trying to find a circuit to control a fail safe relay, 555 timer?

Thread Starter

Chipper

Joined Jul 19, 2018
38
I've not posted to this forum, so I'm not sure what to expect... here goes:
i have been searching for a circuit that will provide a method to trip off a 250A latching contact relay in the charging side of a 13 volt lithium battery pack. The pack is protected with charge control relays directed by a BMS (battery management system) , as well as charging voltage limits on the chargers (solar, alternator, AC rectified). The 250A contactor relay forms the fail safe component. The BMS pulls low (up to 175ma) when the absolute charge limit is reached on any individual cell is reached, I am using this signal to operate a small signal relay. The signal stays low (relay output high) until the over voltage situation is resolved. I would like to use this signal to generate a one shot pulse of sufficient current to operate the contactor coil, (approximately one amp), one second duration would be long enough. I will use a manual reset. I've looked at dozens of schematics and circuits, built a few, but I can't really find what I'm looking for. I know this is pretty basic stuff for most of the users of this forum and I'm trying to learn as I go. Any ideas would be appreciated Thanks for your help, chip
 

Dodgydave

Joined Jun 22, 2012
8,586
So you have a Low going signal from the Bms pcb, at what voltages is it going to and from? Can you provide a drawing?

The low signal can trigger a 555 timer, and this can give out a pulse for any duration upto 10mins.
 

Thread Starter

Chipper

Joined Jul 19, 2018
38
So you have a Low going signal from the Bms pcb, at what voltages is it going to and from? Can you provide a drawing?

The low signal can trigger a 555 timer, and this can give out a pulse for any duration upto 10mins.
Thanks for the response Dd, it's looking like i should bypass the signal relay, and use the 175ma pull low from the BMS (Ewert Orion jr.) can i use something like this circuit:
all voltages are approx 12 volts cj
 

eetech00

Joined Jun 8, 2013
1,740
I've not posted to this forum, so I'm not sure what to expect... here goes:
i have been searching for a circuit that will provide a method to trip off a 250A latching contact relay in the charging side of a 13 volt lithium battery pack. The pack is protected with charge control relays directed by a BMS (battery management system) , as well as charging voltage limits on the chargers (solar, alternator, AC rectified). The 250A contactor relay forms the fail safe component. The BMS pulls low (up to 175ma) when the absolute charge limit is reached on any individual cell is reached, I am using this signal to operate a small signal relay. The signal stays low (relay output high) until the over voltage situation is resolved. I would like to use this signal to generate a one shot pulse of sufficient current to operate the contactor coil, (approximately one amp), one second duration would be long enough. I will use a manual reset. I've looked at dozens of schematics and circuits, built a few, but I can't really find what I'm looking for. I know this is pretty basic stuff for most of the users of this forum and I'm trying to learn as I go. Any ideas would be appreciated Thanks for your help, chip
Hi

One consiideration with relay based fail-safe systems is to avoid the use of de-energized components. Simply because it’s difficult to detect failure. That said, is your contractor coil normally energized and deenergizes when an overvoltage condition occurs?

Do you intend to include a 555 timer to perform a failsafe function? If so...don’t do it...As mentioned above, it’s failure mode is unpredictable.

eT
 

Thread Starter

Chipper

Joined Jul 19, 2018
38
Hi

One consiideration with relay based fail-safe systems is to avoid the use of de-energized components. Simply because it’s difficult to detect failure. That said, is your contractor coil normally energized and deenergizes when an overvoltage condition occurs?

Do you intend to include a 555 timer to perform a failsafe function? If so...don’t do it...As mentioned above, it’s failure mode is unpredictable.

eT
The trigger i think will go low and stay low until the state of charge is lowered to acceptable levels. what are the possible methods of keeping the output pulse to a short duration? I want to keep the contactor coil de-energized as soon as the contactor has latched off.

As configured now the fail safe system consists of a pair of dual coil TE Potter Brumfield contactor relays 1-1414939-4. One for charge on for discharge. Contactors are set to ON with a momentary contact switch and latched to OFF by my proposed fail safe. The Ewert BMS outputs fails "low" in the event a BMS failure. I'm good on the discharge side as the contactor is controlled by energized NC signal relay that will go high and send a pulse to the contactor just before the contactor shuts everything off between the system and the battery. I'm trying to come up with something on the charge side contactor and the 555 circuit seems to be a possibilty
 
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Hymie

Joined Mar 30, 2018
721
In terms of the timer circuit, this 555 timer circuit will do what you want; with the trigger input (pin 2) capacitively de-coupled such that should the trigger input remain low (longer than the timed period), the circuit will give a positive output pulse period, determined by the component values Rt & Ct.

