grouping transistors with same C,E junction - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: July 04, 2017, 04:28:24 pm
content: hello everyone,This post is regarding grouping some transistors with same c E junction as shown in the schematics. Suppose the are two positive inputs fed through 2 similar NPN transistors which drives a 12V LED strip. The two inputs are noncontinuous in nature & must be isolated from each other for some reason. so for each input a separate transistor is used to drive the LEDs. So I would like to know that does this type of configuration possible or not if much larger numbers of transistors are grouped with this arrangement, without damaging the transistors with any reverse voltage or other any other.

author: Zero999
author_rank: Super Contributor
timestamp: July 04, 2017, 05:46:47 pm
content: What you've described should work but don't expect the two transistors to share the current equally. It will be like an OR gate.There's a huge error in your schematic. The BJTs will just short out the 12V battery and you missed the base resistors.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 04, 2017, 07:29:40 pm
content: Thank you Hero999. I'm sorry that i didn't add the base resistors. I'm using p2n2222a transistor with 120 ohm base resistor to limit the collector below 400mA. I didn't understand about what you said about shorting the battery. Could you please explain about it ??Actually i use a 12v line from a dc adapter instead of battery. What should i do inorder to avoid it??

author: madires
author_rank: Super Contributor
timestamp: July 04, 2017, 07:41:04 pm
content: You could use one transistor and two diodes for the inputs.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: July 04, 2017, 08:12:52 pm
content: Quote from: madires on July 04, 2017, 07:41:04 pmThe two transistors are in parallel with the battery so when they turn on they will short the battery.

author: madires
author_rank: Super Contributor
timestamp: July 04, 2017, 08:18:22 pm
content: Yup, that's what Hero999 has mentioned already.

author: 4CX35000
author_rank: Regular Contributor
timestamp: July 04, 2017, 08:24:03 pm
content: Quote from: madires on July 04, 2017, 08:18:22 pmI suppose he will understand when the transistors blow up.

author: Zero999
author_rank: Super Contributor
timestamp: July 04, 2017, 10:02:17 pm
content: Quote from: 4CX35000 on July 04, 2017, 08:24:03 pmI suspect the circuit doesn't represent what he's built. It's probably a drafting error.Quote from: Adhith on July 04, 2017, 07:29:40 pmYour schematic is wrong. If you build it as depicted, the transistors will fry.I've cut around the section of the schematic showing the transistors short circuiting the battery.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: July 04, 2017, 10:58:52 pm
content: Quote from: Adhith on July 04, 2017, 04:28:24 pmWhat exactly do you mean by that? The transistors (and diodes suggested ba some) do provide a functional decoupling, but as the transistors share a common potential, the signals applied to their bases will be coupled through this potential!Coupling signals while avoiding galvanic connections means using a optocoupler, transformer, or a relay.

author: bson
author_rank: Unknown
timestamp: July 05, 2017, 09:23:40 pm
content: Nothing inherently wrong with the circuit. What it does depends on what's connected to the inputs and how it's controlled. As drawn it does nothing at all because the inputs aren't connected to anything. If they're on a connector it still does nothing because the connector lacks a ground reference so can't supply a control voltage or current. What happens also depends on the transistor; something like a 2N3055 on a heatsink can, with its low current gain, function as an adjustable load, wheras a 2N3904 with its high current gain and poor power dissipation will be difficult to control and most likely go up in smoke in short order.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 06, 2017, 07:04:32 am
content: Thank you for your valuable suggestions. Actually what I need was an OR gate with some transistors. I have attached a new schematics here. This would work right?? Its just a representation where the single LED will be replaced by an short LED strip.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 06, 2017, 07:10:43 am
content: Quote from: Neomys Sapiens on July 04, 2017, 10:58:52 pmThe whole Idea of doing this is for a VU meter which could display different patterns in addition to the DOT and BAR display modes. each type of patterns are separately controlled by switches which in turn controls the base terminal voltage and activates specific regions of the LED display which would lit up according to the VU meter.

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2017, 08:33:45 am
content: Quote from: Adhith on July 06, 2017, 07:04:32 amThe transistors are configured as emitter followers so the voltage on the LED strip will now be 5V minus the base-emitter voltage of the transistor. Assuming the LED strips have three white LEDs in series, they would not light because the forward voltage will be a couple of volts each, even at very low currents.The transistors are better switching the low side. The fist circuit would do, if you fixed it by adding the emitter resistors and connected the transistors in series with the LED strip, rather than short circuiting the supply.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 07, 2017, 05:52:39 pm
content: If anyone could help me with a similar circuit diagram it would be great help.

author: Zero999
author_rank: Super Contributor
timestamp: July 07, 2017, 06:07:41 pm
content: Quote from: Adhith on July 07, 2017, 05:52:39 pmWhy not use a single transistor and connect the inputs to the base via diodes? Look up diode transistor logic for some inspiration.Do you want to do PWM?

author: madires
author_rank: Super Contributor
timestamp: July 07, 2017, 06:23:56 pm
content: I'd suggest to start with same basics:https://www.build-electronic-circuits.com/how-transistors-work/.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 13, 2017, 12:19:30 pm
content: Quote from: Hero999 on July 07, 2017, 06:07:41 pmThank you again for you help. Let me check the diode transistor model and see what can i come with. No i dont want do PWM.