SparkFun Forums

[dossjh]

Hello!

I have a 555 timer in astable mode driving a NPN Transistor

this one:
http://www.jameco.com/webapp/wcs/stores ... _211385_-1

When I hook this up to an led to test it this works perfect, but when I try to hook it up to the car the transistor just gets really hot and the lights don't flash. I tested and the lights use 5.5A (per side) I had a 1k ohm base resistor I tried changing it to 330 ohms with no affect.

Also, I am confused why the lights don't even blink, getting really hot I can understand but current is obviously flow why not even a little light??

Any ideas would be greatly appreciated!

Thanks,

Doss

[Chevelle]

Can you post a schematic? That will really help in trying to figure out what is wrong.

[Blackfin]

Yes, a schematic would definitely be helpful.

The datasheet for the TIP3055 shows that it has a minimum hfe of 20 at 4A collector current. At 10A it's just 5.

This means that you'd need a base current of 4/20 or 200mA to properly conduct 4A and you'd need 10/5 or 2A of base current for 10A of collector current. If you have 5.5A per side you're going to need a pretty substantial base drive for this transistor to work. Assuming you're running the 555 at 12V the base current, even with a 330R resistor is going to be, at most, 12-0.7/330 or 34.2 milliamps. This will be fine for an LED but not for 11 amps of incandescent lamps.

Consider using something like a BDW42G Darlington NPN instead. It has an hfe of 250 at 10A collector current, meaning you'd only need a base current of 40mA to switch 10A through the collector.

But to know for sure we need to see your schematic.

[waltr]

Yep, Blackfin nailed the reason it isn't working.
I would use another transistor to drive the TIP3055's base.

[dossjh]

I think Blackfin is exactly right, I started wondering about this earlier. I am going to try to use another transistor to drive the base of the TIP3055 as suggested bu waltr.

Sorry about no schematic, not sure the best way to draw it.

Doss

[Blackfin]

FWIW, the technique of driving the 3055 with a second transistor is fine but it's a discrete implementation of a Darlington transistor, which puts both transistors in one package. IOW, you'll need a series resistor, a couple of pull-down resistors and two transistors to do discretely what you could do with a single series resistor and a single Darlington.

[dossjh]

I want to try to get this working discretely, I have attached a schematic of what I have so far, and its still not working. For testing I have removed the 555 and all of the blinking components and I am just trying to illuminated one side (5.5A). I calculated that I would need a 2 ohm resistor to limit the base current to 7 amps, which i dont have so i placed two 1.5 ohms in series. The lights now turn on, but its still getting really hot.

Am I just trying to source to much current? Should I use a relay? This is my first real world project (you know, not just blinking an LED or something) and I was just trying to use a transistor.

Anyways, With the posted schematic maybe someone can offer a suggestion.

Thanks!

[dossjh]

Q3 is acutually MJE3055T this one:
https://www.jameco.com/webapp/wcs/store ... 1_25857_-1

[Mee_n_Mac]

You're trying to use the NPN's as a "high side" switch. That's fraught with problems. You usually place NPN's btw the load (which is connected to +Vcc) and ground and the NPN acts as a switch closure to ground. If all you have available is the "high side" of the bulbs (and that makes sense as the ground/return for auto circuits is usually the body/frame) then you need to use a PNP transistor(s) to act as a switch from load to +Vcc.

[Mee_n_Mac]

Here's one possibility for your 555 circuit using a pair of transistors to switch the high side to the bulbs (which I think are in parallel).
(click on to open)

Blinker555.jpg

You may have to change the R's and C's to get the right timing and to match whatever transistors you use.
A plot of the 555 output voltage (vs time) and the current through one of the bulbs is below.
(click on to open)

Blink555plot.jpg

[jremington]

You could try something like the following with the transistors you have (where R1 is the equivalent of two lamps, drawing about 10 A from 12V). I chose the base resistor values according to the model for the 2n3055 transistor that comes with LTSpice. This leads to saturation of Q1 with about 10 watts dissipation, so it will still heat up. It is not very efficient to use transistors of this type (BJT) and you may not be able to use this circuit in an automobile because, as Mee_n_Mac pointed out, one lamp terminal is usually grounded to the frame.

darlington.png

[dossjh]

Ok, so I swapped the NPNs for PNPs and got everything working...mostly. It still gets to hot for my liking, I think its just to much current without a heat sink. I was hoping since it was 1/3 of the max it would be ok, but its not

I have decided to just use a relay, but now I have a different problem.

The car is a grand am with an hazard switch and flasher in one, and instead of just having a switch coming in from the blinker switch they had to go and make everything difficult.

light schematic.png

So the way this works is you have to send a signal down the flasher signal line, when the turn signal switch is closed you will get this signal back and you have to send power down the flasher signal line to blink the lamps.

So this is my attempt:

grand-am-turn-signal.png

I can't test this circuit until my girlfriend gets home with the car, so in the mean time I was just wondering if anyone had any better ideas?

Thanks!

Doss

[dossjh]

I just realized there is no way to tell what the 555s are doing. U1 is in astable mode providing the blink. U2 is in monostable mode turning on the transistor to allow that blink to pass to the relay.

[jremington]

There are lots of problems with that circuit; it cannot possibly work. Perhaps you should explain what the various parts are supposed to do. To begin, why are Q1 and Q2 connected in series? What do C1 and C2 do, and how did you choose the value? Wires to nowhere? You need to show the rest of the connections to and parts associated with U1 and U2.

The more fundamental question is, what do you really want to do? Is this circuit intended to replace the flasher circuit in a car?

Since you can draw schematics, I suggest to download and learn to use LTSpice. It is free, easy to learn with on-line tutorials, and best of all, it can simulate a circuit that you enter as a schematic diagram! It includes models for the 555 timer IC and common transistors. It is a fabulous tool for experimentation and learning.

[dossjh]

That is exactly what it is intended to do. The wires to nowhere are connections to the car, I tried to label them to make that clear, the software i was using is online and I didn't see a symbol for connector.

The flasher circuit is easy to replace, its just like flashing a (really high current) led. The problem comes with how the manufacture decided to detect the blinker switch was closed. Thats the purpose of Q2 and U2, and why Q1 and Q2 are in series. Q1 is the amplifier for U1 to drive the relay. While Q2 is only on when U2 is pulsed by the left or right enable lines (coming from the car connector).

Thanks for the tip about LTSpice, I have been using circuitlab.com which is pretty good, and offers some simulation, but only graphs and such.

C1 and C2 are just decoupling capacitors, and i just choose 100uF because thats what I have. I removed one because I only have two left and I need one for U2.

I am posting an updated schematic.

Thanks for the response!

Doss