With Rt = 91kΩ & Ct = 10uF will give a positive output pulse of circa 1 second.

Timer circuit.jpg
 

Thread Starter

Chipper

Joined Jul 19, 2018
38
In terms of the timer circuit, this 555 timer circuit will do what you want; with the trigger input (pin 2) capacitively de-coupled such that should the trigger input remain low (longer than the timed period), the circuit will give a positive output pulse period, determined by the component values Rt & Ct.

With Rt = 91kΩ & Ct = 10uF will give a positive output pulse of circa 1 second.

View attachment 160912
Thanks for the circuit Hymie ,
I breadboarded this with a 9v power source and it worked as you describe. a couple of questions...
(1) when i pull low through a momentary switch held closed a single short duration (1 second) pulse is achieved. when the switch is opened, after a few minutes the timer seems to reset on its own. what controls the duration of this reset? what would be the proper reset method for the circuit?
(2)I didn't have a 1N918 and used a 1N4007 to mock up this circuit, but it seems to work as described. any problem using the 1N4007? Eetech00 seems to imply that the 555 is not a sufficiently reliable solution for a fail safe back-up to my primary charge limitation device.. any thoughts?

Thanks to you Hymie, and Wendy, and Bill for providing some tutoring

PS, when i first power this circuit, there is a very brief output pulse with enough energy to short flash a LED. is there a way to eliminate this pulse or should this circuit powered up continuously
 
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Hymie

Joined Mar 30, 2018
721
Opening the switch is likely to be introducing noise on the input, especially if the cable run to the switch is long – causing the timer to trigger again. You may need to add additional capacitive filtering on the input signal line to eliminate the noise. The circuit is automatically reset (ready to be triggered again) once the timed period has ended and the input goes high.

Using a 1N4007 rather than the 1N914 diode should be OK, the diode is to ensure that the voltage on pin 2 remains within limits, when the positive going input pulse is fed to the pin 2 capacitor.

To eliminate the circuit triggering on power up, connect a 0.1uF capacitor between pin 5 and 0V.
 

eetech00

Joined Jun 8, 2013
1,740
Thanks for the circuit Hymie ,
Eetech00 seems to imply that the 555 is not a sufficiently reliable solution for a fail safe back-up to my primary charge limitation device.. any thoughts?
Hi

I haven't really been following this thread so if the failsafe issue has been addressed, please disregard.

If by "failsafe" you mean a function that must fail in a way that prevents bodily harm, or death, then whatever circuit is used must have predictable failure modes so they can be mitigated. The 555 chip can fail in many ways and unless the circuit accounts for them, it is not "failsafe". For example, the output of the 555 can fail Hi, Low, anywhere in between, or not function at all.

eT
 

Thread Starter

Chipper

Joined Jul 19, 2018
38
Hi

I haven't really been following this thread so if the failsafe issue has been addressed, please disregard.

If by "failsafe" you mean a function that must fail in a way that prevents bodily harm, or death, then whatever circuit is used must have predictable failure modes so they can be mitigated. The 555 chip can fail in many ways and unless the circuit accounts for them, it is not "failsafe". For example, the output of the 555 can fail Hi, Low, anywhere in between, or not function at all.

eT
eT, thanks for your thoughts, please amend my use of "fail safe" to "back-up" c
 

Thread Starter

Chipper

Joined Jul 19, 2018
38
Opening the switch is likely to be introducing noise on the input, especially if the cable run to the switch is long – causing the timer to trigger again. You may need to add additional capacitive filtering on the input signal line to eliminate the noise. The circuit is automatically reset (ready to be triggered again) once the timed period has ended and the input goes high.

Using a 1N4007 rather than the 1N914 diode should be OK, the diode is to ensure that the voltage on pin 2 remains within limits, when the positive going input pulse is fed to the pin 2 capacitor.

To eliminate the circuit triggering on power up, connect a 0.1uF capacitor between pin 5 and 0V.
Thanks again Hymie, I will add the 0.1uF cap at pin 5. I notice that is a common addition to 555 timer circuits. I have decided to power this circuit continuously while charging is enabled. current draw while charging won't be an issue. exactly what do you mean by additional capacitive filtering? a larger capacitor I assume.
 
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Hymie

Joined Mar 30, 2018
721
The additional filtering might consist of a capacitor across the switch contacts providing de-bounce of the switch action – or twisted pair connected wires to the switch, minimising any electrical noise pick-up (which might cause the timer to trigger). All this might not be required within the final circuit connected within your BMS.

In terms of making the monitoring circuit (which switches off the relay) fail-safe; you could add an additional ‘logic circuit’ which monitored the low signal (fed to the timer circuit) and the relay coil voltage. If the monitored signal is low and the relay still energised, then another circuit component/path would de-energise the relay.

