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555 Timer Circuit

crutschow

Well-Known Member
Most Helpful Member
the source you are using in the lower right hand corner with the label reset
That simulates your short reset signal.
What is its amplitude?
 

crutschow

Well-Known Member
Most Helpful Member
if the temporary shorting of the timing cap results in relay drop-out
Why do you think that would occur?
The 555 output will stay high until the capacitor voltage charges to the THRES level, which the Reset signal will delay by discharging the cap to 0V.
 
Last edited:

hhaibert7

New Member
Why do you think that would occur?
The 555 output will stay high until the capacitor voltage charges to the THRES level, which the Reset signal will delay by discharging the cap to 0V.
I think what alec_t was referring to was the issue I told him about me using an OpAmp as a buffer since the output of the timer goes to the SSR but the SSR was loading down the timer and I was having an impedance mismatch. block diagram of the circuit is shown below
Capture.PNG
 

alec_t

Well-Known Member
Most Helpful Member
Why do you think that would occur?
It shouldn't. I noticed a glitch when simulating, but it may have been an artifact (or me mis-reading something :)).
 

crutschow

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Most Helpful Member
the signal is a 60kHz, 100mVp-p signal. However, it gets converted to a 0.6V (typ.) when freq. is within band.
That is new information to me.
Then you would need to amplifier and rectify the signal to give the 5V pulse signal required for the reset.
An op amp could readily perform that task.
What is the output impedance of the 60kHz source?
What is the duration of the signal?
 

hhaibert7

New Member
That simulates your short reset signal.
What is its amplitude?
The schematic you provided me with works!! I am able to reset the time on the timer while keeping the timer on!
I have 1 issues, however; I am not sure of how to resolve this:
When the timer turns on, the timing capacitor starts charging. If it detects the signal it immediately discharges to 0 and then starts charging again. When it charges up to the threshold voltage the timer turns off BUT the timing capacitor discharges very slowly instead of immediately discharging to zero. This ends up effecting the actual time (i.e. 2 minutes) the timer is on for when the timer is turned back on after turning off. In other words, if the timer turns off and I turn it back on right away, the timing capacitor has not fully discharged yet; therefore, now that it is on again, instead of 2 minutes staying on its on for less (i.e. 1 min and 30 seconds). How can I make sure after the timer turns off, the timing cap discharges immediately similar to when it detects the signal which results in resetting the time?
 

hhaibert7

New Member
That is new information to me.
Then you would need to amplifier and rectify the signal to give the 5V pulse signal required for the reset.
An op amp could readily perform that task.
What is the output impedance of the 60kHz source?
What is the duration of the signal?
So I found a test point on the device that does output 5 volts when the signal is detected. Hence the success in the circuit. The only issue I am facing now is the one mentioned above.

Thank you
 

crutschow

Well-Known Member
Most Helpful Member
the timing capacitor discharges very slowly instead of immediately discharging to zero
Don't understand that.
The DIS pin should immediately discharge the capacitor to ground when the pulse naturally ends.
Do you have the DIS pin connected directly to the capacitor?
It would seem you don't.

What turns the timer power off?
 

hhaibert7

New Member
Don't understand that.
The DIS pin should immediately discharge the capacitor to ground when the pulse naturally ends.
Do you have the DIS pin connected directly to the capacitor?
It would seem you don't.

What turns the timer power off?
The DISC is directly connected to the capacitor. Diagram shows how I have connected the circuit. It does seem rather odd to me too that it won't discharge immediately. When I look at the voltage across the capacitor with a multimeter I can see its a rather slow discharge not immediately to zero; whereas, when the transistor momentarily shorts the timing cap, the cap immediately discharges to zero.

What turns the timer power off?
I'd assume once the timing cap reaches 2/3Vcc the internal flip flop resets and turns on the internal transistor which allows timing cap to discharge. I do not have any other way of turning the timer power off.
 

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crutschow

Well-Known Member
Most Helpful Member
If the DISC connection doesn't discharge the cap at the end of the pulse then you may have a faulty 555.
Has the DISC pin every been accidentally connected directly to the V+ (5V) voltage?
 

alec_t

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danadak

Active Member
Item 6 seems to imply we can connect a V source to discharge pin and no damage will occur. I have my doubts.
The rated output V at 200 ma is 2.5 typical implying a sat R of ~ 12 ohms. Further internal bussing of power rails
limitations in this type of part I would look into as well as no mention of that in datasheet.

1637415755228.png


So in short putting a large V source (bulk cap) directly onto pin, eg. a large cap, I would have some concern.
maybe a small series R would add some prudent margin to the application.

Just a thought.


Regards, Dana.
 

danadak

Active Member
Ground >> Cap >> Resistor >> Discharge Pin, all in series, so that R limits max current when discharge transistor
turns on to remove charge from cap.

Regards, Dana.
 

crutschow

Well-Known Member
Most Helpful Member
He means a small value resistor (e.g. 100Ω) in series with the timing capacitor
 

danadak

Active Member
A similar precaution here (general CMOS parts) -


The 555 die looks something like this -

1024px-ST_e3_CHN_TS555CN_KM9603_J_36.jpg



The large structure lower left is the discharge transistor. The spec says typ 2.5 volts @ .2A for the output.
Thats 1/2W and all that power is concentrated in that area of the die. Thats "typical" spec. Think dissipating 1/2W
in a 1/4 resistor, hot ! Compound that with a die attach that is not 100% (read move heat into leadframe)
area coverage and you have a recipe for silicon hotspot = melting. Note above die is CMOS version.

Then think of the ground buss, looks hefty, but takes a low power microscope to look at it. Dump all that current
into ground buss and you get noise margin compromised. Just some thoughts.

Regards, Dana.
 
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hhaibert7

New Member
If the DISC connection doesn't discharge the cap at the end of the pulse then you may have a faulty 555.
Has the DISC pin every been accidentally connected directly to the V+ (5V) voltage?
So I figured one thing out regarding this problem however I am not sure if adding a small resistor in series with the timing cap is the solution or not.
When I set up the timer circuit on its own and turn it on with a +5V source, after timer turning off the timing cap discharges instantaneously. However, when I integrate the timer with the rest of the circuit, the device turns on with a +9V then the timer gets its power from a test point on the device which is a +5V and then the timer's output connects to a Solid State relay, that is when the timing cap does not discharge instantaneously. In other words, when the timer is on its own the timing cap discharges fast, when the timer is integrated with the whole system that is when the discharge slows down. I will try to put a small resistor in series with the timing cap to see if this could help or not. I also took your advice on the possibility of the faulty 555 and changed the chip but clearly that was not the issue since when the timer is set up on its own I do not face any issues as far as discharge is concerned.
 

crutschow

Well-Known Member
Most Helpful Member
when I integrate the timer with the rest of the circuit, the device turns on with a +9V then the timer gets its power from a test point on the device which is a +5V and then the timer's output connects to a Solid State relay, that is when the timing cap does not discharge instantaneously. I
How are the 9V and the 5V connected to the 555?
 

hhaibert7

New Member
for the sake of testing I am using a DC Power Supply but in the practical world it would be a 9V battery supplying power to the device
 

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