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  1. #1
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    Need very basic R-C "delay off" circuit

    I just got some great help on this forum to resolve a logic gate issue, so I thought I would try my luck with what I hope is my last issue.

    Below may be too much context and you may ant to skip to my "question" at the end.

    For background, the context is a battery management system for a 200 Ah LiFePO4 marine battery bank. The system has a commercial battery management board, an Arduino to refine a couple of functions that the board does not allow, and 7 relays. Control signals are all 5v DC.

    To finalize my (now working) prototype I want to trying adding a very simple RC delay in one of the relay control circuits.

    The relay in question is a 12V, 120 Amp DC relay (HDC60D160). It is the main disconnect to separate both the charging bus and the load bus from a LiFePo4 battery bank. It is controlled by the battery management board.

    Most of the "normal" scenarios for disconnecting the buses from the lithium bank are programmed with a short delay after the triggering condition and before disconnect. This delay allows relays to disable the charging sources (alternator field current, solar charge controller, etc.) a few seconds before the battery disconnect. This prevents voltage spiking from disconnecting the charging sources (most worrisome is 100A alternator) from the battery while under load.

    Unlike the delay in the "normal" disconnect scenarios, the emergency disconnect scenarios shut down the charging sources at the same time as the main disconnect is opened. This is probably safe enough, since earlier layers of protection should have shut down charging sources long before emergency conditions are reached and because there is a 12v lead-acid battery in the loop to absorb any unintended/unforeseen transients resulting from main bank disconnects.

    That said, I would prefer to introduce a delay into the main disconnect control circuit if it can be done simply.

    The disconnect relay is normally open and is held closed during normal operation by a 12 volt signal from the drain of a p-channel MOSFET (STP10P6F6).

    The 12v control signal (current) goes to zero in a number of ways, including:
    a) There are thermal shut offs on the individual battery cells, and wired in series into the control line,
    b) there is a manual emergency shut off button in the control line, and
    c) the source of the 12v control signal is a p-channel MOSFET whose drain goes to zero under the control of the battery monitoring module under emergency conditions.

    In each case the 12v control signal to the relay is interrupted and the main disconnect relay opens.

    My question: Is there a simple resistor-capacitor type arrangement I could put in the wire that carries the +12v control signal in order to cause a few seconds delay in the opening of the relay? The "must turn off" voltage of the relay is 4.2 so if there is a resistor-capacitor arrangement that would take 5-10 seconds to decay from 12 to 4.2, that would be perfect. If a general schematic were provided I could probably figure out the values of R and C that are need from the currents in the data sheet (although I would be happy to hear that to). What I really need is to know if this can work for DC and the requisite arrangement of the elements.

    From my hazy 35 year-old memories of a circuits course I once took, I think that this must be an unbelievably simple problem (a resistor in series and a capacitor branching off to ground?) but I cannot figure it out from internet sources.

    I attach a rough sketch of what I have now.

    P.S. I don't want to use IC timers or a time-delay relay, so I am just looking for a passive, simple, physically robust solution if it is possible

    Thanks in advance for your help.

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  2. #2
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    Re: Need very basic R-C "delay off" circuit

    if you use darlington connected xtors (bjt's) to turn on the relay, emitter gnded, then you can have 4u7 on the base to gnd and 220k drive resistor to the base which will give you a delay, reverse schottky diode to speed up turn off ...


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  3. #3
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    Re: Need very basic R-C "delay off" circuit

    Simple diode-resistor-capacitor networks create a gradual ramp-wave when you input a sudden transition.

    The principle behind the downward ramp (lefthand) is obvious.

    It's not as obvious how to create the upward-going ramp. It's possible by connecting to positive supply rail.

    Click image for larger version. 

Name:	one-time delay DRC (downward starts downward-ramp) (upward starts upward-ramp).png 
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    Timeframe is 19 seconds. The potentiometers are 50k. Some resistance is needed in the loop to drain the capacitor and reset it for the next use.

    As stated in post #2, by adding transistors it allows you to reduce capacitor values.



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    Re: Need very basic R-C "delay off" circuit

    Thank you very much. Clear and simple, except where you say:

    Quote Originally Posted by Easy peasy View Post
    ...reverse schottky diode to speed up turn off ...
    I do not understand reference to Schottky. Are you suggesting to use a Schottky for the flywheel protection diode on the relay to speed up turn off, compared to a regular diode? If so, why do I want that when I am seeking delay?

    Or, does the Schottky play some different role?



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    Re: Need very basic R-C "delay off" circuit

    across the timing resistor 220k, if you apply 3v3 or 5V to the 220k it will take a while for the volts to come up to 1.1V necessary to turn on the darlington array of xtors ( 2 x npn ) the reverse diode across the 220k speeds up the turn off - if desired.

    We have used this with 470k & 10uF low leakage electro's too ... ( 1Meg & 1uF give 1 sec to 63% of Vdrive - so easy to calc rough delay )

    the darlington input draws only a few uA.


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  6. #6
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    Re: Need very basic R-C "delay off" circuit

    Thanks ... close to solved. One last question about role of diode.

    You wrote:

    Quote Originally Posted by Easy peasy View Post
    across the timing resistor 220k, if you apply 3v3 or 5V to the 220k it will take a while for the volts to come up to 1.1V necessary to turn on the darlington array of xtors ( 2 x npn ) the reverse diode across the 220k speeds up the turn off - if desired.
    I want fast charging of cap for a fast turn on and slow discharge of cap to delay turn off by 5-10 seconds.

    For fast "on" and slow "off", does the Shottky across the 220 have its cathode at the base of the Darlington or at the 12 volt source?



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    Re: Need very basic R-C "delay off" circuit

    Hi,

    Thanks ... close to solved. One last question
    Just press the "Mark as solved" button.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



  8. #8
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    Re: Need very basic R-C "delay off" circuit

    yes - for a fast turn on the diode points to the base of the Darlington, i.e. cathode to base ...


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