# High current timer help

Discussion in 'The Projects Forum' started by JohnnyAstro81, Sep 4, 2016.

1. ### JohnnyAstro81 Thread Starter New Member

Aug 16, 2016
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I am a complete novice so please please forgive my lack of knowledge. I am trying to build a circuit that will switch on, off, on, etc that will be 6V and approximately 3A. I have attempted to research this on my own and have been working with a 555 IC. I believe that the 555 puts out too little current (100mA) to power my device (3A). I would like the timer intervals to be about 4-5 seconds a piece. Using an online calculator my R1 is 10ohms, R2 is 130ohms, and C1 is .047uF. Is that correct? From what I have seen online I can use the 555 to power a transistor that will trigger the higher current? It says that the transistor will modify the voltage though and I want to keep the output to stay at 6V and 3A. How do I do this? Thank you for your time!!

2. ### #12 Expert

Nov 30, 2010
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Need schematic because 10 ohms is not in the range that causes 5 seconds.
When we see the schematic, we can figure out how to deliver 6.0 volts.
We also need to know what you are delivering 6 volts to.

3. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Nothing like correct . But we can fix that.
Yes. What is your power supply specification?

4. ### tcmtech Distinguished Member

Nov 4, 2013
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At minimum multiply your two resistor values by a factor of at least 50 to keep from burning out the 555 IC.
Then for the output current boost you can either use a transistor or a small 5 volt relay.

5. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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First, make sure you have a complete definition of what you want. Two things:
1. Is it important that the on and off times be *exactly* the same, or only close? The standard 555 astable circuit cannot do 50-50, but a modified version can.

2. Is it ok for the load to be connected directly to the +6 V, and the "bottom" end is switched to Ground, or is one end of the load connected to Ground (the - terminal of the battery or power supply) and the circuit switches the "top" end of the load to +6 Vdc?

3. There are two versions of the 555, standard (LM555, NE555) and CMOS (LMC555). Pick one.

Then start with the 555 datasheet. Read it end-to-end, twice. It has tables and graphs that indicate the preferred range of values for the timing components. Because your on and off times are relatively long, start with larger resistor values and recalculate the capacitor value. Draw and post a schematic, following the layout on the circuits in the datasheet. Hand-drawn and a cell phone photo are fine, but neatness counts.

ak

6. ### dannyf Well-Known Member

Sep 13, 2015
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don't forget about C6 and L2: they are vital in getting your circuit working as desired. Formula 4.1 on Page 25 has to be worked out first, however.

7. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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Are you sure your real name isn't Colin?

ak

8. ### JohnnyAstro81 Thread Starter New Member

Aug 16, 2016
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Thanks for your responses! The schematic of the circuit I was using to base my timer intervals is attached. I am currently using the NE555. The calculator I used to get my R1, R2, and C1 values is as follows: http://www.ohmslawcalculator.com/555-astable-calculator. Assuming the calculator is correct R1 @ 10ohms, R2 @ 130ohms, and C1 @ .047uF would give me a high (on) time of 4.56 seconds and a low (off) time of 4.234 seconds. My on time and off time do not have to be exactly the same but the closer the better. My source is 4x rechargeable AA batteries @ 6.4V and 2800 mAh. I absolutely need all 6.4V and 2800 mAh to go to my load. My second schematic is what I found for increasing the output of the 555 to more closely match that of my load requirements, but from what I have read the transistor will rob me of at least 1V. Thank you all again for your help!!

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9. ### tcmtech Distinguished Member

Nov 4, 2013
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Just for a heads up on simplifying the 555 timer schematic and as built circuit.

If you eliminate R2 and connect R1 between pin 3 (output) and pins 2 and 6 that are tied together you can get full 50/50 duty cycle operation with one less component to work with.

And don't forget that the online calculator is specifying your resistor values in Kilohms not ohms which is a difference by a factor of 1000 times.

Also for a 4 - 5 second on and off time I found R1 value of 68K (68,000 ohms) and C1 value of a 220 uF falls right in that range.
I'm not sure where your 10 ohm, 130 ohm and .047 uF numbers came from for that equivalent timing period being the calculator says that would be ~ 114 Kilohertz (113,720 Hz) although in reality the 10 ohm resistor would cause the IC to burn up near instantly being the discharge pin 7 would be trying to sink something like 600 milliamps.

Last edited: Sep 4, 2016
10. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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That is a good calculator site for the standard circuit, and can be fooled into working for the modified circuit.

Nothing about your calculations is correct, probably because you did not pay attention to the units selection. Your values as stated above calculate to 4.56 and 4.23 m*i*c*r*o seconds, not seconds. Try again.