Although such a circuit is likely to have some common circuit features such that ‘failure mode & effects analysis’ would show the circuit could still fail – it would be less likely to occur with the logic circuit added, and could be considered a minimal risk.
 

Thread Starter

Chipper

Joined Jul 19, 2018
38
The additional filtering might consist of a capacitor across the switch contacts providing de-bounce of the switch action – or twisted pair connected wires to the switch, minimising any electrical noise pick-up (which might cause the timer to trigger). All this might not be required within the final circuit connected within your BMS.

In terms of making the monitoring circuit (which switches off the relay) fail-safe; you could add an additional ‘logic circuit’ which monitored the low signal (fed to the timer circuit) and the relay coil voltage. If the monitored signal is low and the relay still energised, then another circuit component/path would de-energise the relay.

I like your thinking here, Let me get the timer circuit built and installed. Then I'll look at this addition.. it should be pretty easy to accomplish chip


Although such a circuit is likely to have some common circuit features such that ‘failure mode & effects analysis’ would show the circuit could still fail – it would be less likely to occur with the logic circuit added, and could be considered a minimal risk.
 

Hymie

Joined Mar 30, 2018
721
With your additional logic circuit configured such that it indicates that it has tripped/operated (rather than the 555 timer circuit), you would know that the primary protection circuitry had failed for some reason and needed investigation.

If the additional protection circuit was implemented on a commercial unit – then the ability to reset this secondary protection circuit could be a service activity.
 

Thread Starter

Chipper

Joined Jul 19, 2018
38
With your additional logic circuit configured such that it indicates that it has tripped/operated (rather than the 555 timer circuit), you would know that the primary protection circuitry had failed for some reason and needed investigation.

If the additional protection circuit was implemented on a commercial unit – then the ability to reset this secondary protection circuit could be a service activity.
Hymie, the BMS incorporates a user addressable input that will log errors and set alerts, this might be an easy way to connect this additional circuit..

By the way, when a run the mock up circuit at 13 volts the trigger response is different. Do I need to change the values of components?
 

Hymie

Joined Mar 30, 2018
721
The 555 timer will operate with a supply voltage of up to 18Vdc – changes to the supply voltage should not change the operating performance of the circuit in post #7.

However if the trigger input voltage high is less than 13V (Vcc), the circuit might not trigger reliably (or at all) since as a result of the falling input signal, pin 2 needs to fall below 1/3 of Vcc.

If the BMS out low is not a step function, you may need to increase the trigger input capacitor to ensure pin 2 falls below 1/3 Vcc.
 

Thread Starter

Chipper

Joined Jul 19, 2018
38
The 555 timer will operate with a supply voltage of up to 18Vdc – changes to the supply voltage should not change the operating performance of the circuit in post #7.

However if the trigger input voltage high is less than 13V (Vcc), the circuit might not trigger reliably (or at all) since as a result of the falling input signal, pin 2 needs to fall below 1/3 of Vcc.

If the BMS out low is not a step function, you may need to increase the trigger input capacitor to ensure pin 2 falls below 1/3 Vcc.
Hymie, I've built a copy of this circuit on a generic board, and it seems to work fine.. Although there might be one problem I can think of: once triggered the circuit will not trigger again for quite some time. I can see where this might be a problem in the event of a battery cut-off contactor reset (this is manual process in my application) and then the circuit could not trigger properly in the event of an additional over charge situation. Am I making myself clear?
 

Hymie

Joined Mar 30, 2018
721
I can only suggest that you ensure the input is at Vcc when not triggered (switched low). This can be achieved by connecting both sides of the input capacitor to Vcc via a 4k7Ω resistor – with the trigger signal being the input switched to 0V.

Increasing the input capacitor value may help, but to ensure that the circuit is not triggered on power up, the capacitor connected at pin 5 should be greater than that connected to pin 2.
 

Thread Starter

Chipper

Joined Jul 19, 2018
38
I can only suggest that you ensure the input is at Vcc when not triggered (switched low). This can be achieved by connecting both sides of the input capacitor to Vcc via a 4k7Ω resistor – with the trigger signal being the input switched to 0V.

Increasing the input capacitor value may help, but to ensure that the circuit is not triggered on power up, the capacitor connected at pin 5 should be greater than that connected to pin 2.

Thanks for the suggestion, Hymie. Here's what I've done: (1) added 4k7 ohm resistor on both sides of the input capacitor (I used a 0.1uF here as I did not have a .01uF cap) (2) paralleled another 0.1uF cap at pin 5 to 0V
Problem now is that while it works as desired, occasionally it also triggers on release from low and it seems to be a totally random problem. any more thoughts on this (or are you getting totally bored with me ;))
 
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