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Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44
Hey all, I'm in the process of building a 3 stage coil gun with optical triggering. I've built a circuit consisting of several 555 timers, CD4049, CD4081, and CD4043 logic to control the signals received from the opto-couplers and interface with the SCR's
There's adjustable delay built into the circuit so I can fine tune when the trigger is sent to each SCR after the projectile has triggered the optocoupler. I've also added a bit of hysteresis which requires the opto-coupler to be triggered for an adjustable period of time before the circuit actually takes that value to be high. This is so dust particles or other crap doesn't accidentally trigger the opto-couplers and fire the SCR's when they're not supposed to be firing.
I also have logic lines coming into the circuit from other subcircuits in the project that act as enables and resets.
I've built the circuit and tested it and it works exactly as expected - which is good. However, the 555 timers get quite warm. They don't blow (I've left the circuit running for 30+ minutes straight while testing it), but they do get fairly warm to the touch. I can't figure out why.
The outputs of all the timers are fed directly through 4049 NOT gates. They are not switching any power transistors or anything else. The CD4049 chip does not get warm. The 555 timers are acting as single shot delay timers so most of the time their output is steady state high. Is this the problem? Do I need to add pull up resistors on the outputs of the timers?
Here's a slightly outdated Altium screenshot showing one part of the circuit. You can see the timers with their outputs connected directly and only to the 4049 NOT gates. The 2MOhm resistors have been replaced by POTs. Also I'm running at 9V VCC instead of 5V, because I found I get better results with the optocouplers that way. (This is still within the VCC max requirements of the 555 timers - and they still got hot at 5V VCC.)
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
maybe it's the energy from the 10 uF timing capacitors being repeatedly dumped into the 555s via pin7? try a resistor from pin7 to the timing capacitor, 1k for a start.
EDIT: on reflection, the energy x frequency seems too low for serious heating, but you should always limit pin7 discharge current to protect the internal discharge transistor.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
Well, it's not the VCC level. Before I look at the circuit more carefully, is the circuit shown the exact circuit you have wired--AND CHECKED--TWICE. Try placing the pots in about the center position to eliminate shorts (I don't think I saw any series pot resistors. Also, double check that pin 3 on each 555 is feeding an -input- or better, disconnect pin three of each 555, leaving everything else as it is. This will unload them and if there is a wiring error they should run cool. If none of this helps, I will trace the circuit and look harder for you.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
**MODS, PLEASE FORGIVE ME FOR THE DOUBLE POST** but I forgot something vital and don't want AARPCARD to blow the 4049 gate before reading the following addendum:
When you pull pin 3 of the 555 be sure NOT to leave the gate inputs -open- that are driven by the pulled 555 (pin 3)--pull these inputs up or down before disconnecting the 555 driving pin and applying power!!
*I looked at the pulse timing and your astable output pulse is rather long for a 555 timer 22 seconds--but I see no problem, unless... *It is realized that once triggered this 22s high pulse (at 555 pin 3) REMAINS high until this long period has timed out--even if triggered again --during-- the interval. *If you have a scope, then on pins 6 and 7 you should see, when the low-going trigger pulse is applied to 555 pin 2, a slowly exponentially rising voltage (over the 22s interval). ON SECOND THOUGHT: we are talking about a 22s interval, you can just use a voltmeter and look for a voltage rise from 1/3 Vcc to 2/3 Vcc. Time it--if it is about 22s the timers are working. This isolates the 555 circuit in a standard configuration and is almost BOUND to work! * Your connections as shown should cause no malfunction, unless perhaps you are inadvertently driving an output somewhere. *Also, you can just yank the small caps from pin 5 and see what happens--they are just a mild precaution and would actually not be needed for such a long pulse aplication. *finally you can try coupling the trigger input to the 555 (pin 2) through a 0.001uF ceramic capacitor with pin 2 pulled up to Vcc (at the chip) by a 10k resistor. ...hope this helps
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
You complain about a warmed up LM555. "quite warm" is not really an accurate measure, but i expect thats not bunring hot, but noticably hotter than the PCB around it if you touch it with your finger. That means its maybe just 5-10 Kelvin above room temp which would be near body temp then... and i guess you feel "quite warm"
So... you complain that its getting hotter? How much do you actually expect it to warm up ?
At 9V and a specified current consumption 6mA is to be expected. At 106K/W this would raise your chip temp by 6K. Voila, allready explained what you see. Basic math. Of course this could be wrong.. but given what you give us......... The Datasheet also states that you should connect a PullUP-Resistor to ais the output stage. A normally high output should be supportet with a pull up while a normally low output shoult be supported by a pull down. Maybe the lack of this causes more current consumption in the output stage.
