H Bridge - 6V latching solenoid driver

[will_raymo2000]

Hello all,

Some background on my project. This urinal flush controller has broken, although the valve still works fine, and the replacement cost is eye watering. http://www.gentworks.co.uk/Cistermiser-IRC2-sensor-with-ceiling-kit.html?gclid=Cj0KEQiA8orFBRCEpODivaOft_EBEiQAy3mlfbQg024gg2isREVMrlp2wzW4Tl9wm1uKW5tg1pPQoj0aAt3L8P8HAQ I have decided to modify it to just run on a timer rather than be PIR actuated.

I have mocked up an H Bridge design based on this circuit https://electrosome.com/dc-motor-driving-using-h-bridge/ but using TIP31C and 1N4148 diodes plus 220ohm resistors as opposed to the 150ohm on that website. Set up like that and running off a 5V supply (I don't have a 6V to hand), it works absolutely fine although current is only 500mA. The solenoid coil is a single coil and is rated at 7.5w at 6v giving a resistance of 4.8ohms and requiring a 1.25A current to actuate. So under load the solenoid probably wouldn't actuate properly at that low current.

Now my questions are, what is the best way of incorporating capacitance to allow the current to be higher when a pulse is sent through the H bridge? Also, what would be the simplest way to implement a timed pulse? I have a few PICs around which I guess would do the job but I'm not sure how to best implement them. I'd probably want the solenoid on for 10 minutes in an hour.

Thanks,
Will

[Ian.M]

I think the electrosome H-bridge should be taken as a 'Ladybird books' level introduction to the circuit.   When you've only got 6V to play with, you cant afford the Vce_sat drop of bipolars, let alone the Vbe drop of high side NPNs.

Take a look at TI's DRV8838 (or 8837) low voltage MOSFET H-bridges.  They should be a good match to your application.  Unless you are set up for SMD reflow assembly, you'll need a breakout board.  e.g. https://www.pololu.com/product/2990

A PIC makes sense - just make sure you use a modern XLP one if its using batteries.  The code will be nothing special - you could just let it idle along from its low power low speed oscillator at about 7.5K instructions/second, with a 1 second sandwich delay superloop and count loop passes for all timing etc.   The low power INTRC oscillator typically isn't very accurate but I doubt you'll care miuch if there's a +/- 20% timing variation.  If you need better accuracy, use the lowest divider from INTOSC, but then you'll probably need to implement some sort of power saving.

[NivagSwerdna]

I have mocked up an H Bridge design based on this circuit

Ah.  A bit like...



?

Seems these need only a short 20ms pulse to switch direction. 

There is a project here... http://vanderleevineyard.com/1/post/2015/04/arduino-control-for-vineyard-irrigation.html which is a vineyard, slightly more glam than a urinal. 

If you can wait for the slow boat from China on EBAY for <£2 you can get a L298N module and re-use parts as necessary.  You can estimate how much energy you need to expend in 20ms.  It won't require a big capacitor.  The linked article uses L9110S and that's <£3 from a UK seller.

[will_raymo2000]

Thanks for the suggestions, I have ordered a couple of the L9110S drivers from hobbycomponents to have a play with. Will report back.