Same GND

Hello everybody,

I am new in the areas Electronics & Arduino and I have to make a project for school. This project contains 2 motors, those speed I can control through two NPN transistors (look at my attachment) and other stuff (not really important). The problem is that my Arduino UNO hasn’t enough GND for my project. I thought for a solution, and ‘combing’ the GNDs might work. So I did the schematic diagram, which you can see below, and as you can see the two emitters of my transistors have to be connected to a GND pin on my Arduino. So instead of using one GND for each emitter I ‘combined’ them to only one. Now my questions are: Would this work? Would this damage my board ? If it doesn’t work, what else can I do, in order to save one GND pin?

Thanks for your time :slight_smile:

That's fine except that you need a resistor in series with each transistor base to limit the base current to about 1/10 of the motor current.

I hope they are small, low current motors!

Russell.

Hey Russell,

thanks a lot for your time and reply. I want to feed each DC Motor with about 9V. Is this ok ?
As I said I have two transistors and in order to control the velocities of the motors I have to increase or decrease the voltage at the two bases. If there are two different voltages at the both bases, for example transistor A has a voltage of 4V and transistor B one of 2V, would this still work if I have only one GND ?

And if you have time, could you please explain me why does the current at the base have to be 1/10 of the motor current ?

Thanks a lot :slight_smile:

I take it you are trying to use the transistors as analogue controllers to vary the power supplied to the motors. This is not a good way to do it for various reasons (heat, unpredictability of gain etc).

The usual way is to use a PWM signal to switch the transistors fully on and off. There are special output pins which will generate the PWM signal, and you can connect this to the base of the transistors via a resistor. As alluded to by Russell, the resistors are needed to limit the base current to a safe value, whilst providing enough current to saturate the transistors (divide collector current by hfe (the dc gain of the transistor). But MOSFETS are even simpler to connect if you use 'Logic level' driven devices.

I want to feed each DC Motor with about 9V. Is this ok ?

The small rectangular 9V batteries won't work for more than a few minutes, if at all. Use a regulated power supply, like a power brick.

Whether 9V is OK depends on the motor. The small motors found in toys that run from 2 AA batteries will burn out very quickly.

You also need to ensure the transistors you use can handle the maximum current for the motor.

Most small transistors are limited to a fraction of an amp, most motors can pull an amp or even more...

yendis:
enough current to saturate the transistors (divide collector current by hfe (the dc gain of the transistor).

No, no, no! I don't know how many times this has been stated here but for saturation hfe is irrelevant. You typically need 1/10 of the collector current to flow in the base to adequately saturate a bipolar transistor.

Russell.

Russell, sorry but I am new here so not seen your previous posts on this - where does the x10 rule come from?

I was taught that there are two types of Hfe, one is the small signal current gain, dIv/dIb, the other is Hfel or the large signal or DC gain, Ic/Ib. Using Hfel, divide the maximum collector current by Hfel gives Ib value which will start to forward bias the BC junction and saturate the transistor. If Hfel is not given in the data sheet, you can deduce the required base current from the output graph on the datasheet.

I am not saying that this figure will give the optimum base current for lowest saturation voltage, but it will start to saturate the transistor.

Your figure of 10 seems to be a useful typical approximation, but for some power transistors operating at a high collector current, even more base drive current is needed so that 5 or lower would apply to get a low enough saturation voltage.

Your figure of 10 seems to be a useful typical approximation, but for some power transistors operating at a high collector current, even more base drive current is needed so that 5 or lower would apply to get a low enough saturation voltage.

A good data sheet will show typical saturation behavior, and you should base your designs on good data sheets.

I copied and posted chunks of datasheets for the 2N3904 and 2N2222 transistors that clearly show the 1/10th rule for saturation, not just as an approximation.

Newer, higher gain transistors still specify no less than 1/20th.

Some modern designs do better, superbeta devices designed for switching have much more
attention given to the saturation region - checkout the amazing ZTX851 for example:

http://www.redrok.com/NPN_ZTX851_60V_5.0A_1.2W_Hfe75_TO-92.pdf

Note you can drive 2A output with 50mA base drive, for only 0.1V Vsat, and it handles 20A peak
collector current.... The hfe value is only a modest 200 or so.

If only it was as easy to find and as cheap as a 2N2222....

Wow. Now -that- is a switching transistor.

The datasheet modestly suggests emergency lighting circuits as the only application!

yendis:
Russell, sorry but I am new here so not seen your previous posts on this - where does the x10 rule come from?

From 50 odd years of experience. Like you I’m a retired MIEE old school analogue engineer. Of course it’s just a rule of thumb for switching applications and you should, of course, refer to the data sheet for the appropriate values but many beginners here don’t know that.

I was taught that there are two types of Hfe, one is the small signal current gain, dIv/dIb, the other is Hfel or the large signal or DC gain, Ic/Ib. Using Hfel, divide the maximum collector current by Hfel gives Ib value which will start to forward bias the BC junction and saturate the transistor. If Hfel is not given in the data sheet, you can deduce the required base current from the output graph on the datasheet.

Yes, Hfe (large signal) should be used for biasing while hfe (small signal) should be used for gain calculations etc. However in the OPs case he is (whether he knows it or not) using the transistor as a switch to drive a motor so the transistor needs to be well into saturation to minimise the voltage drop.

Russell.

ElectronicsNewbie:
thanks a lot for your time and reply. I want to feed each DC Motor with about 9V. Is this ok ?

Shouldn't be a problem but you should check maximum voltage and current from the data sheet for the transistors you're using.

As I said I have two transistors and in order to control the velocities of the motors I have to increase or decrease the voltage at the two bases. If there are two different voltages at the both bases, for example transistor A has a voltage of 4V and transistor B one of 2V, would this still work if I have only one GND ?

First, you should understand that you are not applying a varying dc voltage to the transistor bases. You are driving them with a PWM signal which at any given time is either 0 V or 5 V. So as you have drawn the circuit you will be trying to apply 5 V to the bases some of the time - not good.

Bipolar transistor bases are current driven not voltage driven so you need a series resistor to convert that voltage into a current.

When you average a PWM signal you get a varying voltage. In your circuit that averaging is being done by the electrical and mechanical characteristics of the motor. The output from the collectors is still a PWM signal.

Hope that helps.

And if you have time, could you please explain me why does the current at the base have to be 1/10 of the motor current ?

Just a general rule of thumb for switching circuits. You really need to read and learn to understand data sheets.

Russell.