Given the internal circuit of the 555 and your ultra low Cap-Charge rate, you might run into some extra current consumption or even slight oscillation with your tinty 10pF cap as Signification said.
Just measure current consumption, calculate the power, caluclate the expected temp rise and come back when this figure is out by more than 50%.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Extemely good thinking! If heat transport is further reduced then the the 106k/W do not apply. then its easily explained why the temperature rises >10 K
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
Sulaiman, Great! I never thought about current-limiting that hefty internal discharge transistor. If I understand correctly wouldn't you have to include this resistor in the pulse-width calculation. How, exactly, would you place it? I would like to derive the formula for the modified pulse width.
This made me think -more- of something that has been bothering me: those pots connected DIRECTLY from pin 7 to Vcc. If no resistance is there, you have a dead short from Vcc through that transistor to ground--that would let the smoke out. So, in my opinion, there should always be a safe minimum resistance from Vcc to pin 7...especially if a pot is used. It WILL get turned to zero ohms!
And of course we are talking about the standard 555 astable mode that ~always works. As I said, isolate it (pin 3 unconnected) AND provide a simple HI-LO-HI trigger pulse. It is true that this pulse must be shorter than the output pulse--but that is 22s! AND he has more than one 555 to play with! I do wonder why such a long pulse is needed in a sequential CG triggering circuit--it has even occurred to me that the long pulse may not be intentional. Another possibility is they are BOTH burned out due to the pot, and it is just assumed that BOTH 555's won't malfunction similarly, thus each needs to be replaced. Anyway...great to wake the old 555 sights.
Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44
Thanks for all the advice/ideas. I tried putting 10k pull ups on the outputs - this didn't resolve the issue. The chips are all socketed and soldered into perfboard. Changing chips is easy, but adding components/changing connections is hard due to the wiring density on the board - I have this board really tightly laid out due the space I need it to fit in for the gun's housing. I'll see if I can disconnect pin 3 while pulling the inputs to the inverters high and see what happens.
DerAlbi wrote ...
You complain about a warmed up LM555. "quite warm" is not really an accurate measure, but i expect thats not bunring hot, but noticably hotter than the PCB around it if you touch it with your finger. That means its maybe just 5-10 Kelvin above room temp which would be near body temp then... and i guess you feel "quite warm"
So... you complain that its getting hotter? How much do you actually expect it to warm up ?
At 9V and a specified current consumption 6mA is to be expected. At 106K/W this would raise your chip temp by 6K. Voila, allready explained what you see. Basic math. Of course this could be wrong.. but given what you give us......... The Datasheet also states that you should connect a PullUP-Resistor to ais the output stage. A normally high output should be supportet with a pull up while a normally low output shoult be supported by a pull down. Maybe the lack of this causes more current consumption in the output stage.
Given the internal circuit of the 555 and your ultra low Cap-Charge rate, you might run into some extra current consumption or even slight oscillation with your tinty 10pF cap as Signification said.
Just measure current consumption, calculate the power, caluclate the expected temp rise and come back when this figure is out by more than 50%.
This was going on in the back of my head - since I couldn't figure out what was wrong with the circuit and it worked as expected, I was wondering if some heat is normal especially since the chips are normally high. I was concerned though because I've never had a 555 get warm in the past unless it was fatally wired. They get noticeably warm - but not burning. I had the chips running for 30 minutes with no issues and temps seemed to stay stable, but warm - I don't have a proper thermometer so it's all by touch.
Like I said, the schematic is not up to date, but the important parts are there for at least looking into this issue. In the built circuit I have an additional two 555 timers with identical wiring going into the last two inverters on the CD4049 - so that's 4 out of 6 inverters on the CD4049 that are sinking current from normally high 555 timers. The CD4049 is cool as you would normally expect when dealing with logic and the outputs of the 555's definitely aren't driving anything else - it's exactly as shown in the schematic. If the problem was due to the 555's sourcing too much current, I would have expected the CD4049 to get considerably warm as well as it would be sinking all of the current that was causing the 4 555's to get warm, but the CD4049 is nice and cool.
. . . So that makes me think the problem is not on the output side of the 555's and troubleshooting that I'm at a loss which is why I came here.
I just noticed I don't have pull ups on pin 4 (Reset) - I know that's best practice to include them, but I've never had this issue before with just putting pin 4 on VCC. What do you think?
I assumed that since they were driving logic there's no reason for them to get warm and I want to avoid a slow burnout situation.
I'll try as many of the things you guys suggested and let you know what happens.
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555 Timer Gets Hot - Coil Gun Related
