The problem of unnecessary switching delay in relay

Thread Starter

teresa123

Joined Jan 10, 2021
6
Hi,

I use 12v charger (1 amp) to connect (for testing purpose) to the coil points of the relay. The relay works nicely as it should be. I did not use any diod yet because I am just testing the relay.

But after switching off the 12v charger, the relay switches after 5-7 seconds. I know it is due to the electric charge collected in the capacitors of 12v charger.

How can I dismiss the charge of capacitors to switch the relay immediately after turning off the power of 12-charger?
May this problem (switching delay) be solved by using the diod and transistor as shown in the attached image?

Thanks in advance,

Note:
I am using the following relay:
https://www.daraz.pk/products/spdt-relay-12v-i2138213.html
 

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peterdeco

Joined Oct 8, 2019
330
I don't think this is going to work. If the power supply caps are powering the relay, they may keep the transistor ON for possibly, an even longer time.
 

Yaakov

Joined Jan 27, 2019
2,362
Hi,

I use 12v charger (1 amp) to connect (for testing purpose) to the coil points of the relay. The relay works nicely as it should be. I did not use any diod yet because I am just testing the relay.

But after switching off the 12v charger, the relay switches after 5-7 seconds. I know it is due to the electric charge collected in the capacitors of 12v charger.

How can I dismiss the charge of capacitors to switch the relay immediately after turning off the power of 12-charger?
May this problem (switching delay) be solved by using the diod and transistor as shown in the attached image?

Thanks in advance,

Note:
I am using the following relay:
https://www.daraz.pk/products/spdt-relay-12v-i2138213.html
The capacitors discharge according to the RC time constant of the circuit that includes the caps, any bleeder resistors across them in the supply, and the relay. Though it would be very inefficient, if you reduced the resistance across the caps (increased the load) the time would be shorter. This is going to loose power to heat while it is running.

If this is only for testing, why to you need to solve the problem? If you need it to go off immediately, place a switch between the PS output and the relay and switch with that. It will be instantaneous.
 

sagor

Joined Mar 10, 2019
394
As long as there is power, things will run. You cannot stop that.
One possibility is to check the charger circuit secondary, and see if you can measure the AC voltage going to the diodes. That would be an "almost" instant off, as the diodes would block any charge from the capacitors from being measured. Or, you could add one diode off the AC secondary winding and measure that with only a small capacitor ( a couple of uF only) to filter the pulsing DC. That capacitor should have a good voltage dropping resistor on it, to discharge the cap in less than a second. Take that resulting DC voltage and feed it to your cut off circuit/detection.
 

LesJones

Joined Jan 8, 2017
3,078
I am guessing that the relay is being used to disconnect the charger from the battery when the mains is switched off to the charger. What I do is to add a diode between the output of the bridge rectifier and the reservoir capacitor and supply the relay coil from the rectifier end of the diode.

Les.
 

Thread Starter

teresa123

Joined Jan 10, 2021
6
I don't think this is going to work. If the power supply caps are powering the relay, they may keep the transistor ON for possibly, an even longer time.
I also think so.
After switching off the 12-volt charger, the charge of its capacitors must be dismissed immediately to switch the relay immediately.

If this is only for testing, why to you need to solve the problem? If you need it to go off immediately, place a switch between the PS output and the relay and switch with that. It will be instantaneous.
It is not just to test how the relay works. I will also use the relay for switching later after solving this problem.
 

Thread Starter

teresa123

Joined Jan 10, 2021
6
As long as there is power, things will run. You cannot stop that.
One possibility is to check the charger circuit secondary, and see if you can measure the AC voltage going to the diodes. That would be an "almost" instant off, as the diodes would block any charge from the capacitors from being measured. Or, you could add one diode off the AC secondary winding and measure that with only a small capacitor ( a couple of uF only) to filter the pulsing DC. That capacitor should have a good voltage dropping resistor on it, to discharge the cap in less than a second. Take that resulting DC voltage and feed it to your cut off circuit/detection.
Yes, it's good idea. I will open the 12-volt charger and see it.
Thanks :)
 

LesJones

Joined Jan 8, 2017
3,078
Can you clarify if the charger is providing the just power to the relay, Just base drive to the transistor or both. What is the final purpose of the relay as you say you are just testing ?

Les.
 

Thread Starter

teresa123

Joined Jan 10, 2021
6
Disconnect the circuit from the power supply when you want the relays to be "off".
Yes, it solves the problem but I cannot do it manually every time.

Can you clarify if the charger is providing the just power to the relay, Just base drive to the transistor or both. What is the final purpose of the relay as you say you are just testing ?
Les.
Yes, I am using 12-volt (1 amp) charger to power the relay coil. I am not just testing how the relay works. I will use it later for auto-switching purpose.

At this time, I am not using diode or transistor.

