pwm water methanol pump

Hi all, Apologies for the wall of text, but wanted to give as much relevant information as I could. I'm not 100% if this is the correct forum, but as my questions relate to the use of a solid state relay I thought the relay forum to be most relevant. After some help regarding a project of mine for my car. I am fitting the car with water methanol injection, and want to run the setup roughly proportional to fuel. I have a nozzle sized to 15% of the total fuel injector size, and a 12 volt 10 amp (rated) DC pump that I wish to PWM off the fuel injector earth signal from the ECU. As a result, as the injector signal increases (anywhere from 0-100% and 0-60hz as an example) the pump's duty cycle and pressure will roughly match, giving a more proportional flow than an on-off system. To do this, and after some reading, I have sourced a "Mager" 40amp DC-DC solid state relay that I think should do the job. The system also uses a pressure switch to power the relay, so the relay will turn only about 10% of the time the car is running, and the switching signal and load will be "disconnected" the remaining 90%. Some points I need help with: -I intend to supply 12V/battery power from the pressure switch to the positive switching side of the relay, and the injector earth signal to the negative, after using an oscilloscope to view the signal on the earth (worrying about the quality of the signal and spikes from the injector when closing), I established the following: 1.The earth has a battery voltage supply most of the time, equivalent to the input or positive side. 2. The PCM then applies earth to open the injector, dropping the voltage near 0volts, (this is the bit I need to "copy") time at 0 volts varies with injector opening time. 3. Once the earth is released, the injector causes what I assume is EMF, a spike lasting less than 2ms up to 26 volts, then returning to battery voltage before the next opening. After some more searching I found out this spike could damage the relay and destroy it. My questions: 1. Will I need to protect the input side of the relay with a diode against the 26V spike? If so, what size/type etc. diode should I use and where? Or will the duration of the spike being within the relays operating range (3-32V) not cause problems? 2. Will the 10 amp DC pump on the output side cause any issues for the relay as it is an inductive load? Or will the relays 40 amp rating withstand this? 3. Are there any other problems that I may encounter with this design? Such as an effect on the injector opening time or PCM operation? Or damage else where? If you read through the whole post, thank-you for your time and any input you have to offer.

Any time the voltage is removed from the coil of a relay, the coil itself produces a spike that is much more violent than the 26 volts you are seeing. That will not damage the relay. The diode will protect the controller from the spike caused by the relay. Any 1 amp diode will be sufficient. 1n4001, and any number between and including 1n4007.

Thanks for the reply! The relay i'm using is a photovoltaic ssr, not a standard mechanical coil-operated relay, will this relay still produce a spike?

A solid state relay will not produce this voltage.

Ok, but will I need to protect the input of the SSR from the voltage spike on the injector earth? And will the pump cause problems on the output side?

I have no idea, do you have a spec sheet on the relay?

Load Current: 40A Load Voltage: 5-220V DC Actual (24-220V DC MArked) Control Voltage: 3-32V DC Control Current: 5-25mA DC On Voltage: ≤2.3VDC Off Leakage Current: ≤2mA On-off Time: ≤10mS Dielectric Strength: 2000VAC Insulation Resistance : 500M ohm/500VDC Ambient Temperature.: -30℃～75℃ Working indicators: LED Certification: CE

Your 26 volts is below the 32 volt upper end of the control voltage.

That's what I was hoping for, that the relay would cope with it. Will it matter having battery voltage on the positive side (~14volts) then the 26v spike on the negative side for 1ms connected to the switching terminals of the relay? I'm concerned it will force voltage the wrong way through the relay.