Electro-hydraulic actuator

[NickMu]

Replace the SPDT unit with two SPNO units.
Your logic should actuate none or only one of the SPNO units at any given time.
Both SPNO units ON is what you want to stay away from.

Nick

[mackrackit]

I always seem to end up up using automotive SPDT for things like this, mainly because they are easy to find and reliable.
Use two and have the coils wired together so they are both energized at the same time giving you a DPDT. Be sure to use a fuse just in case....

[HenrikOlsson]

Hi,
I think you are making it more complicated than it really is....
If all you need is to be able to drive the motor in both directions as well as not drive it at all you need two SPDT relays (RE1 and RE2 in the below (very crude) schematic), each driven by an output on your PIC.

Here's the crude picture:



Both relays are, as usual, drawn in their "inactive" state, both sides of the motor is connected to the positive side of the supply and no current will flow thru the motor - it won't go anywhere. To run the motor in one direction activate one of the relays, to run it in the other activate the other relay. If you activate both relays both poles of the motor will be connected to the negative side of the supply and the motor will not run.

/Henrik.

[amgen]

yes, and a good ckt for spike reduction and inductive energy dampening for bi-directional ckt, using 1 full wave bridge.

[Steve_88]

Thanks for the explanations/examples, that is a huge help
I'm not completely clear on using the full wave bridge though, are you using just two of the four diodes?
Thanks again

[Chirpy]

the 4 diodes in full wave bridge will convert the positive and negative parts of AC waveform into a positive only waveform that has both of them bouncing on the positive side. to convert it to a clean dc signal, you need a large capacitor which will take care of all of the ripples and will create a clean dc signal. if you only used two diodes (half wave), it just cuts the ac signal's negative off and only gives out the positive pulses from the ac. if you look at the waveform through an oscilloscope of a half wave bridge, where the negative pulses were at, it's just ground/0v.

easier way to think of it is full wave bridge's waveform is "bump bump bump etc" while half wave is "bump nothing bump nothing etc".

[HenrikOlsson]

Chirpy,
That's what a full wave rectifier normally is used for but it has nothing to do with this application.

Here it is being used to provide a circulating path for the motor current when the contact on either side switches from one state to the other.
* Lets say the left contact is in the "up position" and the right contact in the "down position".
* The current is flowing from left to right thru the motor.
* Now the left hand side contact is switched to its down position in order to stop the motor.
* Just as the contact opens the inductance of the winding in the motor will cause the voltage on the "left side" to rise abrubtly, if the diodes wasn't there this voltage would casue an arc to develop across the now open left contact. Because of the diodes the voltage spike is clamped because there's a path thru the motor, down thru the right hand contact and back up thru the left hand side diode.

The reason for using a bridge rectifier like that is, I guess, due the spade terminals making it easy to connect when using an automotive type relay like the one in the picture. Two single diodes would work just as fine (and be cheaper).

/Henrik.