What resistor do I need for the base of this transistor pls? - Page 1
Forum section: electronics / projects

author: BrightSpark2022
author_rank: Newbie
timestamp: June 21, 2022, 05:26:37 pm
content: Hi guys,My first post! - Very pleased to be here, as I have already worked out that I'm going to need some assistance from more experienced electro wizards out there.I wonder if you can help me on this?I am struggling with, what I imagine is a pretty simple explanation on a circuit.I am trying to figure out what resistor I need for the base connection of my transistor on the attached circuit diagram?I have done a bit of Googling, but tbh I am now so confused that I thought I would ask.The transistor is a D45H8G (The data sheet says that the Bce is 1.5v max)The ignition coil is a fairly standard 12v type.This circuit is supposed to be an 'amplifier' circuit using a PNP transistor to drive an ignition coil.The input voltage is 12vdc, and I think that is what voltage comes out of the PWM connection to the transistor base.Also...Is the ignition coil a 'load'? Do I need to know what mA it draws?Or can I work out the base resistor without that?Any help much appreciated!Thanks,BrightSpark(Not so bright)

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: June 22, 2022, 12:45:41 am
content: Without info on the feed a safe guess would be 1K ohm.Jeff

author: moffy
author_rank: Super Contributor
timestamp: June 22, 2022, 01:47:18 am
content: The transistor is rated at 10A with a nominal gain of 40-60, the current pulse into the coil can be quite high say 5A, so dividing by 20 you want about 0.25A drive in the base, so 10R base resistor will drop 2.5V, 100R will drop 25V so you can work it out from there. Best to do some measurements like the resistance of the coil and the pulse voltage driving the base of the transistor. My calculations are pure guess work and sometimes I am completely wrong.

author: Zero999
author_rank: Super Contributor
timestamp: June 22, 2022, 06:47:53 am
content: What's the output impedance of the signal generator? Presumably around 50 Ohm? If so, I wouldn't use a base resistor at all. The internal resistance of the signal generator will limit the base current.The data sheet specifies a minimum hFEof 40 at 4A with a VCEof 1V, but that's the worse case and it'll probably increase with temperature, so you'll probably get around 50, at 5A.I wouldn't recommend this circuit. It will probably work, but not drive the ignition coil to its full potential. A MOSFET would be a better solution.

author: BrightSpark2022
author_rank: Newbie
timestamp: June 22, 2022, 07:33:06 am
content: Thanks Moffy,I will put my maths hat on and start cranking that calculation.BS

author: BrightSpark2022
author_rank: Newbie
timestamp: June 22, 2022, 07:37:19 am
content: Hi Zero999,Thanks for your help.I am not sure about the output impedance, as it is a small cheap PWM module and not a larger desktop unit.It might be the standard 50 Ohm, but I am not sure?I agree that a MOSFET would be better, but I think the PNP is being used deliberately for safety, in order to limit the output a bit. I know that messing about with these coils and very high voltages can be dangerous.Having said that...I do have an 800v rated fast switching MOSFET which I might try, once I feel more confident with what I'm doing!BS

author: Zero999
author_rank: Super Contributor
timestamp: June 22, 2022, 08:45:50 am
content: Quote from: BrightSpark2022 on June 22, 2022, 07:37:19 amThe manual/data sheet and search engine are your friends.QuoteHow on earth is a BJT any safer than a MOSFET?

author: mikerj
author_rank: Super Contributor
timestamp: June 22, 2022, 09:40:41 am
content: Driving an ignition coil from the high side is pretty unusual, you do realise the pulse generator output will have to go up to the supply rail (or close to) to turn off the transistor? For instance if you have a 5v pulse generator with push/pull output the transistor won't switch off.Also you have no avalanche protection for the transistor. When the transistor switches off there will be a high voltage developed across the transistor (secondary voltage multiplied by turns ratio) which can easily be 300v or more (especially if the secondary gap is made large enough that no arc can form). Usually there would be something to prevent the voltage rising above some level that is safe for the transistor. This may be built into transistors that are designed for use in this application.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 22, 2022, 09:51:02 am
content: Be careful, this circuit can damage your signal generator.Also, better drive the coil on the negative side and use an ignition IGBT instead of a transistor.The IGBT can be driven directly by the signal generator.