On-time delay circuit using LM555

[mozikluv]

hello everybody & good day,

i have made an on-time delay timer circuit using the LM555 with R-1.5Mohms and 4.7uf as time determinant. have configured it in astable mode with pin 5 & 7 open.

circuit is used to drive 2 relays in parallel. it functioned as expected. both relays were turned on simultaneously at 9v regulated and 12v unregulated.

now here's the problem: at 12v unregulated, when the circuit is turned off both relay does not cut off simultaneously. but at 9v it does! the time difference between cdut off is about 0.5sec. why :? :? :?

solutions or explanation will be highly appreciated.

heres some voltage and current reading that i have obtained from the circuit:
1. both relay draw 24ma. at 9v and 42ma. at 12v
2. pin 3 voltage is 0.2v w/relay off and 7.6v w/relay on
3. pin 6 voltage w/relay on at initial reading - 7.2v and settles at 4.5v

i know that if i power the circuit with 9v regulated it would solve the problem or i use a DPDT relay would also do. but i would like to use the above set-up and have it powered by the supply of the unit which is 12v unregulated. i dont want to put a 7809 regulated on the circuit board of the delay circuit. pls. dont ask me why.

am willing to try other ways in solving this problem.

thanx in advance.

 

[captainhannes]

Just a wild guess, but let's assume the relay needs at least 8V to switch, so if you power the circuit with 9V the time to discharge the capacitor in the power supply to 8V is much less than the time to discharge the capacitor from 12V (?)

Greetings,
Hannes.

 

[crust]

Let me see if I understand: Both relays are in parallel and cut off at different times? What happens if you try each relay independently, do they seem to have similar characteristics? Also, do you have diodes across the coil of the relays? If not the inductive kick from the relay could cause unwanted side effects when one of the relays turns off.

 

[mozikluv]

on-time delay circuit

thanx for the replies guys,

here's some more data that i forgot to mention;
1. each relay has a resistor (1.0ohm) in series with the coil at the supply side.
2. relay 1 has a resistance of 398ohms
3. relay 2 has a resistance of 395ohms
4. between both relays there is a difference of 3ohms

question: does the 3 ohms difference cause the time difference in its cut-off? :? :? if it does, then why do they cut-on at the same time?

pls. correct me if i'm wrong with this analysis;
1. a 398ohms coil would take a longer time to energize than the 395ohms
2. a 398ohms coil would take a longer time to de-energize than the 395ohms (which is happening now)
3. since a 398ohms coil takes a longer time to energize then it would cut-on later than the 395ohms coil (which is not happening, they turn on at the same time)

more replies would be highly appreciated

 

[captainhannes]

ahhh...now I get it... your problem is that the 2 relays do not cut off at the same time!

Well, if you have a 3rd relay, try to exchange the first or the second and try it again.

Do you have a schematic ?

Hannes.

 

[mozikluv]

relays not cutting off at the same time



well i have not tried that approach, i'll try that.

well here's another question, could it be that the 555 is not capable of not functioning properly considering that it is driving directly 2 relays at the same time. am not using transistor or scr to drive the relay.

 

[k7elp60]

It appears that the two relays are not identical. Once a relay is energized on or near it's rated coil voltage it will remained energized until the voltage across the coil reaches the dropout voltage. This varies from relay to relay and from manufacture to manufacturer.
Do you have a diode across the coils of the relays to absorb the counter EMF when the relays are deenergized? Do you have a diode from pin 3 of the 555 timer to each relay?
If the relay coils are rated at 9V I would put a larger resitor in series with the coil when you are using the unregulated 12V. If they are rated at 12 Volts no extra resistor is neeeded.
Ned

 

[mozikluv]

555 timer

hi ned,

yes, there is a shunt diode across the coil, no, i did not install a series diode from pin 3. would placing a series diode cure the cut-off time difference?

thanx for your reply

 

[k7elp60]

I don't think adding the diode will change the dropout time. You could try one diode for each relay, but I don't think it will do any good. The only thing I can think of is to add some more resistance in series with the fast relay to decrease the overall magnetic field it has. That may slow it down to match the slow releasing one.
Ned

 

[mozikluv]

lm555

hi ned,

thats what i have been mulling to do, add a resistor on the fast one to equal the resistance of the slow one, as can be gleaned from my previous post there is a 3ohm difference between the two relays. again my question, would this 3ohm diff. really make that kind of cut-off difference?

another approach i would like to try is increase the series R on the supply side of the faster one. i dont know if this is a sound approach.

thanx