Is there any way to discharge the capacitors of the charger immediately after switching off the charger, so that, the relay also could switch immediately?
 

peterdeco

Joined Oct 8, 2019
330
This might jolt the PS for a few ms but a normally closed relay across the caps would open up when the power was switched on. Guys does this sound feasible?
 

LesJones

Joined Jan 8, 2017
3,078
Why does the relay have to be connected DIRECTLY to the charger output ? The drive to a relay coil would normally be controlled by a switch or relay contact or the output of some type of sensor. (Or drive to the base of the transistor in your original circuit.)

Les.
 

djsfantasi

Joined Apr 11, 2010
7,551
As was asked before, what is the relay switching?
What is providing power to the switched relay contacts?

In the final application, how is power interrupted to the charger coil? Test for a power fail before the supply caps. This test (a properly rated NO relay might work) would immediately cut power from the coil supply to the relay, and bypass the effects of the caps.

Reading this before I post it, it sounds vague. But that’s because you haven’t provided enough detail. Answer my questions, first.

Secondly, I’m left with the impression that you may be trying to solve a problem that won’t exist in your final application. That is, you’re searching to solve a problem that only exists in your test.
 

BobaMosfet

Joined Jul 1, 2009
1,502
Hi,

I use 12v charger (1 amp) to connect (for testing purpose) to the coil points of the relay. The relay works nicely as it should be. I did not use any diod yet because I am just testing the relay.

But after switching off the 12v charger, the relay switches after 5-7 seconds. I know it is due to the electric charge collected in the capacitors of 12v charger.

How can I dismiss the charge of capacitors to switch the relay immediately after turning off the power of 12-charger?
May this problem (switching delay) be solved by using the diod and transistor as shown in the attached image?

Thanks in advance,

Note:
I am using the following relay:
https://www.daraz.pk/products/spdt-relay-12v-i2138213.html
Hi- There is a general misunderstanding of what a relay is. A relay is a coil- an inductor. As such, it builds up a charge to activate, and it has a charge to release when deactivated. This is apart from any capacitors. This is usually what causes the delay. If you have caps, that may add additional delays. You need to control the flow of current using diodes etc.

If you want something faster, use an Triac/SCR/Thyristor.
 
Last edited:

djsfantasi

Joined Apr 11, 2010
7,551
Hi- There is a general misunderstanding of what a relay is. A relay is a coil- an inductor. As such, it builds up a charge to activate, and it has a charge to release when deactivated. This is apart from any capacitors. This is usually what causes the delay.

If you want something faster, use an SCR.
I’ve yet to find a relay that takes 5-7 seconds to drop out. For high speed switching, the phenomenon you refer to needs to be considered. But 5-7 seconds? There is something else going on.

We don’t know what “faster” means. Is it 2-3 seconds? 2-3ms? 0.2-0.3ms?

The TS is focusing on a solution rather than the problem. I maintain that a solution to her “test” may bear little resemblance to the solution to her problem.

We've asked the TS several times to describe the final application. That may help define “faster”. But in absence of a reply, we can’t help her.
 

Jon Hoover

Joined Oct 10, 2019
31
This is a confusing situation. If you disconnect the power supply (PS) from line voltage, the PS will discharge over time into the relay delaying it turning off. I am curious how you are disconnecting the PS from line voltage. Are you unplugging it or are you using a switch? If you are using a switch, then maybe you could operate a 2nd relay with the switch that disconnects the load relay from the PS when the PS is turned off. If you are not using a switch, then you may spend more time and trouble trying to solve this problem than it's worth. Pick your battles wisely. Don't get stuck in a rabbit hole trying to correct something that may not be an issue if you approach the entire situation a little differently. If it was me, I would put a switch between the PS and the relay if possible. But, if you are trying to sense when there is a power loss, then you need to look at your PS and get rid of any capacitors that may be holding a charge and delaying the relay shut off. The output voltage will be noisy and will have a lot of ripple, but if all you are doing is operating a relay with it, you should have no problems. Even simpler is to use an ac power supply switching an ac relay. If the line voltage, ac, goes away, the relay switches off.
 

djsfantasi

Joined Apr 11, 2010
7,551
and get rid of any capacitors that may be holding a charge and delaying the relay shut off. The output voltage will be noisy and will have a lot of ripple, but if all you are doing is operating a relay with it, you should have no problems. Even simpler is to use an ac power supply switching an ac relay. If the line voltage, ac, goes away, the relay switches off.
Hear, hear!!!
 

sghioto

Joined Dec 31, 2017
2,264
How can I dismiss the charge of capacitors to switch the relay immediately after turning off the power of 12-charger?
This might jolt the PS for a few ms but a normally closed relay across the caps would open up when the power was switched on. Guys does this sound feasible?
Actually it is feasible with a slight modification.
The relay the TS is using is a 400 ohm coil, SPDT. This can be accomplished using one or two relays depending on what the contacts are used for. Effective resistance is 40 ohms. Relay dropout although not instantaneous should be less then 1 second.
1616542583643.png
 
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