Latching relay driver

[baba_sanfur]

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

 

[BeanieBots]

You don't actually need both sides of the bridge.
Just use one half that can switch between 5v and 0v.
Then put a cap in series with the coil.
The cap needs to be large enough to energise the relay when charged to 5v.

Switch to 5v, the cap charges through the rely and it energises.
Switch to 0v, the cap discharges through the relay (current flow is reversed) and the relay de-energises.

 

[leftyretro]

You don't actually need both sides of the bridge.
Just use one half that can switch between 5v and 0v.
Then put a cap in series with the coil.
The cap needs to be large enough to energise the relay when charged to 5v.

Switch to 5v, the cap charges through the rely and it energises.
Switch to 0v, the cap discharges through the relay (current flow is reversed) and the relay de-energises.

I used that method and it worked fine for me on a 30ma rated 5v single coil latching relay: http://www.picaxeforum.co.uk/attachment.php?attachmentid=320&d=1197268121

However the OP states that it's a 200ma rated coil so it may take a rather large capacitor and there will be a charge/recharge time factor also to be considered that could limit how fast it could set,reset and set again.

Lefty