Hey guys,
Im needing some help with a project im building. I am trying to run a tiny 12v motor for a couple of seconds and then shut it off for a couple of seconds and then back on.... I searched the net and found my best bet would to be with a 555 timer and so it began. So i sat down and figured up what supplies i needed and came up with the attatched drawing. Sorry this is my first circuit build. Anyway what you see is basically how i made it. I didn't have a 30000 ohm resistor so i put two 15000s in series. Maybe that was wrong...I don't know. So I had everything soldered up and the 555 circuit was working flawlessly. The motor was running for a little over 2 sec and shut off for around 2. I thought i had everything fine so I was letting it run to see how long my 1.2 amp/hr battery would last with it running. 10 mins or so into the run i went to check on it and the transistor was pretty hot so i shut it off to let it cool down. I came back in a little while to try it again and the motor would barely spin. I checked the timer and evadently fried it. It seems the transistor is still ok though. Honestly, I just guessed at the current my motor pulls because it pegged my 250 mA multimeter out with a load but ran free drawing only 40 mA or so. I figured there's no way its drawing over 500 mA. There was a formula on a site that gave me the base resistor value. It was Rb= (V*hFe)/(5*Amps). (12*50)/(5*.500)= 240 ohms. 200 ohm resistor was the closest I had so i used it. From what I've read I wasn't really scared about "over" saturating the resistor, but maybe thats what i did. Maybe that wasn't even the right formula. Another thing i thought it might have been was the fact that i had the load on the emitter side of the transistor. I didn't think it made that big of a difference. One more thing i thought it could have been was that i didn't have a diode across my motor to prevent voltage spikes when shutting off. I dont think that would've destroyed the 555 but probably wasn't good on the transistor Anyways I would just like to get some insight from people who know what there doing on these type of things. I am totally new to circuit building and am in over my head i guess. Thanks in advance.
Adam

 

Part of the problem is you don't have a diode to suppress high voltage spikes. I don't have a exact schematic in my files, but I have one that is close enough.

This configuration for the driver will make the transistor much less hot, since the motor will very closely connected to ground. You used a mode called common collector. This is a comment emitter mode.

 

Thanks for the quick reply. So not having the diode on there made the transistor hot and also having it wired in a common collector configuration made it even hotter and that took out the 555 timer?

 

I just guessed at the current my motor pulls because it pegged my 250 mA multimeter out with a load but ran free drawing only 40 mA or so. I figured there's no way its drawing over 500 mA.

If the motor is drawing more than the transistor maximum of 600mA, and with the slow switching speed of once every two seconds you could try a relay switch. I bought a 5VDC SPDT Relay 44.7mA relay switch that can switch up to 3A@120VAC for only $2.00 - breadboard mountable. It's sort of noisy though.

 

The high voltage spikes are very similar to ESD damage, and is cumulative.

Len Whistler could be right about the transistor rating vs. motor though. Connect the motor under load and see what current it takes to turn.

What is the transistor number?

More than likely the common collector was the problem. This is an analog mode, a standard 555 drops 1.4 volts from the plus power supply voltage, plus the 0.7V drop on the transistor adds a 2.1 volt drop on the transistor, which adds up to heat. The Common Emitter configuration doesn't have this problem.

 

use a microcontroller and program it...........

 

Well guys, I found my main problem I guess. I bought a digital multimeter today so I could check my motors current draw. It turns out that my motor is pulling 1.05 amps with a load on it. Far above what I thought it was. With it drawing so much I figured it would burn up my transistor instead of my 555 timer though. Any suggestion on a transistor I should use? I would use a relay but noise is an issue. It seems that the next step up would be a tip31 3amp. It has a min hFe of 30 I think. What size base resistor should I use. 100 ma should just saturate the transistor at the min hFe. So with that, I need 120 ohm base resistor correct? Or do I need less than 100 mA to it? Thanks for all your help.

 

You sound like you have the basics down. A general rule of thumb, it takes 1/10 the collector current to reliably turn a transistor on (saturation). A MOSFET turns on with much less ohmage when it is on, so it may be a superior solution.

 

Thanks for the Mosfet recommendation. What size pull-down resistor do you think i should use? It seems Radio Shack only carries one Mosfet - IRF510 - I think that will be suitable for what i need. right?

 

Yep, and while technically a MOSFET doesn't need a resistor on the gate a small resistor is recommended to prevent ringing.

LEDs, 555s, Flashers, and Light Chasers

Chapter 10 - Transistor Drivers

BTW, the IRF510 is about the worst example of MOSFETS, they come much better, but it will work well for your application.

 

Ok, I need some more help. lol So i made another circit tonight. It worked when i connected it , disconnected it, connected it again, and motor started to spin and then shut off. Now nothing is working. so frustrated. I ended up using a tip31 transistor with a 150ohm base resistor. R1 is 2200ohm, r2 is 32,000 ohm. I had to use a low power 555 (output 100 mA) because thats all radioshack had. I checked around the circuit for voltage and im not getting anything after r1 so im thinking i blew the resistor. ??? Im using 1/4 watt resistors which i thought were plenty but now im not so sure. Again i didn't use a diode across the motor, i know i should have but i didn't think 10 cycles could hurt. Maybe i just fried the timer with a voltage spike. I really appreciate you guys helping me. Thanks
Adam

 

I would take the motor part out and get the 555 timer going with a LED to represent the motor. Once it's working modify it and add the motor. This is a good 555 tutorial, which might need adjustments for 12 volts.

http://www.horrorseek.com/home/halloween/wolfstone/TechBase/com555_555TimerCalc.html



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You do realize I have a tutorial on 555s in the experiments area (Volume 6 of the ACC book)?

I'll draw up what I think the OP is doing in a bit.

 

Wow Bill! That is a lot of information you have there. Its gonna take me a while to go through it all. Very nice work. I really thought i had everything figured out but now im lost. Im gonna just start from scratch and try to re-think everything. So you guys don't think it was the resistor? One more question. I saw on Bills page( i think ) that the duty cycle needs to be 55% or more. A few of the sites I saw, said above 50% duty cycle. With my resistor values my duty cycle is about 51%. Is that a problem that it's below 55%?

 

Actually, the more I learn about 555's the less critical duty cycle is. There is a circuit to do everything after you learn them all.

My suggestion, work with the design you have, and improve on it until it works. Treat the two resistors in series (a perfectly valid approach) as one resistor.

BRB, drawing...

 

You used a TLC555? Those can sink up to 100mA, but can only source about 10mA. That's not going to work very well.

 

I was going to bring that up. Radio Shack also carries the LM555, which is the part you need. The TLC555 (the part you have, a CMOS 555) can be useful, but it is a low power version. If you need batteries to last a long while, the CMOS is great. But if you need power, the LM555 is the right chip. It can output 200ma (use ½ that).

Anyhow, this is what I think you are doing. The transistor you had in example 1 was starving for drive on the base. R3 is a minimum.

Try the MOSFET shown in example 2, I think you'll like it much better.

I took this from my cookbook, one of many projects in process (not completed).

Bill's Index

My Cookbook

Don't get overwhelmed. It is easy to do, but most of the guys here are glad to help. Set your design goals, and we'll get you a working circuit. Hopefully you'll have fun and learn something.

.

 

Well I guess the low power 555 was my problem on the last one. I re-used everything except for the low power 555 replaced with a high power one and now its working like a dream. I had no idea it could only sink 100mA. Oh well! I want to thank you all for being so patient and being so helpful. I really appreciate it. Thanks
Adam

 

It's what we do here.