I have an application where I currently use 2 STDT mechanical ralays to switch to solenoid control wires between the existing controller and the solenoids that control hydraulic pressure.
Basically, the relays take the following circuit
Cntrl 1 ----------------> Sol 1
Cntrl 2 ----------------> Sol 2
And when the relays are switched on the circuit becomes
Cntrl 1 ----------------> Sol 2
Cntrl 2 ----------------> Sol 1
This mechanically binds the device as long as the operator seems necessary via a pushbutton.
This works fine for the current use, but I would like to apply PWM to the relay control so that the device can be "partially" unbound and allowed to turn slowly. To do this would require the relays to pulse on approx 50hz and 25% duty cycle, but that can be adjusted to the individual machine/operator using trim pots if need be. Cannot see it ever needing more than 60hz and 75% duty.
Obviously I first looked into solid state relays. To re-create my circuit with SSRs would require 2 NO, and 2 NC, setup to switch simultaneously. There are plenty that fit my needs with a couple exceptions I'm not clear on. Current leakage and current drop.
In majority of the machine operating time, the relays connecting the circuits would be NC. But there will be normally open relays on standby ready to connect the feeds to opposite circuits. That current leakage I worry about as the solenoids are fed a 500mA to 1.1A current depending on the controllers programming for the conditions. Will current leakage from NO circuits then add to the current on the NC circuits? And vice versa?
Also, in NC condition, will the voltage drop of a SSR relay be enough to significantly alter the supplied current from the controller?
I am not concerned with gate voltage as I can use opto isolated SSR's.
The controllers outputs are not easily altered to compensate for current changes introduced by my circuit, so for machine parts life, the current passed to the solenoids needs to be minimally altered. However with that said, std 12v automotive relays have served me well without PWM.
So basically I need the existing controller to operate as normal until a button is pressed, then the ssr will feed each solenoid with opposite signals. When the operator presses a 2nd button momentarily, he should be able to pulse the relays to allow the machine to creep at slower than normal operating speed by remaining partially bound hydraulically.
This is all kind of like a transmission brake on a race car, but with a pair of current controlled solenoids rather than a single 12v on/off solenoid. (That's where I got the idea for my first implementation and the current idea for pwm the solenoids) but this is on a transmission driven jet drive for a boat.
I'll add a diagram showing the current mechanical relays as soon as I get in front of a pc and off my phone.