What is the most economical way to drive an array of single coil latching relays?

Question

Please read the question to the end. The answer to this question may be non-trivial.

Replacing a number of 2-coil latching relays with single coil latching relays to save space on the board. The trouble: single coil relays are a lot more trouble to drive with the current reversal requirement.

Currently low-side switches such as darlington arrays or mosfets are doing the work and they cost pennies. Here are the common solutions I've thought of which do not work in our case:

1. An array of h-bridges: Too expensive and not enough room on the pbc board.

2. The method shown in the picture below (from Maxim) for every relay driven by two low-side switches: The relay coil voltage is close to Vcc (3V) which yeilds low R values and a total current draw above what the PS can provide.

3. Capacitor Charge and discharge drive: This method requires large capacitors that are physically too large to fit in the enclosure (defeats the purpose of using single coil relays).

4. An array of analog switches (one per coil as an on/off switch and two to flip Vcc and Gnd and reverse pol): This might work, however analog switches with low on-resistance are expensive compared to low-side mosfet or bjt switches.

What other solutions are recommended in this case?

Answer 1

One approach would be to control N relays using N+1 half-H-bridges. One of them connects to one wire which is common to all the relays, and the others connect to one relay each. A variation on this would be to arrange the relays into a matrix, with a half-H driver for each row or column (the N+1-half-bridge solution is simply a 1xN matrix). One caveat with this approach is that other relays--especially those in the same row or column as the driven relays may receive a substantial fraction of the drive voltage. In an NxN matrix, the voltage fraction will be (N-1)/(2N-1), which would be 1/3 for a 2x2 matrix, 2/5 for a 3x3 matrix, and would approach 50% as a matrix got bigger. For a non-square matrix, things would be worse.

If you can be sure that relays will not fire unless they receive more than half the rated drive voltage, a square matrix approach might work. Even the relays which don't fire would gobble significant current (as the matrix gets bigger, the voltage across the relays in the same row and column as the driven row would approach half the drive voltage, so figure current similarly); if one used an 8x8 matrix, the total current gobbled by the inadvertently-driven relays would be about 7x the current going to the deliberately-driven one. Not great, but being able to drive 64 relays with 16 half-bridge drivers might be worth it.

Answer 2

Someone asked a question about a mystery circuit here a while ago, that lead to a pretty damn clever way to single-coil latching relay with one IO line.

the 100Ω resistor is the relay-coil. Note that you will need to size the capacitor (220 uF in the image) to properly drive your relay coil, and you should test the circuit with a fairly worn-out relay, in case the required coil current increases as the relay ages.

Image is a link to a simulation of the circuit.

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FWIW, I think your design constraints as you have outlined in the question are kind of ridiculous. You've pretty much ruled out all the ways there are to drive relays.

1. An array of h-bridges: Too expensive and not enough room on the pbc board.

   You can't afford 4 cheap BJTs? How can you afford the relays? You do know you don't need to buy pre-built ICs for this, right?
   Since you really don't need much current gain, you can always build your own h-bridge using two NPN transistors, and two PNP transistors. It sounds like you're only looking at ICs for some reason.

   Also, it sounds like you really haven't done much shopping in this area. You can buy low-power integrated H-bridge ICs for ridiculously low prices. For example, the LV8548MC-AH is a dual h-brige driver IC for $1.41 each in single quantities. The MPC17C724 is similar, and is $1.15 in single quantities (though a slightly less easily assembled package).
   Each of these devices could drive two relays.

   If you're really, REALLY price constrained there are even cheaper alternatives out of china, but in that case, why are you using expensive latching relays?

2. The method shown in the picture below (from Maxim) for every relay driven by two low-side switches: The relay coil voltage is close to Vcc (3V) which yeilds low R values and a total current draw above what the PS can provide.

   This seems like a really silly way to drive the relays. The only reason I can see for doing this is if you have to use a demultiplexer to drive the relays, and can therefore only set one IO line low at a time.

3. Capacitor Charge and discharge drive: This method requires large capacitors that are physically too large to fit in the enclosure (defeats the purpose of using single coil relays).

