How to drive latching relais ? - Page 1
Forum section: electronics / projects

author: Kleinstein
author_rank: Super Contributor
timestamp: July 10, 2020, 03:55:10 pm
content: For a circuit I may need some 4 (possibly more) latching relays. For the 2 coil version driving is relatively simple with one sided drivers (e.g. ULN2003 or similar).However some of the smaller relays are one coil only and here driving is more tricky. Are there special chips for this ?A see mainly 2 options:a) 1 full bridge bridge with 2 control lines.b) a single sink/source driver and a series capacitor (e.g. some 10-100 µF) and only 1 control.So far my idea would be an OP with sufficient output current, e.g. an TS912 driving a 12 V 15 mA coil.The idle power ( ~1 mA for 2 OPs) is still a little on the high side.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: July 10, 2020, 04:03:39 pm
content: Quick search turned up this:https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3288.html

author: poorchava
author_rank: Super Contributor
timestamp: July 10, 2020, 04:10:13 pm
content: I use a dual lowside mosfet drivers for this. By using a complementary driver such as tc4428 you can turn this relay effectively into a normal non latching one. Tc4427 required 2 control lines but reduces power consumption

author: Kleinstein
author_rank: Super Contributor
timestamp: July 10, 2020, 04:51:23 pm
content: 2 nice solutions, I did not have on my list. The gate drive could also be used with a capacitor.The maxim app. note has a nice idea to use single sided drivers and extra resistors. It need more current for a short time, but only a rather short time, so it should be OK in may case, especially as I have a ~20 V supply and could thus use relatively large resistors.

author: Simon
author_rank: Unknown
timestamp: July 10, 2020, 04:56:15 pm
content: Why not use a relay that is easier to drive. I am sure there are ones that you pulse to turn on and pulse to turn off. If you have to drive reverse polarity into the same coil then yes a H-Bridge that also turns off.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 10, 2020, 05:20:12 pm
content: Quote from: Simon on July 10, 2020, 04:56:15 pmThe small relays I have in mind are only available with 1 coil. The gate drives look good. A bit of an overkill to have a 1-4 A driver for a relay that only needs some 15 mA, but they are reasonable cheap and some (e.g. DGD0215) also have low power consumption (e.g. 50 µA) in static mode.I think I will go with a power shift register like STPIC6C595 or NPIC6C595 and resistors like in the Maxim app note.So I could relatively easy change from 1 coil to 2 coil.

author: Simon
author_rank: Unknown
timestamp: July 10, 2020, 05:29:15 pm
content: If you link to the relay it will be easier to see your requirements. Is size an issue with the project?

author: Benta
author_rank: Super Contributor
timestamp: July 10, 2020, 06:24:21 pm
content: For 15 mA, I'd consider analog switches, eg, DG 419, DG6xx etc. cheap and easy.

author: Simon
author_rank: Unknown
timestamp: July 10, 2020, 06:41:08 pm
content: it depends on what you want. i have used 2A brushed motor driver to supply signal power level PWM simply because there is nothing that delivers less with the same functionality. I actually worked out the best discreet designed I could and it was nowhere near as good and cost more....

author: Kleinstein
author_rank: Super Contributor
timestamp: July 10, 2020, 07:17:46 pm
content: Size is not really an issue, the board just gets larger.But the small relays ( FUJITSU FTR-B4CB012Z) are also relatively cheap and have gold contacts.https://www.tme.eu/en/details/ftr-b4cb012z/miniature-electromagnetic-relays/fujitsu/With other small form factor relays the 2 coil version can the contacts closer together and thus less isolation. So there is an advantage of using the 1 coil version, especially with small form factors.With 2 single sided drivers and 2 resistors the circuit and even the software side is essentially that same as with a 2 coil relay. It just needs a higher (e.g. 1.5 - 2 times) current to operate and the 2 extra resistors.

author: Simon
author_rank: Unknown
timestamp: July 10, 2020, 07:36:01 pm
content: Well you can drive this off a micro-controller with 2 small N channel and 2 small P channel MOSFET's, you can use the N channel in each pair to drive the P channel.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 10, 2020, 07:54:57 pm
content: I did this on a regular basis using the outputs of a 'HC595 shift register (the relays were single 5V sensitive coil), and a 100uF capacitor in series. Up to eight relays driven by a single IC, but one should not switch more than one at the same time. Wait a few ms until the next relay switches, to not create too large peak currents within the 'HC595 circuit.There was quite an amout of relays involved within the device, so at power-on there was a storm of clicking noise while the software switched all the relays on and off again to bring them into an known position.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 10, 2020, 08:50:49 pm
content: With some relays, especially the smaller, more sensitive one it may work directly from a 5 V powered 74HC595 with a series capacitor. It may need some 15 mA, but this can be OK for just 1 channel for a short time.It may not work wit all types though.

author: Simon
author_rank: Unknown
timestamp: July 10, 2020, 10:07:07 pm
content: i think the one used is 9V? 5V may be a little low, why not use the 5V version but this does want 30mA

author: Calvin
author_rank: Regular Contributor
timestamp: July 12, 2020, 06:22:34 pm
content: Hi,here´s a simple discrete circuit for a 12V TX-S Panasonic relay (2 diodes, Resistor, Cap and Transistor).The Cap may be chosen considerably smaller in value,depending on the current demand of the relay.regardsCalvin

author: retrolefty
author_rank: Super Contributor
timestamp: July 12, 2020, 06:33:08 pm
content: That's what I did on a project I made using a 5v low current single coil latching relay by just driving them directly from two digital output pins from an avr328 mc. I worried a little about reverse coil collapsing current spikes but didn't seem to cause any problems even after several years. Still probably not a recommended best practice method but it worked for me and my application.