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# Question Driving BiStable Relay

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#### xeeshan74

##### New Member
Hello
Recently I have been experimenting with one of my hobby project to control a relay with some Micro Controller and that part is been taken care of.

What I want to know is that I have a Bistable relay this one RT314A12 Product Details - Tyco Electronics

and I saw this circuit posted
EDN PDF

which does exactly what I am trying to acheive. Though my question is as per the relay above I need to change the resistors connected to the Darlington array , if Some one could explain to me the math behind it as to what we are trying to acheive and how it will work and what i sthe principle behind it. Please donot repond to this question if you dont know exactly what you are talking about and I dont want suggestion like , Well you can use some other Hbridge Driver or Blah Blah Blah .

and I am using ULN28003 and not ULN2003 , I believe the difference between these 2 is the number of outputs plus some perating voltages

Bluroom look at the PDF link I posted its the same symantic but driving the relay that I posted the link RT314A12
Thiis relay has following specs
* Contact Current Rating (Amps.) = 16
* Coil Resistance (Ω) = 360
* Coil Power, Nominal (mW) = 400
* Actuating System = DC
* Coil Power, Nominal (W) = 0.40
* Input Voltage (VDC) = 12
* Dielectric Strength - Coil Contacts (V rms) = 5000
* Tracking Resistance of Relay Base = PTI250
* Coil Rating = Sensitive

What i was asking what needs to be changed in the posted PDF symantec to drive this Bistable relay??

Ah it's latching opps. The resistors are the + path for each side of the coil, drop a resistor on your relay 4.7 to 47 may be fine. It's a really inefficient design as the 2803 will also have to drive current through the resistor too.

The TC4427 is probably a better IC for such a relay, or better yet use a dual coil relay.
https://www.robotroom.com/HBridge.html

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So your are saying drop the resistors to 47k instead of 4.7k on each side correct?
also what would you suggest to drive a single coil Latching relay??

Design constraint are
It has to be single coil not more than 12 v for coil volt.

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No 47K is way too high. 4.7ohms thru 47 ohms. You can get dual coil 12V relays, very easy to drive.

What design constraint? Why not use the TC4427?

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what exactly you mean by "4.7ohms thru 47 ohms" .

As I said earlier its a design constraint to use only single coil latching

Try energizing the coil with a resistor inline. Too low a value is wasteful, too high won't energize the coil.

Is it a contest? School project? Again the TC4427 would be my choice vs the UNL2803

Try energizing the coil with a resistor inline. Too low a value is wasteful, too high won't energize the coil.

Is it a contest? School project? Again the TC4427 would be my choice vs the UNL2803
Its not a contest nor a school project i am way too old for that.lol

I am an engineer (software) and its a design that we are pitching .. you can check my company @ **broken link removed** and I am Zeeshan you can check my profile there under about->management.

I can further discuss this with you but not in a public forum. If you feel like drop me an email @ zeeshan.taj@gmail.com.

What microcontroller are you using? Are you a software or hardware company?

We are software primarily.

Ok then, what microcontroller are you using and why not consider the TC4427?
Recently I have been experimenting with one of my hobby project to control a relay with some Micro Controller and that part is been taken care of.
Since it's a hobby project why not switch to dual coil relays?

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We are using Atmega168 along with this darlington driver to drive relay. There is a whole concept behind this.. All communication with avr is radio based.
So here is the design--- Radio Transciever --- AVR ---- Darlington driver...

Would you mind telling me the logic behind using resistors to drive this Bistable relay and when we are connection both sides of the coil to +(positive) and when opening drain on either end lacting the relay.. I know its all about Current flow through the coil but I really want to know the Math behind it. I mean the calculations of resistor to acheive this HBridge sort of operation.

It's a kludge, the 2803 can only pull down to GND. You need a positive voltage on the other side of the relay and the 2803 can't do that so that's what the resistors are for.

The correct value will depend on the relays minimum pull in voltage. Try 47ohms first and see if the relay pulls in, if not decrease to 33ohms etc...

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ohh so its a kludge .. interesting!
Could you give me a short symantic for TC4427 driving bistable relay??

Try reading the datasheet. It's a simple 1A to 3A dual half bridge, that and 4 diodes can run your relay no problem. Just hook it up like the robot room link I sent you.

Its really very simple and elegent I love the simplicity.
though I have 4 Relays to operate minimum 1 and max 4 .. independently.. do you think they have another version with more output lines??

A pair of SN754110 or L293D quad half bridges will work and they both have built in clamp diodes.

Hi,

there is another way controlling a latching relay with least pin count.

Check out the high speed dual MosFet driver type MC34151 (Motorola).

It's an independently controllable dual gate with 1.5A totem pole output.

It allows omitting the resistors in series with the relay coil, thus not deteriorating switching capability.

Datasheet attached.

Boncuk

#### Attachments

• MC34151.pdf
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Driving Bistable relay using ULN28003 Trouble

Ok Boncuk and blueroom I made the symantic to show you my connections and as per Blueroom I reduced the resistors step by step to nearly 20 ohms and the relay would not respond. the Only way I made it to latch was directly applying power to it from the adaptor.
Boncuk I remeber you have solved this problem before , and I cant seem to remember which forum was it , this was the exact problem and you gave some suggestion which eventually solved the issue. Please look at the symatic and let me know what I am doing wrong . I have included all the related data in this JPG file. Thanks in advance

#### Attachments

• RelayDriver.jpg
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