   How small are your relays? I've shopped around a fair bit for latching relays, and I don't think I've seen any that are even similarly sized to reasonably priced large-value low-voltage capacitors.

4. An array of analog switches (one per coil as an on/off switch and two to flip Vcc and Gnd and reverse pol): This might work, however analog switches with low on-resistance are expensive compared to low-side mosfet or bjt switches.

   This is going to be MUCH more expensive then the H-bridge option, and offer negligible benefits.

Answer 3

Number 2, in my oppinion, is the way to go. When you're saying that you are rather limited on the power supply side, perhaps you could add a resistor in series after the power supply, so that you don't load it too much. This would of course require some rather large capacitance to be placed after the resistor.

Please forgive the ascii art schematic, CircuitLab is not working as expected let's say...

V+ ---\/\/\------ To Relays
               |
              ___
              ---
               |
              GND

You're saying that you are also limited regarding the size of the device. The capacitor does not have to have gazilion Farads, a few hundred micro would do in my opinion. How big these relays are / How much current do they need to latch in?

Answer 4

This is an old topic, but I hope my answer is still useful.

The schematic below shows how to drive a single-coil latching relay from one logic pin, provided that there is a symmetric power supply for the relays. I don't know whether such a supply could be made available by the original poster, but if so the circuit can be built very cheaply.

The circuit can be adapted to circumstances by selecting the appropriate values for the resistors and the capacitors, depending on the logic voltage, the available current from the logic output, the relay coil voltage, and the required duration of the pulse. R1 & C1 determine the pulse duration, while R2 determines the base drive current for Q1 or Q2.

Relay voltage and logic voltage can be chosen independently, the 12V for the coil I've shown is just an example. Also, the ground potential for the logic and for the relay can differ to a certain extent.

The transistors will switch off relatively slowly, therefore there's a good chance that freewheeling diodes for the coil won't be necessary. Check the circuit with the actual relay in order to determine the resulting pulse shape.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 5

Similar to your option 4:

Look at a TS3A5223RSWR 0.45Ω Dual SPDT Bidirectional Analog Switch from TI:

This device will operate with supply voltages up to 3.6V. Note maximum DC current is +- 300mA and VIN-MAX is 4.3V (Absolute Maximum Ratings)

Wire it like an H-bridge: Connect the coil across the COM1 and COM2 pins. Connect NC1 and NC2 to your positive coil supply (via current limiting resistor if needed). Connect NO1 and NO2 to GND. Use the truth-table in the data sheet to control direction and ON/OFF (probably using 3.3V logic).

If you wish to control the direction with a single I/O, you could connect NC1 and NO2 to your positive coil supply (via current limiting resistor if needed) with NC2 and NO1 connected to GND. You would then tie SEL1 to SEL2 and connect this to your I/O. With this method, you would need to control ON/OFF with another device (possibly a single device for all coils).

Note that this device features break before make so shoot-through is not an issue.

Being in a tiny 1.8 x 1.4mm QFN-10 package, it is very economical in terms of space. At $0.213 (US Dollars, 1000 piece price - Avnet - 11/18/2013), it is not overly pricey.

Note that since you are switching coils, you should clamp the voltage on each end of the coils to the supply and GND (DC common). Although the pins are ESD protected, the datasheet mentions that you should not rely on that for normal conditions beyond the Absolute Maximum Ratings.

Answer 6

The issue with the resistive solution is that the switch sinks current from both the relay coil and the resistor to +V.

If that resistor could be arranged to increase when the switch is closed, the excess current could be eliminated. Or if the resistor were initially high and the opposing one could be arranged to decrease. Perhaps something like this...

^{simulate this circuit – Schematic created using CircuitLab}

I realise that this is tantamount to an H-bridge with simplified drive logic, but given a suitable transistor choice I don't see how you could call it large or expensive. Perhaps it just needs a different name?

(EDIT: added protection diodes)

Answer 7

This is a solution I've ben using for a number of years

Answer 8

For an H bridge solution needing only moderate output power, something like the TI ABT or ACT basic logic series can drive about 5V and +/-30 mA. Very cheap and compact. A little bit more expensive, but the TCA series of IO expanders can also do the same job.