SIngle Capacitor providing reversing pulse for latching solinoid

[Water_Dr]

Hello,

I am building a timmer control for an irrigator and need to control a latching solinoid.  I am using latching as the system must be powered by batteries (and I am trying to avoid the additional cost of solar). 

I have many times created simple switched control of the solinoids that require a short pulse + - to latch and a short pulse - + to delatch using a 2200uf capacitor in series to one of the lines and a SPDT to charge (create latchin pulse) and discharge (create delatching pulse).  I could simply use a SPDT relay to activate the solinoid but am trying to keep the current consumption down.

The attached pdf shows the SPDT switch that works fine.  The other diagrams shows a transistor arrangement I thought would work but does not.  The NPN is a BC547 and the PNP is a BC556.  I am no electronics expert and really a novice, but this transistor circuit does not appear to be creating enough pulse as the solinoid only makes a dull thud noise and does not latch, even though the capacitor is charged.

Would some sort of N and P Mosfet arrangement work better possibly?

Any suggestions would be much appreciated.

Regards Blair

solinoid_control.pdf

 

[Hero999]

Both transistors will turn on simultaneously and if the base resistors are low enough the transistors will overheat.

How much current does the relay need?

 

[Water_Dr]

I thought as 1 transistor was NPN and the other PNP only one would be on at a time as the base input was alternated between VCC and G?

Regarding the relay.  It is a latching solinoid that takes a short pulse to latch and a similar but reverse polarity pulse to unlatch.  I am struggling to find any data on them as they are normally hooked up to the manufacturers controllers, but these are not suitiable for this application.  I only measure about 5 ohms across the 2 wires, but do not know the configutation inside as I beleive there are a couple of coils inside these things.

Thanks for any help you can offer, Blair

 

[Hero999]

I thought as 1 transistor was NPN and the other PNP only one would be on at a time as the base input was alternated between VCC and G?

Yes but when the base is connected to neither ground nor +V, both transistors will be on at the same time, allowing a large current to flow.

Regarding the relay.  It is a latching solinoid that takes a short pulse to latch and a similar but reverse polarity pulse to unlatch.  I am struggling to find any data on them as they are normally hooked up to the manufacturers controllers, but these are not suitiable for this application.  I only measure about 5 ohms across the 2 wires, but do not know the configutation inside as I beleive there are a couple of coils inside these things.

What's the operating voltage?

You should then be able to work out the current using Ohm's law.

 

[Water_Dr]

Its 12 volts so if the coil is 5 ohms (which seems very low?) it must be drawing 2.4 amps, which is probably why the circuit won't work.

I did play with a couple of mosfets, 1 P-Channel and the other N-Channel.  I managed to get them to latch and delatch the Solinoid individually but could not use a common control line.  I have attached a drawing that shows when I commoned the gates to one point and run high or low signal to them from a SPDT switch is fails to operate (it all got very hot at one stage and melted and I think my drawing has the source and drains around they way that was working - still do not know what I did wrong and have no more mosfets at the moment to try). 

I think the mosfets are interacting with each other some how as the capcitor is discharged/charged still but the pulse must be dampened or something.  I do not have a scope to check.

Thanks Blair

View attachment 41108

 

[Hero999]

The MOSFETs are configured as source followers. so there's a lot of voltage loss then there's the risk they might both turn on.

I'd recommend using an h-bridge IC such as the L293.

Link to datasheet.
http://www.st.com/stonline/books/pdf/docs/1328.pdf

Rather than using a large capacitor, use a full bridge (see the motor reversing circuit on the datasheet). Unfortunately, the maximum peak current is only 2A per channel but it's easy to get round this by connecting two outputs in parallel.