Latching relay driver

[baba_sanfur]

transistor relay driver

Hi All,
I need help in designing a latching relay driver. The relay is single coil and need 5v @ 200ma. The problem is that my MCU uses 3.3v GPIO.
The system must be extremely low cost, and consume as little power as possible.
I implement it by 4 MOSFET (2xN-ch, 2xP-ch) as an H bridge. The problem is with the voltage levels: to change one set of FETs (P + N) I need 5v or 0v – which I don’t have (MCU GPIO=3.3v). I tried to add 2xnpn transistor to drive the MOSFETs (ugly solution) and it didn’t work at all.
I added the circuit that I used; can anyone help my in designing this driver? Or have other solution to my problem (remember – low cost and power).

Thanks,
Nir

 

[pkuhar]

latching relay driver

what is the vcc voltage?
your circuit should work, but i'd use a lower value for pull-up. ex. 10k

 

[baba_sanfur]

latching relay drive circuit

the Vcc = 5v.
I tried to change the pull up to 10k, but it still doesn't work.
The current needed to operate it is more than the current limitation of my DC power supply (i set the limit to 400ma, otherwise - the transistors might toasted..) and the relay does not change state.

any idea way?

 

[pkuhar]

latching relay transistor

What type of a relay do you use?

Can you measure the voltages on the transistors with all 4 combinations and post it back.

400mA should be more than enough. Most relays are 200-400mW. That is max 80mA @ 5V

 

[antedeluvian]

relay driver (latching)

If you can use a 2 pole relay you can simplify the circuit quite a bit.

Connect your output to the base of an NPN transistor through a resistor as in your circuit. Connect the emitter to ground and the collector to the one side of the coil. Connect the other side of the coil to 5V. Connect the common of the second set of contacts to ground. Connect the normally open contact of the second set of contacts to the collector of the transistor. When the relay is energised, the contacts close and the relay latches itsef on, irrespective of future states of the transistor. The current used to drive the base should be 10 times greater than the collector current divided by the forward current gain to guarantee that the transistor is saturated.

B.Ib=10 Ic

The current is determined by the base resistor: Ib=(3.3-0.6)/R, assuming that the micro does guarantee 3.3 Volts output. Your micro must be capable of supplying this current.


If you need to be able to reset the relay, you will need a switch like a transistor to open the circuit to the coil. You could switch the 5V to the coil through a PNP transistor, or the ground connection to the contact common using an NPN.