Drving a SPDT 2 coil latching relay - Page 1
Forum section: electronics / projects

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 16, 2019, 06:49:21 am
content: Hi, I am designing a PSU (18V 1A).To switch the ouput on and off I am going to use a latching relay.As i know nothing about latching relays can anyone help me to create a circuit so i can drive it from an MCU(ATmega328P).Here is the relay I am going to use.I can only find this circuit in thedatasheetThanks in advance.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 16, 2019, 07:05:01 am
content: Can anyone simply explain what this A1,A2 and A3 are and what is the small box with negative marking on it mean.

author: soldar
author_rank: Super Contributor
timestamp: June 16, 2019, 10:40:36 am
content: My guess is that A3 is common and if you apply voltage to A1 it latches one way and if you apply voltage to A2 it latches the other way.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 16, 2019, 10:47:26 am
content: Thanks for replying .I also thought so, but wasn't sure .

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 16, 2019, 04:36:28 pm
content: Yes. According to page 1 of the datasheet, the coils are "polarized" as I get it.So take care of polarity: look at the "Bistable coils - operation" table. A3 is indeed the common, but is the positive common.The simplest way to control it would be to tie A3 to VDC and use two N-channel mosfets (drain to A1/A2, source to GND, gate controlled by your application). Also don't forget the freewheeling diodes.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 16, 2019, 04:40:00 pm
content: Quote from: SiliconWizard on June 16, 2019, 04:36:28 pmis using a npn transistor ok.In my case 2n3904 with max collector current 200mA.The latching relay only consumes 600mW(12V,50mA)

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 16, 2019, 04:50:29 pm
content: Sure. Since the relay is bistable, it only requires short pulses anyway. You only need to (and should) drive the coils for a very limited amount of time to get the relay in either state.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 16, 2019, 04:53:04 pm
content: Quote from: SiliconWizard on June 16, 2019, 04:50:29 pmWill be 100-500 ms enough?

author: Kjelt
author_rank: Super Contributor
timestamp: June 16, 2019, 05:00:07 pm
content: Yes the datasheet says minimum 30ms max. 1 minute.So 250ms should be enough then you switch the transistor or fet off again.BTW since the microcontroller can not check in which state the relay is in, you need to keep track of this state.And in your startup routine you should set the relay in a defined state.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 16, 2019, 05:02:38 pm
content: Quote from: Kjelt on June 16, 2019, 05:00:07 pmOopse, I didn't read the datasheet carefully

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 11:26:59 am
content: Quote from: Kjelt on June 16, 2019, 05:00:07 pmWhy does it sat maximum one minute.what happens if I power the coil more than that?

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 11:28:31 am
content: I will configure current state(off) of relay when programming

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2019, 11:30:55 am
content: Quote from: Arjunan M R on June 17, 2019, 11:26:59 amThe coil isn't rated to run continuously and will overheat if run for much longer than the maximum recommended time.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 11:51:11 am
content: Quote from: Zero999 on June 17, 2019, 11:30:55 amOh, so it can only pass 50mA for max 1 miniute.

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2019, 12:28:16 pm
content: Quote from: Arjunan M R on June 17, 2019, 11:51:11 amYes, electrical energy is converted to heat in the coils, 600mW according to the datasheet. When power is applied to the coil, it takes time for the coil to heat up. After 1 minute the coil will reach its maximum temperature rating. If it's powered for longer, it will heat up more, which will risk it exceeding its maximum temperature rating and damaging it.The duty factor is 10%, which means it's only rated to be powered for 10% of the time, so if the coil is powered for 1 minute, it will need to be left to cool down for 9 minutes, before it can be safely powered again.Looking at the data sheet again, there are two different variants: one with a single coil, which will turn the relay on and off, depending on the polarity and the one which you have, with two coils: one off and another on. It's specified with the common connection connected to +V, but I wonder if it will work with the common connected to -V and with the logic reversed? It's quite likelu two coils wound in opposite directions, on the same bobbin.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 12:31:16 pm
content: Does i need to care about back EMF in this relay?

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2019, 12:50:15 pm
content: Quote from: Arjunan M R on June 17, 2019, 12:31:16 pmYes, back-EMF protection diodes will be required, as the datasheet doesn't say they're built-in.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 12:54:22 pm
content: Quote from: Zero999 on June 17, 2019, 12:50:15 pmDo i need to use a schotky diode?If so will this be enough?Here is the datasheet.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 12:55:51 pm
content: Quote from: Arjunan M R on June 17, 2019, 12:54:22 pmI am already using that schotky diode if it's enough I don't have to increase the BOM cost.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 01:00:49 pm
content: Does this mean that I can only energize the coil for max. of 10 ms.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 17, 2019, 01:26:00 pm
content: Quote from: Arjunan M R on June 17, 2019, 01:00:49 pmNo, as I get it, this is purely about the contacts themselves. I understand it as the max time from fully open to fully close and vice versa.

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2019, 01:47:56 pm
content: Quote from: Arjunan M R on June 17, 2019, 12:54:22 pmThat diode is more than adequate. It doesn't need to be Schottky. Ordinary silicon will do and it doesn't even need to be rated to the full coil current, as it only carries a short pulse, when the coil current is interrupted. The only requirement is the reverse voltage exceeds the power supply voltage.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 03:30:16 pm
content: Quote from: Zero999 on June 17, 2019, 01:47:56 pmIs ther any disadvantage using Schottky.I am already using that diode in the PSU.It will be good if I can get away with it.

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2019, 03:38:09 pm
content: Quote from: Arjunan M R on June 17, 2019, 03:30:16 pmThere is no disadvantage of using a Schottky diode. All the diode does is provide a path for the current to keep flowing, when the coil is de-energised. That diode is perfectly adequate and if you can cut down on the BoM cost by using the same part, then good.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 03:42:03 pm
content: Quote from: Zero999 on June 17, 2019, 03:38:09 pmYes I can cut down Bom cost because the minimum quantity i can order is 10.I only need about 6 of them.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 17, 2019, 03:43:25 pm
content: Also space in PCB.