Designing Transistor-based switching circuitry for 68HC1x Interfacing:
(Seat of the pants engineering)


This page will take you through several practical design problems illustrating the design of simple transistor circuits used to allow digital 68HC1x outputs to control high current devices such as LEDs, relay coils, motors etc.. In these discussions, we will primarily dwell on NPN bipolar transistors, although PNP and MOSFET transistors will also be discussed.

Starting point: Don't expect the 68HC1x outputs to source/sink loads of more than about 1mA. To do so may well result in eventual failure of the 68HC1x port output circuitry. We will first look at a relatively low current application (driving an LED or opto-isolator input), and discuss how to size various resistors and to choose suitable transistors for buffering the current. The following figure displays two common methods of buffering a microcontroller output to enable driving of a high current load. Note that the circuit shown in (b.) is crossed out. Although this circuit uses fewer components (1 less resistor), it does not assure protection of the 68HC1x. Should the collector-to-base junction of the transistor breakdown (e.g. due to a large back EMF spike, should the load be an inductive motor instead of an LED, or due to transistor failure due to a short circuit of the load etc.), the output of the 68HC1x could be destroyed. The base resistor (R_b) of figure a.) assures effective isolation against such a catastrophe.

Typical transistor circuits for buffering of 68HC1x outputs.

Now lets talk about "seat of the pants" engineering. We will commonly use wise "practical" practices such as derating specified component values in order to assure the long term reliability of the circuit. Consider the following specifications: LED from Radio Shack:
(I_LED)_max = 20mA (we'll derate this to 10mA).
(V_LED)_forward = 1.7V .

2N2222 General Purpose (Low Power) Transistor from Radio Shack
Case style (TO-92 plastic case -- low power!)
Max. power dissapation ~ 500mW --- definitely derate this to, say, 250mW (to be safe)
I_c max ~ 100mA -- derate to 67mA.
Breakdown voltage- collector to emitter ~ 40V --> shouldn't be a problem as we're only using 5V !

When the transistor is off - it dissapates no power. When the transistor is on (assume it's in saturation) then P_D = (V_CE)_sat x I_C ~ 0.2V x 10mA = 2mW.
Since 2mW << 250mW, power dissapation is obviously not an issue.

Calculating a value for R_C: Use Ohm's law -- R = V/I.
V_Rc = 5V - (V)_LED - (V_CE)_sat ~ 5V - 1.7V - 0.2V ~ 3.1V .
Now choose R_C = 3.1V/10mA ~ 330 Ohms (closest standard resistor value).

Calculate a value for R_B:
Although h_fe is advertised to be on the order of 100 for the 2N2222 or 2N3904, this current gain tends to drop significantly when I_c is large (i.e. when the transistor is conducting large collector current) or when the transistor is well within the saturation region. Thus, in calculating R_B we will use h_fe = 100, then to be certain we drive the transistor hard into saturation, we will reduce R_B to 1/3 of the previously calculated value. To assure good isolation, however, we will not reduce R_B below a value of 2.2kohm.

Thus, I_B = I_C / h_fe ==> I_B ~ 10mA/100 = 0.1mA. Now we calculate the voltage across R_B as V_h(68HC1x pin) - V_BE. The 68HC1x output high voltage [V_h(68HC1x) ], (under load - sourcing about 0.1 mA), I would guess to be about 4.5V. One could actually look this up in the Motorola spec sheets, but I'm sure this guess will get the job done for our purposes. Thus the voltage across R_B ~ 4.5V-0.7V ~ 3.8V. Thus by ohm's law, R_B = 3.8V/0.1mA = 38kohms. To assure that we drive the transistor hard into saturation, we divide this value by three, and use an actual value of:
R_B ~ 10 kohms.
This will still provide good isolation of the 68HC1x pin should the collector-to-base junction of the transistor breakdown. {Imagine that the junction did break down while the 68HC1x was in the "low" state (V_low approaches 0 volts)-- (68HC11 output low voltage). Then R_B would have as much as 5 volts across it, but using a minimum level of R_B = 2.2k would limit the pin current to about 2.25 mA, which is safely below the "absolute maximum" 10mA rating published in the Motorola 68HC1x specifications.}

Now the prudent thing would be to prototype the circuit, and actually measure pertinent voltages and currents, and adjust values if necessary! For example, if we have the LED turned on, the voltage across the transistors collector-to-emitter should be less than 0.2 Volts. If not, we would make R_B smaller and see if the collector-to-emitter voltage drops. Thus we would experimentally assure that the transistor goes well into the saturation mode.

