Pulse circuit to operate Latching relay

Thread Starter

KiwiOm

Joined Sep 13, 2017
11
Gidday Forum,
Im needing to control a Latching relay that is operated by a pulse.

Im using it to disconnect power from my solar regulator to my LiFEPO4 battery when it reaches its full voltage, and is controlled by bi-stable relays on the BMS.

I need a circuit that is powered from the relays on the BMS to send a pulse to operate the Latching relay.
Im looking for something that will not consume much power and once the pulse is sent doesn't consume anything.
The BMS relays are 12V@ 2 amp

I want the pulse to happen when the relay comes on and only once and then reset when the power goes off so its ready to send a pulse again when the BMS relay is powered on again.

I will include pdf details about the Latching relay

Thank you in advance
Kryz
 

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Thread Starter

KiwiOm

Joined Sep 13, 2017
11
The power for the pulse circuit would be provided by the relay on the BMS that is mounted on the battery. The relay gets turned on when the battery reaches its full charge voltage and stays on until the battery voltage drops again.
The latching relays I want to use require only this pulse and not the constant power coming from the relays on the BMS.
Hence I'm looking for a circuit to provide the pulse
 

Thread Starter

KiwiOm

Joined Sep 13, 2017
11
I am charging 4 cells of a 12V LiFePO4 battery and the BMS controls the charging by using the on board relays to turn on and turn off when the battery voltage reaches preset levels.
These relays are therefore either on or off.
I want a circuit that will send a pulse when the relay first comes on so I can operate the Latching relay

I want the pulse to happen when the relay comes on and only once and then reset when the BMS relay goes off so its ready to send a pulse again when the BMS relay is powered on again.
 

Alec_t

Joined Sep 17, 2013
12,249
A latching relay (LR) needs one pulse to set it and another pulse to reset it.
So, just to confirm, you want a first pulse to set LR when the BMS relay turns on, and a second pulse to reset LR when the BMS relay turns off?
 

Thread Starter

KiwiOm

Joined Sep 13, 2017
11
yeah, sounds about right
My original idea was to put one circuit on the NO side of the BMS relay and another circuit on the NC side as the relay just toggles between
them.
Just looking for something simple that only uses power to produce the pulse and then consumes nothing.

If I dont use the Latching relay I have to use a relay that is using 0.5A and that is going to really eat up my battery capacity, something which for me is limited especially in winter when some days I may be only loading 3A into my batteries from the solar!

Thanks for your interest and help with my little project
 

Sensacell

Joined Jun 19, 2012
2,904
You have not provided a diagram of the final system, but a latching relay seems very expensive and complicated?

You might be able to use a MOSFET - which consumes almost zero power when activated?
 

Thread Starter

KiwiOm

Joined Sep 13, 2017
11
It's my end of charge voltage
Not charging LiFePO4 battery to 100% you can get much longer cycle rates
So iI'm opting for lower cut off voltage between the voltages I mentioned
 

AnalogKid

Joined Aug 1, 2013
9,385
Relay coil: 12 V nominal, 9 V minimum = 3 V differential
0.3 A, 0.03 ms (minimum operate time)

E x C - i x t
C = (i x t) / E
C = (0.3 x 0.03) / 3 = 3000 uF

A 3000 uF capacitor in series with the relay coil should pulse it long enough to operate, then reduce the coil current to almost zero (some leakage current). 3300 uF works on paper, 4700 uF should work in the real world.

Without more information about how your system is wired...

ak
 

Alec_t

Joined Sep 17, 2013
12,249
0.03 ms (minimum operate time)
I take that to mean the contact change-over time, but I could be wrong. The datasheet also specifies the operating pulse duration as 100mS. It's unclear whether that's a minimum, maximum or recommended pulse duration :confused:. Unless it's a maximum, perhaps a bigger cap would be advisable?
 

crutschow

Joined Mar 14, 2008
27,903
11.2V or 11.4 V
Then using AK's calculation in post #14, the minimum series capacitance to generate the required 100ms (assumed nominal time) pulse into the latching relay coil is
(0.3 x 0.1)/ (11.2-9) = 13,600μF at the minimum battery voltage.

Note that, in addition to adding this capacitor in series with the coil, you will also need to add a resistor to ground at the other end of the capacitor (opposite the end connected to the coil) to discharge the capacitor when the voltage is removed.
The capacitor recovery time is determined by the resistor value.
A 10kΩ resistor will give a recovery time of several minutes, but that should not be a problem in your application.
A lower value resistor can be used to speed up the recovery at the expense of more current when the applied voltage is at the high level.
 

Thread Starter

KiwiOm

Joined Sep 13, 2017
11
Thanks for all the help here.
Could some one draw me the circuit here please so that I get the + and - round the correct way

Thanks so much
 
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