In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Jesse,

I agree with Chris - you need the serial resistors - they limit the current that flows from the SX outputs (when high) into the bases of the driver transistors.

I'm not sure what transistor types you use to drive the TIP120s, so I can't tell if these drivers invert the logic, i.e. if the TIPs will conduct, (i.e. the solenoids will be activated) when the SX pin is at low level, or has high impedance. This might cause dangerous situations.

As you know, all SX port pins are configured as inputs (having high impedance) on reset or power-on. With an inverting driver, this would mean that all solenoids would be activated whenever the SX is in reset state, or when it is "running wild" by some reason, leaving the port pins at high-Z (inputs). Therefore, such circuits should always be designed in a way that makes sure that solenoids, relays, etc. will only get activated with a high level at the driver inputs. You could simply pull the SX out of its socket (I assume it has one :-) ), and see if the solenoids are being activated then, or not (they should not).

In one of my SX-based projects, I had to PWM-control the speeds of DC motors, each of them rated at 24 Volts, 10 Amps max. For this project, I used IGBTs (Isolated Gate Bipolar Transistors) to drive the motors. The nice feature of IGBTs is that they have a relatively high input impedance, similar to FETs (i.e. low input power), combined with the low Uce saturation voltage of bipolar transistors when "turned on". I have attached a modified schematic of my project, showing how the IGBT could drive a solenoid instead of a motor. The IRG4PC40KD I'm using can handle up to 600 Volts with a collector current of up to 25 Amps, a saturation Uce of 2.1 Volts at a gate-emitter voltage of 15V.

The schematic shows a driver circuit with a 2N3906 and a 2N3904. I'm using two transistors to make sure that the IGBT only conducts when the SX output is at high level (see above). The IGBT I'm using requires about 15 Volts at the gate to get "turned on". As the maximum allowed gate-to-emitter voltage for this type is 20 Volts, I'm using the 1.5K and 1K resistors to limit that gate voltage.

Please also note the diode in parallel to the solenoid. This diode should be a fast switching diode, capable to short the reverse current generated by the solenoid when being turned off. Without such a diode, the IGBT (or any other driver transistor) might be killed. As you can see, this IGBT has a "built-in" recovery diode but this can't kill the solenoid's reverse current, it is only useful when the IGBT is used in bridge configurations.

This type of IGBT would definitely be an "overkill" for your application (maybe not for the "large" solenoid you had mentioned), but there are other types available from various manufacturers that can be directly controlled from an SX output pin, i.e. which are "turned on" with 5 Volts Uge.

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