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Design Example for a Higher Power Load

Let's say we need to drive a 5V relay coil that pulls 150mA. The transistor, in saturation, would have to dissapate as much as V_CE (sat) x I_C ~ 0.2V x 150mA = 30mW. The 2N2222 or 2N3904 can certainly handle this, however the 150mA exceeds their maximum specified collector current of ~90mA. Thus we need a higher power transistor. We also note, that with a collector current of 150mA, even with an h_fe of 100, the 68HC1x pin would have to source 1.5mA of current, which is too much. A good solution is to consider a "Darlington pair" transistor[such as the TIP120 - available at Radio Shack] - (which has an "effective" h_fe of well over 1000! Another effective solution is to use an "N-channel" MOSFET enhancement-mode transistor as a replacement for the NPN transistor. [Even though a MOSFET draws no steady state current from the 68HC1x output pin, its still prudent to use a resistor between the 68HC1x output pin and the "gate" of the MOSFET, should the Drain-to-gate junction of the MOSFET break down]. The TIP120, e.g., also comes in a TO-5 package which contains a large metal heat sink tab. If a heatsink is attached, the TIP120 is capable of disspating several watts of power! Thus either a TIP120 or power MOSFET (in a TO-5 package) is a good choice for switching high power/current loads. The figure below illustrates use of a TIP120 darlington transistor to switch the 150mA relay coil. Note that in addition to the "free- wheeling" diode - absolutely essential when driving any sort of inductive load, we have also placed a small capacitor (0.1 microfarads) across the coil. The capacitor helps absorb the initial back EMF spike generated, as the diode cannot "turn on" instantaneously. Figuring an effective h_fe of 1000, we calculate R_B to be as follows:

Since the darlington contains two base-emitter junctions in series, the total voltage from base to emitter is ~ 1.4 volts. Figuring again on V_h(68HC1x) ~ 4.5V, the voltage across R_B comes out to 4.5V - 1.4 V ~ 3.1 V. Now figure I_B = 150mA/1000 = 150 microamps, thus R_B can be as large as 3.1V/150 microamps = 20kohms. We use a value of, say R_B = 6.8k to assure that the transistor is driven hard into saturation. With this large of a base resistance, the 68HC1x output is now really well isolated against a catastophic breakdown of the transistors collector-to-base junction! Of course, if we wanted to feel even better about ourselves, we could also add an optoisolator between the 68HC1x output and the transistor base.


Figure: Darlington transistor circuit for driving a high current inductive load.

Again, note that we could also replace the TIP120 Darlington transistor with a suitable N-channel MOSFET transistor. The MOSFET transistors often perform better in such switching applications. Since the gate of a MOSFET does not draw any steady state current from the 68HC1x output pin, we can generally replace R_B with something like 4.7k or 6.8k to merely protect the 68HC1x output in the event that the Drain-to-gate junction should break down. When designing using MOSFETS, one would also go thru the above procedures for checking for power dissapation. MOSFETs are usually specified with a "maximum channel resistance" as (R_on)_max, thus one could calculate the maximum MOSFET power disspation as I_D^2 x (R_on)_max, and make sure that this is well under the maximum rated power disspation. If driving, say, an LED, the resistor placed in series with the LED (for the purpose of limiting the LED current) would also be calculated as was done above.

Finally we show other circuits below. In part a.) we have replaced the NPN bipolar transistor with an N-channel enhancement MOSFET (avail. @ Radio Shack - the IFR510 power mosfet in TO-220 package). Here, when the gate-to-source voltage exceeds the V_GS threshold voltage, the MOSFET turns on. Although this threshold voltage varies from one transistor to the next, it is typically around 2 - 4V, and the 68HC1x output pins hopefully will generally have no problem switching the MOSFET on. Often, however, some MOSFETS will have a higher Gate-to-source threshold voltage, and the 68HC1x will not be able to turn the MOSFET on fully, in which case a substantial voltage drop exists across the Drain-to-Source terminals, and the MOSFET ends up "gittin real hot". You will want to check this by turning your MOSFET on (while driving a load) and measuring this drain-to-source voltage. If the problem persists, you may need to find MOSFETS with a lower V_GS threshold, or take other measures to raise the applied Gate-to-source voltage.

The configurations shown in b.) and c.) below differ in that they use PNP and P-channel MOSFET (enhancement mode) transistors, such that the logic is reversed. That is, the LEDs would be turned on by writing a "0" out to the 68HC1x port pin, and would be turned off by writing out a "1".


Other transistor circuits for buffering of 68HC1x outputs - note b.) & c.) are controlled by reverse logic (applying a low voltage turns on the LEDs).



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