Circuit to delay cut off

Thread Starter

abdulk79

Joined Jan 8, 2016
13
Hi,

I have 2 relay's which are controlled by toggle switch, I want some circuit which would delay cut-off of relay B when switch is moved to off state.



Thanks,
Abdul
 

AnalogKid

Joined Aug 1, 2013
8,229
Thank you for telling us what you want. Unfortunately for you, it's not all about you. What about my wants, my needs, my fulfillment? Engineer does not equal psychic. Relay part number, manufacturer, coil resistance - and DELAY TIME...???

ak
 

Thread Starter

abdulk79

Joined Jan 8, 2016
13
its written 10A 24VDC on relay, so which capacitor should i use?

Only capacitor will do the work or i need more components?
 

TheButtonThief

Joined Feb 26, 2011
233
Relay A and relay B are pulled in by the same switch, therefore on the same circuit. This way, whatever delay you introduce to Relay B will knock-on to relay A. You'd have to separate them by switching their control voltages independently, either by using two different toggle switches or a single DPDT toggle switch.

As for the delay, we don't have enough information to help you. What's the coil resistance of the relays? What time delay do you require? Are you limited to a modular solution or are you ok with using discrete components? How long will the relay be pulled in before the supply is removed? The answers to these questions may even raise more questions.

Our ability to help you depends on the scope of data you provide.
 

AnalogKid

Joined Aug 1, 2013
8,229
its written 10A 24VDC on relay, so which capacitor should i use?
Only capacitor will do the work or i need more components?
Do you really think that the relay *coil* draws 10 amps when it is operating? Think again.
Relay part number, manufacturer, coil resistance. Consider for a moment that I know what I'm doing and asking for a reason.

ak
 

AnalogKid

Joined Aug 1, 2013
8,229
Relay A and relay B are pulled in by the same switch, therefore on the same circuit. This way, whatever delay you introduce to Relay B will knock-on to relay A. You'd have to separate them by switching their control voltages independently, either by using two different toggle switches or a single DPDT toggle switch.
Or by leaving the switch alone and putting a diode in series with the delay circuit to Relay B.

ak
 

Thread Starter

abdulk79

Joined Jan 8, 2016
13
Thanks for your inputs.

I am using relay JQC-3FC/T73 12VDC and below is the datasheet.
MAX. SWITCHING CURRENT:7A,10A MAX.
SWITCHING VOLTAGE:28V DC/ 250V AC
DIELECTRIC STRENGTH VR.M.S: BETWEEN OPEN CONTACTS =750VAC;
BETWEEN COIL AND CONTACTS =1000VAC;
BETWEEN CONTACTS FORM =1000VAC;
AMBIENT TEMPERATURE: -40-+85oC;
OPERATION/RELEASE TIME:=10/8MS
CONTACT CAPACITY: 10A 125V, 7A 250V
 

TheButtonThief

Joined Feb 26, 2011
233
Now we're getting somewhere. However, you still need to answer another question:

Are you limited to a modular solution or are you ok with using discrete components?
Also, with regards to your specification of 1 minute, is that an approximate value or does it need to be 60.000000 seconds? Do you have a budget?
 

AnalogKid

Joined Aug 1, 2013
8,229
The relay coil resistance is 400 ohms (now, was that so hard?) and the dropout voltage is 9 V, so the current that must be supplied by the delay circuit is 30 mA and over that time the voltage can sag 3 V. Using a first order approximation:

E x C = I x t
(11.3-9) x C = 30 mA x 60 sec
2.3 x C = 1.8
C = 0.78

So, a diode and a 780,000 uF capacitor will delay the turnoff of the 2nd relay by approximately 60 seconds.

This is a completely impracticable, unrealistic, and unreliable approach, but it's good to rule out the simple approach by considering it carefully first. A variation of this is to use a much higher impedance timing network followed by a relay coil driver. This will bring the size of the capacitor down to something realistic. For example, a 604K resistor with 100 uF capacitor has a time constant of 60 seconds. A 2N7002 threshold voltage is much less than 63% of 12 V, so a smaller resistor will work.

In this sim, R1 is the relay coil, and V1 simulates closing and opening the switch. The switch opens at 2 sec. and the voltage across the relay coil decreases to 9 V at 59 sec.

ak
Relay-Delay-1.gif
 
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