Hello everyone,

Just wanted to say what a great and helpful site... lots of info.
I am new at putting circuits together and would love some help on finishing this one. I did search through the forums and couldn't find the exact answer although I did find some info.

I would like to pulse DC with a 555 timer and a TIP31 npn transistor. I chose the 555 because it seems easy and parts are available. That particular transistor fits the 1a range I would like to pulse.

I have successfully setup the 555 by itself blinking LEDs and pulsing dc to some coils I am experimenting on but the 200ma max is just not enough for what I want to test. I have changed around the capacitors and resistors to adjust the frequency as well.

So I basically took the working 555 timer and setup the transistor from what I found online.
Base to pin3 on 555 with 220ohm resistor
Emitter to ground
Collector to coil

But when I turn my power supply on I get almost no current.
So I tried changing out resistor between 555 and TIP and it just got REAL hot.

So I am wondering what I am doing wrong. I just want to be able to set this circuit up to pulse dc at different frequencies up to 1amp. Adjusting like I normally would with the C,R1,and R2.

Thanks for all of your help!!!

 

I just want to be able to set this circuit up to pulse dc at different frequencies up to 1amp.

The base resistor needs to be sized to allow about 100mA of current to be sure your transistor is fully turned on. Pin 3 max voltage minus 0.7 ( the base-emitter ∆V) will be dropped at 100mA. Use Ohm's law to calculate the right resistor. It'll need to be rated to ~2X the expected power dissipation to avoid burning up.

 

I must have did something wrong. Using his original resistor and a supply voltage of 5 V, I calculated a base current of just under 20 mA. The datasheet for TIP31 states that the minimum hFE is 25, so he should have seen about 0.5 A through the transistor. He said he hardly got any current. Am I incorrect?

 

The datasheet for TIP31 states that the minimum hFE is 25...

I haven't read the details of that, but I bet the devil is in that detail. It's much safer to assume base = 10% of collector current.

 

I haven't read the details of that, but I bet the devil is in that detail. It's much safer to assume base = 10% of collector current.

Only if you want to use the transistor as an on-off switch (as cplouffe wants to). I'm asking because there should be a higher current unless I made a calculation error.
So, did I make an error?

 

Thanks for the replies. I will figure out the base resistor and see what I get.

The TIP31 is from radioshack so I will have to look again at what is required for it to be fully turned on.

 

The pinout for the TIP31 is 1 = base, 2 = collector, and 3 = emitter, left to right viewed from the front of the transistor.

What is the resistance of your coil?

 

The 2 configurations of coils I am working with are 5.4ohms and 7.8ohms.

Is that part of the problem is there isn't enough draw or load?

The goal of pulsing is to make the coils electromagnets and see the different fields that I can measure. Then compare to regular DC.

Thanks!

 

Depending on your supply voltage, the current may be exceeding the capability of the TIP31, which would explain why it's hot.

 

I must have did something wrong. Using his original resistor and a supply voltage of 5 V, I calculated a base current of just under 20 mA...

With a 5V supply, a 555 will output something like 3.3V in the high state. So the the OP was dropping 3.3-0.7=2.6 volts over the 220Ω base resistor, giving ~12mA. Maybe 150-200mA thru his coil. If he was expecting 1A, maybe this looked like "almost no current". And maybe the supply voltage sagged when it was suddenly faced with the load, dropping the base voltage and current even more.

Or he got the transistor pins wrong as Tracecom is suggesting. We've all done that.

 

Thanks guys... I will check the pins again on the transistor
I know I can pull out the base resistor and transistor and circuit works fine.

Just to clarify the TIP wasn't getting hot the base resistor between pin 3 on 555 and TIP was really hot. I was trying different resistors just to see. Started with 220 then 1k then 100. The 100 is the one I noticed really hot but it the others could have been too, just didn't check. While it was running I checked temp on TIP and it barely moved.

--tracecom... The TIP is rated to handle alot higher current, right? Or is the high 3A rating just the output?
--wayne... I will measure the current in the coils next but I know the power supply draw was reading very low if that matters.

 

Thanks guys... I will check the pins again on the transistor
I know I can pull out the base resistor and transistor and circuit works fine.

Just to clarify the TIP wasn't getting hot the base resistor between pin 3 on 555 and TIP was really hot. I was trying different resistors just to see. Started with 220 then 1k then 100. The 100 is the one I noticed really hot but it the others could have been too, just didn't check. While it was running I checked temp on TIP and it barely moved.

--tracecom... The TIP is rated to handle alot higher current, right? Or is the high 3A rating just the output?
--wayne... I will measure the current in the coils next but I know the power supply draw was reading very low if that matters.

I actually didn't look at the current rating on the TIP31 (I should have), but rather assumed (from your first post) that it was 1 A. Anyway, if it's not the transistor that's getting hot, my comment is irrelevant.

It could be that you have the transistor wired wrong and the base resistor is actually connected to the load and overheating.

Post a schematic.

 

I actually didn't look at the current rating on the TIP31 (I should have), but rather assumed (from your first post) that it was 1 A. Anyway, if it's not the transistor that's getting hot, my comment is irrelevant.

It could be that you have the transistor wired wrong and the base resistor is actually connected to the load and overheating.

Post a schematic.

Sorry I am wanting up to 1A to work with. The packaging says 3A.

I will post one soon. The multi-meter I have allows me to plug in transistors and test them and I am getting a 0 reading no matter which way I plug it in.

By the way it sounds you guys must be right about it being wired wrong and I probably burnt it up? I can run by and pick another up and test first before I run it then make sure it is wired correctly.

 

ok I think it is working ok now. When I was running with just the 555 I had one lead of the coil connected to the ground/negative end on the breadboard. But after looking at the circuits with the TIP, I needed to have the other end of the coil on the positive. Once I did that it started working fine.

So why is that, for future use? I can disconnect the tip and add an LED from the 220/pin 3 straight to ground on breadboard and lights up fine. THanks

 

So why is that, for future use?

As soon as current begins to flow in the coil, the voltage on top of it increases. The transistor base needs to exceed the emitter voltage by 0.7V. So if the emitter is on top of the coil instead of at ground, the emitter voltage goes up and the transistor may be turned off. It CAN be made to work - it's called an emitter follower, but it's usually better to avoid if you just want a switch.

 

As soon as current begins to flow in the coil, the voltage on top of it increases. The transistor base needs to exceed the emitter voltage by 0.7V. So if the emitter is on top of the coil instead of at ground, the emitter voltage goes up and the transistor may be turned off. It CAN be made to work - it's called an emitter follower, but it's usually better to avoid if you just want a switch.

Great! Thanks