Transistors completely fine with base high current  - Page 1
Forum section: electronics / projects

author: abdulbadii
author_rank: Frequent Contributor
timestamp: March 31, 2023, 01:11:11 am
content: Please anyone expert enough to list or tell which transistors that is completely fine to have base high current i.e. no base resistor in order to use it / Q as switch?

author: amyk
author_rank: Super Contributor
timestamp: March 31, 2023, 01:14:10 am
content: Why not just use a MOSFET?

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 31, 2023, 01:54:22 am
content: Define "high"....

author: srb1954
author_rank: Super Contributor
timestamp: March 31, 2023, 02:36:42 am
content: Quote from: abdulbadii on March 31, 2023, 01:11:11 amThe transistor datasheet will have a specification figure for the maximum allowable base current. Stick within that specification limit.Having excessive base current can cause problems with the turn-off performance of the transistor being slower while excess charge is swept out of the base region. Also, remember to take account the power dissipated in the base emitter junction when operating at high base currents.

author: KubaSO
author_rank: Contributor
timestamp: March 31, 2023, 03:05:32 am
content: If you feed the base of a small signal transistor from a supply line, unless you're running on a coin cell, you'll smoke it up even from a couple AA cells. So in general the number of such transistors is zero.In general- thereareexceptions.If you feed the base from an MCU pin, or from a CMOS output - those inherently act as current sources. Except, you then need to worry not about the transistor, but about the dissipation in the GPIO driver in the MCU.Assuming - I apologize if I'm wrong - that such questions typically originate from beginner MCU aficionados, I'd say you'll probably overstress the MCU first. Each transistor will be dissipating the same power - about 15-50mW just from the base. Pretty much any small-signal general purpose type will not even notice that too much.But the MCU will be dissipating progressively more the more transistors you attach that way. Even with just one transistor the 3.3V (I/O) MCU will be dissipating almost 4x more power than the base junction of the transistor. With high-drive 3.3V GPIO, the GPIO output structure will be dissipating in the ballpark of 100mW, and there will be as many of those dissipating as there are transistors driven on. So, drive 10 transistors, and suddenly there's a tiny fraction of a mm2on the MCU that will be dissipating 1W of power. That becomes somewhat problematic for longevity and such. Relatively huge localized heat loads on the die.The transistors so driven will be saturated good and proper and they'll take an order of magnitude longer to turn off than they did to turn on. That may be of no consequence, of course, just something to pay attention to.Abusing MCU pins as current sources is better suited to LEDs, although even then you should set the drive level to the lowest available in the GPIO configuration, and still make sure you don't exceed the maximum VCC(IO) current or else the bond wire(s) may be the fuse. At least the LED will be dissipating more power than the GPIO output stage.I still question the question, though: if you think those base resistors are somehow inconvenient to either calculate or mess with... just buy transistors that have them built-in! Problem solved.In any case, the stouter the transistor, the more unhappy whatever is driving it will be about standing between the positive supply rail and the diode to ground that the B-E junction is in such cases. Power transistors can take a couple amps through the base. If you connect *all* MCU output pins to the base, emitter to 0V, and turn all the outputs on - the MCU will not last very long. The transistor won't even get a chance to get properly warmed up.

author: bdunham7
author_rank: Super Contributor
timestamp: March 31, 2023, 03:17:25 am
content: Quote from: abdulbadii on March 31, 2023, 01:11:11 amMany Darlington transistors have an internal base resistor as well as an additional resistor to prevent deep saturation. So you can use them as a switch without any base resistor, but they don't actually have excessively high base current.An example:https://www.ti.com/lit/ds/symlink/uln2803c.pdf?ts=1680183363584

author: 2N3055
author_rank: Super Contributor
timestamp: March 31, 2023, 08:50:04 am
content: Bipolar transistors shouldn't be used that way.There are special "switch transistors" either darlington or single transistors that are actually small IC with integrated resistors... also called Resistor Equipped Transistors (RETs)... They look like simple transistors but they are not.

author: voltsandjolts
author_rank: Unknown
timestamp: March 31, 2023, 09:19:08 am
content: also called "digital transistors"

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: March 31, 2023, 09:48:33 am
content: Quote from: abdulbadii on March 31, 2023, 01:11:11 amyes, there are switching BJTs that can handle 10 A base current with no problem (if that fits your definition of "high current"). search for BJTs that are specified for Ic = 100 A or more.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: March 31, 2023, 10:25:44 am
content: actually it's just for the simplest H bridge only with 4 Q and a switchof my own toughtful concept design

author: mariush
author_rank: Super Contributor
timestamp: March 31, 2023, 10:39:37 am
content: You can buy transistors that have built in resistors.Look at "pre-biased" transistors , for example here's this category on Digikey :https://www.digikey.com/en/products/filter/transistors/bipolar-bjt/single-pre-biased-bipolar-transistors/292Also pre-biased transistor arrays :https://www.digikey.com/en/products/filter/transistors/bipolar-bjt/bipolar-transistor-arrays-pre-biased/293

author: Terry Bites
author_rank: Super Contributor
timestamp: March 31, 2023, 03:06:47 pm
content: Even if you don't melt your device first time, you will engourage premature death... high currents cause dopants to migrate over time.The characteristics of your Q will change. The higher the base current the more charge needs to be shifted out of the junction at switch off.This will slow down switching and increase heating.Be kind to your designs.

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 31, 2023, 03:17:09 pm
content: Quote from: abdulbadii on March 31, 2023, 10:25:44 amWhat do you mean by "switch"? Like a toggle switch? Because you can make an "H-bridge" with just a single DPDT, center off, toggle switch..

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 31, 2023, 04:46:34 pm
content: For H-Bridge without resistor, use npn on high side / pnp on low side, the transistor will self-limit as the only current sink/source is the load itself, IB will be roughly IC/gain.But this setup is far from optimal, higher ON loses, and you need to drive the high side with the same voltage as the load.Can't see the point, saving 4 resistors won't be a big deal.