You Can Have my TIPs When You Pry them from my Cold, Dead Hands

We’ve seen a growing number of posts and recommendations around the net regarding components, specifically transistors. “Don’t use old parts” they cry,  “Go with newer components.”  You can often find these recommendations on Arduino forums. This all came to a head with a page called “Do Not TIP,” which was linked in the Arduino subreddit.  This page belongs to [Tom Jennings], creator of Fidonet, and one of the early authors of what would become Phoenix BIOS. [Tom] and a few others have been calling for everyone to send their old parts to the landfill – not use them, nor gift them to new experimenters. Get them out of the food chain. No offense to [Tom], but we have to disagree. These parts are still perfectly usable for experienced designers, and have a lot to offer new hardware hackers.

TIP is the part number prefix for a series of power transistors created by Texas Instruments.  In fact, “TIP” stands for Texas Instruments Power. The series was originally released in 1969. Yes, that’s right, 1969. Why are we still using parts designed when man first walked on the moon? The same reason people are still using the 555 timer: they’re simple, they’re easily available, they’re robust, and most of all, they get the job done. The TIP series has been used in thousands of classes, tutorials both online and off, and millions of projects over the years. Much of that documentation is already out there on the internet. The TIP series is also out in the distribution channel – they’ve been used for 40 years. Any retail shop that stocks a few electronics parts will have at least one of the TIP series.

The TIP series aren’t always the best transistors for the job. However, for most hobbyist-designed circuits, we don’t need the best performance, nor the best price – we’re going to use the parts we have on hand. There is always room to improve once you get the basic circuit working.  

In [Tom’s] specific example, he’s using a TIP120 to control a motor at 5 volts drawing 1 amp of current. [Tom’s] big problem with the TIP120 is that it’s inefficient when running the motor. That’s because the TIP120 isn’t a transistor. It’s two transistors configured as a Darlington pair. Like everything else in life, Darlington pairs have trade offs. To achieve high gain, you end up with higher voltage drop. In high current designs, that translates into heat. In this case, 2 watts of heat, which [Tom] claims will result in melted parts and fire. It turns out that the datasheet shows 2 watts is the upper limit for thermal dissipation on the TIP120’s TO-220 case. It will get very hot, but it will not catch fire. Want to be on the safe side? Add a heatsink, which is as easy as attaching a piece of metal using the convenient screw hole in the TO-220 case.

Just for fun, we created our own version of [Tom’s] example. We connected a TIP120 to a 12V lab supply. Rather than connect a motor, we grabbed our Re:Load Pro and set it for 1 amp. We use a 680 ohm base resistor to ensure the TIP120 was in saturation. The Re:Load Pro indicated that it was indeed seeing 1 amp of current flow, at 10.9 volts. This means that the TIP120 was only dropping 1.1V, rather than the 2V quoted on the datasheet. Were we just lucky? We tried a few TIP120s we had around the lab from a couple of manufacturers, and all of them were pretty close – well below the worst case 2V. Obviously you can’t design beyond the specs called on in the datasheet, but sometimes things work out in your favor. With the current set to 1 amp, the math is easy. The Re:Load Pro was converting 10.9 watts of power to heat. The TIP120 was dissipating 1.1 watts. The TIP120 did get hot – we measured up to 60°C. But it never went beyond that. A heatsink would have cooled things down, but we were shooting for worst case scenario.  We ran this setup for 2 hours and there was no smoke, fire, or failure.

Can you do better with a different part? Absolutely. [Tom] suggests a MOSFET such as the NTD4906N. FETs are great, we use them all the time. However, they come with a completely different set of rules and pitfalls compared to BJTs. Learning the rules, the design trade offs and pitfalls of both families of devices are key factors when learning electronics design. Every component a designer learns is a new color on their design palette. On the code side we worry about people becoming “cut and paste” coders. The same thing happens on the hardware side when a designer doesn’t learn how to use different types of parts.

So don’t throw away your old parts. Use them, learn from them, and become a better designer for it!

Filed under: classic hacks, misc hacks