Timeline for Smoking Transistor in Low-Side Switch. What Am I Doing Wrong?
Current License: CC BY-SA 3.0
17 events
| when toggle format | what | by | license | comment | |
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| Apr 13, 2017 at 12:32 | history | edited | CommunityBot | replaced http://electronics.stackexchange.com/ with https://electronics.stackexchange.com/ | |
| Oct 3, 2015 at 12:27 | vote | accept | parrowdice | ||
| Oct 3, 2015 at 11:40 | comment | added | parrowdice | @RespawnedFluff: It was my fault for not mentioning I wanted to drive it from a PIC. I was just trying to keep the question concise, but I guess more details are better. | |
| Oct 3, 2015 at 11:30 | comment | added | parrowdice | @RespawnedFluff: You're previous advice was very useful, and you can see from the first diagram in my post that I planned for 23mA from the PIC, a 1.0V Vbe, and 280mV Vce. The second diagram shows what I am actually seeing, which is 26mA, 0.8V Vbe, and 4.4V Vce. It seems like the Ic/Ib is the bit I was missing! | |
| Oct 3, 2015 at 6:32 | comment | added | copper.hat | @ScottSeidman: It is permitted for medicinal purposes, but only in states that are saturated. | |
| Oct 3, 2015 at 6:21 | comment | added | got trolled too much this week | There are some MCUs, e.g. iND80231 that can source/sink 200mA on some of their IO pins, but these are specialty items. | |
| Oct 3, 2015 at 5:53 | comment | added | got trolled too much this week | I do feel bad I didn't also emphasize that Ic/Ib (Hfe) is only 10 in saturation for this transistor (actually this is a very typical value for a power transistor)... so the resulting base current (200mA) at 2A collector current is outside the realm of what you can directly drive with most MCU IO pins directly. | |
| Oct 3, 2015 at 5:37 | comment | added | got trolled too much this week | And this bit is perhaps confusing to you: what you may have read in general about a Vbe junction (or diode) drop being 0.7V is not accurate enough for power electronics and for power transistors in particular. The Vbe/junction drop can be more for these (at high currents, like in this example 1V Vbe at 2A collector current) that's why you need refer to the datasheet for a better estimation. | |
| Oct 3, 2015 at 5:26 | comment | added | got trolled too much this week | Also you've only planned for 0.8V Vbe (with that base current/resistor), even though I specifically wrote in my answer to your previous question "As for the transistor's base, you need to raise its potential to approximately 1.0V (at 2A load), not 0.8 as you have your schematic. (A bit more doesn't hurt.) Otherwise the transistor will not be fully saturated and will use more power as well as exhibit higher Vce than calculated above." You've basically only put the transistor in the active region, not in saturation. In the active region it dissipates way more power. | |
| Oct 3, 2015 at 5:24 | comment | added | got trolled too much this week | Note that your planned 26mA base drive was at the limit what PIC can generally provide. Again you've included no details, even in this question about the specific PIC model, so don't know if it even supplied that current. | |
| Oct 3, 2015 at 5:13 | comment | added | got trolled too much this week | I'm sorry to hear you smoked a transistor. I did advise a small heatsink, just in case... I didn't get from your previous question enough details on what you intended to drive the transistor with (PIC). Also LEDs have highly variable resistance. It's not a constant with voltage! Your previous question didn't even say you wanted to drive LEDs. The LEDs you link to have no datasheet whatsoever. Measuring the current through LEDs at 12V is no guarantee for the current at 8V. See what @The Photon said. | |
| Oct 2, 2015 at 22:13 | comment | added | Scott Seidman | Even though smoking transistors is very satisfying, it can be very bad for you in the long run if you develop a lifelong habit. | |
| Oct 2, 2015 at 21:08 | comment | added | user16324 | More base current! | |
| Oct 2, 2015 at 21:03 | comment | added | The Photon | Your load must be lower in impedance than you expected. You thought it would draw 1.8 A with 11 V across it, but instead it is drawing 2.1 A with only 8 V across it. Since your load is demanding more current then you designed for, it's not surprising that the driving circuit is overheating. | |
| Oct 2, 2015 at 21:02 | answer | added | Kevin White | timeline score: 10 | |
| Oct 2, 2015 at 21:02 | comment | added | kabZX | Your transistor is hot because it is dissipating 4v *2A = 8watts. You need to increase the base current to drive the transistor into saturation and thereby reduce Vce. Try reducing the 100 Ohm base resistor to 47ohms and see if Vce reduces. If the PIC is not able to supply more current, use a second transistor and make a Darlington pair. | |
| Oct 2, 2015 at 20:38 | history | asked | parrowdice | CC BY-SA 3.0 |