This application is for an autononoumous agricultural field planter. I need to turn the power on to a 24VDC water pump on the planter with a BeagleBone Black so long as the water pressure, monitored by the BeagleBone, is within range. The pump has a pressure switch that runs the pump to keep the water pressure in the tank in range so we just turn the power on to the circuit with the BeagleBone. It’s essentially a safety mechanism. Once turned on, the power stays on unless the BeagleBone’s pressure sensor detects an issue.
I’ve been trying to design a MOSFET based control circuit but found the Crydom CMX60D20 SSR. I chose this one because the temperatures could get high in the field and when I derate this unit per the spec sheet it should be able to handle the 6 amp normal running current of the pump motor (@24VDC) even if temps in the box get in the 160F range. (We’ll shade the box but otherwise not have any extra cooling planned at this time.)
So, will this SSR do the job?
Is there a recommended method for cooling it? It doesn’t appear to have any heat sink attachment capability. What is best practice?
Hello @walterc,
Excellent, focus on the worst-case situations.
Keep going with failure modes:
Thermal: The steady state power is trivial at 6 A * 0.0033 Ω. That is, unless you PWM the motor as that would add switching losses.
Technology: Why an SSR and not a mechanical contactor?
Sincerely,
Aaron
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I sure appreciate the fast response!
I do not know how long before the fuse blows if the pump were to completely sieze. This is a diaphragm pump so that isn’t usually a problem though. We do have a fuse in the circuit and I think it would blow quickly.
This is a completely isolated, self-contained system so we aren’t concerned with lightning in the manner we would be if it were connected to mains. EMI from a nearby strike could cause damage but I suspect if that happened the whole system (MCUs, motor drivers, everything) would go kaput. This system is optically isolated from other modules so surges are only coming from the motor and switching.
We don’t PWM the motor. So what is causing the heat I’m seeing? What causes this kind of heating in a MOSFET?
We are concerned about vibration affecting a mechanical contactor and the arcing from the contacts opening and closing.
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I also want the optical isolation from the BeagleBone Black. I could do that still with the same optical isolator if required
Conduction losses in a FET are modeled as a resistance, with a resulting power dissipation equal to I2R like any other resistor. Cooling of a device like the one in question would typically occur by convection. Presence of a fan providing even a small amount of forced airflow can substantially improve heat transfer.
Device ratings seem reasonable for steady state, though effects of startup current and any transients that occur during turn-off due to load inductance should be evaluated. Ultimately, it’s up to you to determine whether or not a given item is suitable for your specific application.
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