Help with Transistor

Hi all

I'm having some trouble using my 3.3v arduino to switch a 5v load using the following transistor:
Link

I'm not sure whats happening :frowning: but I'm not getting the expected results. My schematic is as follows:

When measuring the voltage at the emitter I get a reading of 2.9v.... so unless the transistor has a voltage drop of about 2.1v I'm doing something very wrong, which I'm about 95% sure is the case :slight_smile: or I have completely misunderstood how transistors work...

Could someone help me out here? I'd really appreciate it :slight_smile:

You're using the transistor as a switch. Are you sure you want to switch (short) 5V to GND??

So… Where is the load? You didn’t draw it on your schematic… So as you said, if this is your real circuit, you’re doing something very wrong :stuck_out_tongue:

For such a circuit (NPN transistor commuting a load), you have to place the load between the +5V and the transistor’s collector.

When eveything is ok, the emitter voltage is 0V (because it’s connected to the ground), the base voltage is around 0.6-0.8V depensing on both the transistor and the current and the collector voltage is close to 0V (a little more, depending on both the transistor and the current as well).

So the base resistor (R1 here) has a voltage drop of about 3.3-0.7 = 2.7 V. Using Ohm’s law, you can calculate its value in order to have the desired base current.
Knowing the current needed for the load, the base current can be calculated : IB >> IC/hFE ; hFE being detailed in the transistor’s datasheet (consider its value to be around 100 for low collector current).

Are the grounds of your transistor circuit and the arduino tied together?

You definitely need a load in there or your transistor will either die, or your power supply will. When you are measuring the emitter, your are measuring ground, it should be 0V, even if you already blew your transistor. Since it isn't measuring 0V, the grounds must not be the same electrical point.

You need a load (like a resistor) in the collector for the same reason you need a resistor in series with an LED...to limit the current through the transistor to a safe current. What are you trying to switch?

-fab

using the following transistor:

And what transistor might that be ?

Hi Guys

Thanks for your responses! I think I'm beginning to see the error of my ways!!!
Sorry about not showing rest of my circuit in my schematic, it was pretty late last night :slight_smile:

I should point out that I did at one point connect the transistor as shown in the above circuit but I noticed the leds on my board dimmed which usually means there is a short so I disconnected it straight away!

I'm building up to use the transistor to switch a 5V 100mAh charger but to be safe I decided to start small and test everything with an led :slight_smile:

So my test circuit had an LED (with resistor) connected to the emitter which then when to ground... everything seemed fine e.g. the LED lit up as expected but was rather dim compared to not using the transistor and going straight from 5v, which is when i started probing. Is this where I'm going wrong? the load to be at the collector and not the emitter?

I'm building up to use the transistor to switch a 5V 100mAh charger

Battery charger?

Regards

vffgaston:
Battery charger?

Regards

Hi vffgaston

It is indeed, but its a (micro) usb charger so my intention is to plug it into a female usb breakout board and have the transistor in place to control connection, so there won’t be any need to build a charging circuit because its already built into the charger.

Seems like a pretty safe idea to me no?

but its a (micro) usb charger

Can you post a picture?

Regards

vffgaston:
Can you post a picture?

Regards

I can but I'm at work at the moment so i'll have to do it later. But its just like your standard phone charger (unless you using an iphone)
like this one here (link)

As I said, the load should be placed between the +5V and the transistor’s collector. If you place it between emitter and ground, it might work for low-voltage drop devices but it’s not the way to do and it might fail with other loads.

I should point out that I did at one point connect the transistor as shown in the above circuit but I noticed the leds on my board dimmed which usually means there is a short so I disconnected it straight away!

That transistor might be toast.

Usually then can handle some abusive treatment... If the LEDs on the Arduino board start to fade it's because of the fuse on the board, the voltage drop and the current is limited.
If the short is removed fast, the transistor is probably still good.

But it sounds like you want a high side switch. That requires more than one transistor as your Arduino is using only 3V versus the 5V charger voltage.

Using an NPN and PNP transistor:

Using a logic level P channel MOSFET and NPN transistor: