Using a transistor as a supply switch

[DigiTan]

I'm having trouble getting a very simple circuit to work. It's my understanding that the circuit shown below is supposed to saturate and drive the collector voltage down to almost zero so that the LED will illuminate. When I try it on my breadboards and PCBs with 2N2222A transistors, the collector voltage stays close to 5V and the collector current is miniscule (less than 1mA). What the heck is going on here? Are the transistors fried or is it the circuit? (I've had these transistors for almost 12 years, so damage may be a factor. They all tested fine, though.)

**broken link removed**

Also, I want to replace the LED with an LCD display backlight. Can this system be used to switch this kind of load as well?

 

[Dean Huster]

There's absolutely nothing wrong with your circuit. If the transistors check good, we'll eliminate them as the suspect for now, assuming that you have them wired correctly (emitter is the lead nearest the case tab, base in the middle, etc.). Is it possible that you accidentally popped a 220K resistor into the circuit vs. a 220 ohm resistor? Easy to do if you assume too much when you grab a part from a drawer full of "220 ohm" parts. Been there. Done that. Have several T-shirts to parade around in. Is the LED in the right direction? Flat side of bottom rim is cathode; longer lead is anode.

Dean

 

[zachtheterrible]

How many transistors did you try? Maybe your breadboard is shot. Try the circuit at a different position on the board. Measure the resistor values. maybe they're shot or like dean said youve got the wrong ones. Maybe the LED is fried. It might still let some current through if it is maybe?

Ya gotta love how the simplest stupidest things just WONT WORK sometimes :lol:

As for using it to light an LCD, I don't see why not. 2n3904 transistors can handle 40 volts with 200mA collector to emitter current, which is a lot more than the LCD will need I'm sure. 2n222a transistors are very similar to 2n3904's so I'm sure this applies to them as well.

 

[ljcox]

What happens if you put a short from the collector to the emitter? The LED should glow.

R1 and R2 are rather low - so the LED will be rather bright and the current may exceed the safe limit. So it is possible that the LED is damaged.

I would use 390 or 470 for R1 and 4k7 for R2.

 

[Screech]

Flat battery ?

 

[mstechca]

I'm having trouble getting a very simple circuit to work. It's my understanding that the circuit shown below is supposed to saturate and drive the collector voltage down to almost zero so that the LED will illuminate. When I try it on my breadboards and PCBs with 2N2222A transistors, the collector voltage stays close to 5V and the collector current is miniscule (less than 1mA). What the heck is going on here? Are the transistors fried or is it the circuit? (I've had these transistors for almost 12 years, so damage may be a factor. They all tested fine, though.)

Check the LED's maximal current capabilities. and use the equation (voltage)/(resistor in series with LED), and adjust the resistor and/or voltage until the current and voltage feeding EVERY device (including the LED) is less than the maximum ratings for each device.

Also, make sure everything is connected properly. If you cannot make a motor work by connecting it to the breadboard, and the battery, then your breadboard has a problem.

Sometimes, you just have to ram stuff in the board, or press on the other side of it so that the closepin things (whatever you call them) inside the breadboard make good contact.

You could maybe lower the resistor feeding into the NPN's base, but don't make it too low to burn the transistor.

Have you also checked about wattage? Your resistor values are high enough to not worry about buying higher wattage resistors, but that is something to keep in mind should you decide to use ridiculously low values.

 

[stevez]

A friend was struggling with a relatively simple circuit using either a 2N2222A or 2N3904/06 (can't recall). He asked for my help. After tracing the circuit I found nothing wrong but it clearly wasn't working. I decided to check the pinouts. The pinouts on the transistors were different than he expected. We pulled two datasheets from two manufacturers for the specific part number and they were different. We put transistors into my DVM and got the right pinout - everyything was fine. I don't know how common a problem this is.

What I try to do, as a hobbyist, is purchase low cost new parts and use the datasheets for that manufacturer - at least for the active devices.

 

[RDL2004]

Depending on what kind of LED you are using, with only a 5 volt supply the current limiting resistor for the LED (R1) may actually be too high to allow sufficient current to light the LED.


- Rick

 

[DigiTan]

I'll recheck the datasheets (for the specific manufacturer this time). It gave me the same problem after I used a 2222A on a PCB.

I checked the LED and R1 by shorting some wire across the collector and emitter and got the LED to illuminate, so its definately some sort of transistor issue. It's almost as if the base pin just isn't contributing anything. This happened with the entire batch of like 12 transistors.

 

[zachtheterrible]

When you say batch, did you get the 12 transistors from the same place and same time? Batches of transistors can go bad.

 

[Dean Huster]

R1 and R2 are rather low - so the LED will be rather bright and the current may exceed the safe limit.

Worst case current in this circuit, assuming a common red LED with the 220 ohm resistor would be around 16mA. R1 shouldn't contribute to the saturated collector current.

What's your source for the transistors? If they are "bargain bag" types (what they used to call "floor sweepings") where you get 500 transistors for $4.95, you may have the makings for some nice electronic jewelry, but not circuits.

Now that you've determined that the collector circuit works by shorting C-E, check to make sure you have 1K base resistors (I like 10K better myself) and not 1M values in there.

Dean




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