The transistor will rob you of some output voltage, but 1 V seems high for 3 A. Where did you get that number. You are using the transistor as a saturated switch, so it is the saturation voltage (Vcesat) that matters. Many transistor datasheets have a plot of Vcesat vs. Ic.

ak

11. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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You can use that calculator for the 50/50 circuit by setting R1 = 1 ohm.

ak

12. ### tcmtech Distinguished Member

Nov 4, 2013
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Yea. In reality however....... hows that IC going to take sinking 6 amps through pin 7 when running on a 6 volt power source?

I'm surprised that that calculator will let you put that low of numbers in being in reality they would kill the timer in fractions of a second.

You just know that if the calculator lets those numbers go through someone will try and build it that way.

13. ### JohnnyAstro81 Thread Starter New Member

Aug 16, 2016
18
0
Thanks you! Ok! Sorry this http://www.555-timer-circuits.com/calculator.html is the website that I used. For some reason this site says seconds while the other one says microseconds? (That is why I made the assumption that microseconds and seconds were the same units) I honestly do not truly know how many volts the transistor will rob me of, but I absolutely need the modified circuit, whatever form that will take, to not rob me of any volts whatsoever (will it rob me of current also?). I used the above sites as reference points but I have never built a circuit myself. This is my first attempt ever. The device I am powering consists of peltiers and fans all wired parallel to each other. Each component requires 5-7V and approximately 2.8A at 6V. The device works well, but I want to keep the peltiers from getting too hot or too cold. I could use some sort of thermostat but that would be way more complicated and bulky compared to the timer circuit. All I need is the diagram of a circuit timer that will power my device at 4 second intervals. It does not have to be a 555 based timer. The 555 just happened to be the only chip that I have seen that will be the easiest to configure. I am confused on how to either make the voltage and current to work with the 555 or what would be a better equivalent. Thanks again for the help!!

14. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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It's a calculator, not a simulator. Dangerous, but I find it handy.

ak

15. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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I'll get back to your circuit later. But first...
How old are you? That is not a snotty question. Granted you are a newbie, but there is a minimum tech level we assume in communicating with someone who has found this site, and your statement changes that. Nothing that can't be adapted to.

ak

16. ### Alec_t AAC Fanatic!

Sep 17, 2013
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How did you decide on the 4 sec intervals? Most Peltier devices have a large thermal inertia, so the mark/space ratio of their supply switching is far more important than the switching frequency/intervals, unless you are deliberately trying to ramp the heating/cooling up and down.

17. ### JohnnyAstro81 Thread Starter New Member

Aug 16, 2016
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I am 35. I am a Paramedic by profession though. What I have learned so far has been from researching things completely on my own without guidance of any kind. This site was the most recommended for its helpfulness which is why I chose it. I hope you all can forgive my ignorance but as I tell all new Paramedics that ride on the squad, we all have to start somewhere. If someone hadn't took pity on me when I started and helped me learn I would not be as confident and competent as I am today. So I am really just beginning to get into ICs, but only because so far my project has not required anything besides some switches, LEDs, a power supply, resistors, and the basic components I have mentioned above (i.e. Peltiers and fans). I am using the peltiers out where it's possible they l could come in contact with skin. From what I have researched and personally observed is that the peltiers can get as cold as -15F and heat to about 158F (optimally) both being obviously harmful if they come into contact with skin. Rather than putting in a bulky thermostat to control those extremes I have opted for the cheaper and easier solution of using a timer so that the chips are not allowed to reach either extreme. As this is a research project 4-5 seconds is merely a starting point to test how hot and cold the chips are allowed to get. I will more than likely want to increase that time interval as I observe the maximum and minimum temperatures.

18. ### JohnnyAstro81 Thread Starter New Member

Aug 16, 2016
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I apologize for the confusion. Regarding the calculator (http://www.555-timer-circuits.com/calculator.html), you had said earlier that these figures were not correct. Does your quote above mean that the figures are correct then?

19. ### #12 Expert

Nov 30, 2010
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I try to answer on the level of the questioner, but sometimes it's hard to tell. The fact that you're a beginner doesn't matter. Whether we know you're a beginner does matter.

JohnnyAstro81 likes this.
20. ### KeepItSimpleStupid Distinguished Member

Mar 4, 2014
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What #12 said.

delta T matters and it's USUALLY no more that about 60 deg C. Max temperature matters too. Low temperatures in free are generally causes condensation. See Dew Point. It's a function of air temperature, RH and surface temperature.

I think you really want a Pulse Width Modulator or PWM. This changes the duty cycle. If you want heating and cooling, you would likely need a circuit called an H-bridge. H-bridges can be made from FETS which have a lower voltage drop. H-bridges are used to reverse the polarity of device powered by a single supply, e.g 12 V rather than +-12V power supplies.

This http://www.jameco.com/z/K8004-Velleman-DC-to-Pulse-Width-Modulator-Kit-Control-DC-Motors_120539.html does not meet your goals, but it's something to look at.

Here http://www.vishay.com/docs/70007/si9986.pdf is a low-current H-bridge on IC form.

Micro-controllers can easily generate PWM waveforms.