it could be oscillating due to the breadboard coupling of pins 2 and 3.It's only used as a comparator to detect when C3 reaches 2/3V so Reset and CV decoupling are not needed, use them caps to decouple the 555's supply pins instead.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: July 28, 2018, 03:51:35 pm
content: I think the problem is highlighted in the attached image. You've taken care to run the +12V for the lamp through a separate breadboard's power rail, but the ground return for the lamp follows the same rail as the 555. When the lamp draws current, the voltage drop on the ground rail would likely cause the 555 to misbehave. If instead the MOSFET's source is connected to the left breadboard's ground rail, this wouldn't be a problem. Also, the source node of the FET is being shared with the gate pull-down resistor - have that go straight to the ground rail.Also, consider moving the power input barrel connector to the bottom of the board (near the lamp) instead. Now the load current and the 555's supply share a much smaller path. As @sourcecharge already pointed out, the layout is way too complicated for the circuit you're trying to implement. Simplify the number of connections and it will likely work a lot better.

author: Unordung
author_rank: Contributor
timestamp: July 29, 2018, 01:06:51 am
content: Quote from: ArthurDent on July 28, 2018, 01:05:30 pmSadly this didn't solve my problem, it was, however, very interesting. I have tied the Discharge pin to vcc via a 10K just as good practice.Quote from: mikerj on July 28, 2018, 11:07:54 amSadly I thought of this and the same fault happened without the mosfet connected.Quote from: StillTrying on July 28, 2018, 03:32:46 pmI have now added a .1uf cap across the VCC input to the 555 still no dice.Quote from: radar_macgyver on July 28, 2018, 03:51:35 pmI gave this a shot, and no dice sadly. The advice, however, was very good and I will keep it in mind for future projects.

author: Unordung
author_rank: Contributor
timestamp: July 29, 2018, 01:13:21 am
content: Quote from: Hero999 on July 28, 2018, 01:25:56 pmI have tested the mosfet in a tester and everything seems within spec, I may have got a bad batch of 555's as they were an eBay purchase.....Quote from: Hero999 on July 28, 2018, 01:25:56 pmThe datasheet says 200mA, whilst it's highly likely the coil will draw less than this I want the added confidence of the mosfet.Quote from: Hero999 on July 28, 2018, 01:25:56 pmYeah It will have a flyback / catch diode once the relay is delivered.I will give the circuit you provided a go and see if it produces better results, thanks for your help.

author: sourcecharge
author_rank: Regular Contributor
timestamp: July 29, 2018, 04:28:53 am
content: I think your discharge pin 7 is connected somehow to Vcc pin 8.Pick the chip out, inspect the legs to make sure there is no damage, then check under that yellow wire that crosses where pin 7 and 8 were for a single row wire bridge hidden underneath it, and if there isn't one, check the resistance between the two rows on the bread board for a short to see if there is a problem with the bread board.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 29, 2018, 11:20:20 am
content: It's frightening to contemplate that anyone would actually fake a 555, but this chip sure looks suspicious to me.Maybe it's just the light, I dunno.

author: exe
author_rank: Unknown
timestamp: July 29, 2018, 09:22:35 pm
content: Quote from: alsetalokin4017 on July 29, 2018, 11:20:20 amActually, there are fake 555:https://zeptobars.com/en/read/Ti-555-NE555-real-vs-face-china-chinese. But may be the IC was abused and damaged, I'd try swapping it...