Saturday, January 14, 2012

NPN Darlington test

Am currently modding a signal tower I bought the nephews months ago and converting it into a multi-featured traffic light for them to play with. The tower has 12VDC red, amber and green incandescent lamps. Back when I bought it I just wired individual rocker switches for each of the lamps and powered it via a 12VDC wall cube. It was a fully manual system where any combination of lights could be turned on.

I've removed all the switches and will be adding a brain so it can function automatically. Initial measurements showed the incandescents draw about half an ampere each. To switch them I'd need either relays or power transistors. I've opted to use NPN Darlingtons instead of relays even if Darlingtons will have a substantial voltage drop across the collector-emitter. A more major concern, however, is the power dissipation because of a Darlington's high voltage drop. Dissipation translates to heat which means high temperatures which, if high enough, would require the use of heatsinks--something I would rather not have to resort to. So in order to find out whether I'd be better off using miniature PCB mounted relays, I bought a few Darlingtons and tested their VCE and the temperature of their cases.

The corner store from which I get my parts doesn't have a good range of Darlingtons and so the parts I ended up testing were the TIP102, TIP112, and TIP120. I breadboarded the following circuit and powered it off my bench supply.

Resistor R5 represents the incandescent lamp. The following tables show the measurements I obtained for each of the bulbs and transistors. VCE is the voltage across the collector and emitter. IC is the current going the lamp. Case temperature was measured using a Fluke 62 infrared thermometer around two inches away from the Darlington. I recorded the maximum reading. VCE drifted as the transistor temperature went up so I gave the circuit a settling time of between two to three minutes before taking the measurements. I previously took some measurements of the inrush current (without using the Darlingtons) using a Fluke 87V's Peak-Min-Max function and the highest value I obtained was 5.2A. 

Red Amber Green
VCE (volts) 0.735 0.744 0.745
IC (amperes) 0.363 0.467 0.469
Case Temp. (Celsius) 41 43 43

Red Amber Green
VCE (volts) 0.714 0.728 0.730
IC (amperes) 0.360 0.467 0.469
Case Temp. (Celsius) 40 43 43

Red Amber Green
VCE (volts) 0.691 0.700 0.697
IC (amperes) 0.362 0.468 0.470
Case Temp. (Celsius) 40 41 42

The TIP102 clearly stands out as having the least VCE and consequently the lowest power dissipation. The temperature as measured by the IR thermometer is of course hardly accurate (I could've used the 87V's thermocouple but I wasn't in the mood to go through the hassle). Since we have all the values necessary let's compute for the (near actual) case temperature. The TO-220 package has a junction to ambient thermal resistance of 62.5°C/W. Current draw of the green bulb is 0.47A. VCE = 0.697V. Power dissipation is therefore 0.697 x 0.47 = 0.328W. Temperature rise of the case is therefore 62.5 x 0.328 = 20.5°C. Ambient temperature was measured to be 30°C. So case temp. ought to be approximately 30 + 20.5 = 50.5°C.

The TIP102 is the most expensive amongst the three Darlingtons, but by less than 10% over the cheapest. Given how I'm averse to designing in heatsinks for this toy I'll use the TIP102, despite the fact that its temperature will be a mere 1.4° less than if I use the TIP120.

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