Transistor self current limiting, why? - Page 1
Forum section: electronics / projects

author: Yamin
author_rank: Frequent Contributor
timestamp: September 20, 2024, 04:59:16 pm
content: Hi guys,I have attached part of an over voltage circuit design that we were making for a college project. So the idea was when VCC goes above a certain threshold the transistor starts to conduct R1 and R2 were chosen like that (I know that it would've been better to maybe add a Zener). So the circuits were working fine except for one unit.The collector of the transistor was connected to a n-channel mosfet which cuts the power for the load when the npn starts to conduct and the collector is at ground potential. In this particular circuit the LED lights up but the n-channel mosfet does not turn off. Measuring voltage at the collector shows around 5-6 volts. Not what we expected.We removed the npn bit (schematic attached) and bread boarded it. Same result as before. After troubleshooting it we found out that RC was the wrong value. It should have been 1.5k but it was 1.5ohms!My question is why did the transistor limit the current, and there was voltage dropped across VCE. During testing 1 LED did burn, but we were troubleshooting it for a very long time. After realizing the mistake we more puzzled as to why the transistor didn't smoke and take out the LED sooner. We expected the transistor to fully turn on/saturate. What parameter/feature makes the transistor perform like this? We did further experiment with slightly higher resistor values and noticed that there was voltage dropped across VCE. The higher the RC value the lower the VCE.Thanks in advance.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 20, 2024, 05:30:54 pm
content: There is no RC in the schematic.The collector current can never be more than the base current multiplied by the trsnsistor's current gain. R1 and R2 impose some limit on the base current. Ergo the collector current will be limited too.

author: jwet
author_rank: Frequent Contributor
timestamp: September 20, 2024, 05:38:41 pm
content: i understand that you mean R3 when you say RC- no problem.If the transistor was infinitely big and had infinite current gain, etc, it would conduct 10 amps to ground. A big MOSFET can easily do this- they have essentially infinite current gain. The current gain, beta is what's limiting you, but in saturation, its gets a bit more complex. Beta in deep saturation might be only 10% of its low current value. The reason that the transistor didn't burn up is that the led let go or partially let go sooner. Look at some transistor curves for IV vs base current on an old data sheet.I hope this makes sense, Good luck with your project

author: magic
author_rank: Super Contributor
timestamp: September 20, 2024, 07:24:11 pm
content: For the record, this transistor wasn't saturated, its BC junction stayed reverse biased.Basically what others said: limited β and limited base current, plus limited overall collector current that the device can conduct even with crazy high base current.Better datasheets show curves of β vs Ic and/or Ic vs Vbe, you will see that there is typically a point where Ic pretty much stops increasing no matter how hard you try.

author: PGPG
author_rank: Frequent Contributor
timestamp: September 20, 2024, 08:49:04 pm
content: Quote from: Yamin on September 20, 2024, 04:59:16 pmDon't you think it would be wise to provide more information (R1, R2 values, Q1 type)?QuoteDid you measured if 15V was 15V when you had 5-6V at collector?(15-5)/1.5=6.67A.Do your 15V source had such current efficiency?

author: magic
author_rank: Super Contributor
timestamp: September 20, 2024, 08:58:42 pm
content: I would guess 2N3904.By the way, using a resistive divider and an NPN for this won't provide stable threshold voltage over temperature. You may consider using TL431 instead. But beware that TL431's output can only go down to ~2V, which may not be enough to reliably turn off some N-ch FETs.

author: Yamin
author_rank: Frequent Contributor
timestamp: September 29, 2024, 05:07:25 pm
content: Thanks everyone for the help. So my key take away from this is that it was all to do with Beta limit. IC can't got above IB*beta.Quote from: PGPG on September 20, 2024, 08:49:04 pmOh this is a good point, we did not measure that. However I noticed that bridge rectifier diodes were very warm to touch. The VCC for this OV circuit comes from the power supply we designed upstream. To test the circuit the regulation part of our power supply is bypassed which should engage the overvoltage circuit.Sorry I didn't share the values for the components before since my question was about transistors in general and yes you are right it was a 2N3904.

author: TimFox
author_rank: Super Contributor
timestamp: September 29, 2024, 05:10:16 pm
content: Quote from: magic on September 20, 2024, 08:58:42 pmIf needed, there is a lower-voltage version of the TL431 that works down to 1.25 V, the TLV431.https://www.ti.com/lit/ds/symlink/tlv431a.pdf?ts=1727611635811&ref_url=https%253A%252F%252Fwww.ti.com%252Flit%252Fgpn%252FTLV431A

author: Yamin
author_rank: Frequent Contributor
timestamp: September 30, 2024, 04:13:29 pm
content: Quote from: TimFox on September 29, 2024, 05:10:16 pmAh nice thanks for that info, should come in handy in the future.