The calculators are fine, but no calculator in the world can answer this question:
Why do *you* want the oscillator frequency to be 0.125 Hz?

ak

Here is another situation where I am lost. I do not understand frequency here. I would like my on/off cycles to be roughly the same and I would like the "high" to be between 5-10 seconds with the "low" obviously being the same. I know I am dumbing down the fancy words but I am unsure how to put it another way.

 

Two different things, but sort of the same terminology.

Frequency has little bearing on PWM. A non-audio frequency is good. An RF frequency is bad.
So, you can pick nearly anything. Let's just say it's 20 kHz, so you can't hear it. PWM of that frequnecy varies it's duty cycle so the power supply EFFICIENTLY delivers an average DC voltage.

The other part of the equation, let's rename as Dwell. So, it could be say 1 min of PWM(50%) followed by off for 1 min and another minute of PWM(50%). We can say this is 50% "power" loosely.

I think that there is a big problem here and that is the response time of the Peltier is really really long at least in my experience.

 

I do not understand frequency here.

If you are aiming to run the Peltiers at 'half power', then the duty cycle is 50%. The Peltiers don't care a hoot whether they are on for 5 sec, off for 5 sec or on for 1 millisec off for 1 millisec; the averaged heating/cooling effect will be the same because of thermal inertia. However, it is easier to make a timer with short periods rather than long ones.

 

I do not understand frequency here. I would like my on/off cycles to be roughly the same and I would like the "high" to be between 5-10 seconds with the "low" obviously being the same.

Frequency is just eh inverse of period. For example. a 10 Hz waveform has a single-cycle period of 1/10th second. In your case, 4 seconds on and 4 seconds off is a 8 second period, which is 1/8th cycle per second, or 0.125 Hz.

But the real question asked several times is why do you want the on and off times to be sooooooo long? Yes, it will decrease the average power through the device by 50%, but you will see considerable temperature swings as it has 5 seconds to heat up and 5 seconds to cool down. If you want to assure that the peaks of the temperature swings do not approach that of full on, then a better way to go is to stay with a 50% on-50% off cycle, but a lot faster. At anything above 20 Hz for example, or 50 milliseconds on/50 milliseconds off, the power dissipation will be 50%, the average temperature will be whatever that equates to, and the peak temperature swing above that average will be very small.

ak

 

Frequency is just eh inverse of period. For example. a 10 Hz waveform has a single-cycle period of 1/10th second. In your case, 4 seconds on and 4 seconds off is a 8 second period, which is 1/8th cycle per second, or 0.125 Hz.

But the real question asked several times is why do you want the on and off times to be sooooooo long? Yes, it will decrease the average power through the device by 50%, but you will see considerable temperature swings as it has 5 seconds to heat up and 5 seconds to cool down. If you want to assure that the peaks of the temperature swings do not approach that of full on, then a better way to go is to stay with a 50% on-50% off cycle, but a lot faster. At anything above 20 Hz for example, or 50 milliseconds on/50 milliseconds off, the power dissipation will be 50%, the average temperature will be whatever that equates to, and the peak temperature swing above that average will be very small.

ak

Ok. I think I understand a little of what you guys are saying. You're saying that only giving the peltiers 50% power for short periods would be better than 100% power for longer periods? Basically the peltiers at 100% for long periods will have larger spikes in temperature and would it would be way smoother at 50% for smaller intervals? I would basically need to continue using the 555 timer at short intervals but this time instead limit the amount of current going to the peltiers? Would my type of materials needed be the same (i.e. 555 timer - with R1 and C1 -, relay with resistors to limit the flow of current)?

 

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.

How would this look on a breadboard?

 

I don't think peltier devices need resistors in series to limit current (like a diode). A series resistor will limit the current, and along with it the temperature, but at the cost of significant power dissipation in the series resistor. PWM is more complex, but achieves the same thing without the extra power dissipation.

ak

 

How would this look on a breadboard?

If you want PWM in a simple circuit maybe this would be of interest to you.

 

I'm not looking to be controversial here, but as often seems to be the case, I suspect the OP has been completely overwhelmed with far more technical information than he really needs! Can we just take a step backwards...

As I understand it, you have a requirement for a device which uses Peltier units to provide either heat or cold. In order to limit the amount of heat or cold you have proposed an interrupted power source. Am I right so far?

It would help enormously to know what the end use of this device is intended to be. Why do I say that? Let me offer an example:

You come to the forum and say I need a vehicle to move me from A to B that uses the minimum amount of fuel and is non-polluting. People have a think and suggest a bio-diesel car, an electric car, a gas-driven golf buggy, a mobility scooter etc.

When what you actually need is a bike!

Not knowing what the intended end use is, I can't be sure, but I have a suspicion that your requirement can be met in a much simpler way. If it's not subject to national security, can you be a bit more forthcoming?

As a matter of interest, a few other gotchas have come to mind: I don't know how long you want this to run for, but with the power source you describe, you'll get about half an hour at best. You also seem paranoid about getting every last 'volt' from your power supply - but if you are running it at half power, one way to do that is to reduce the voltage! You see what I mean?

More info needed.

 

It would help enormously to know what the end use of this device is intended to be.

We get lots of people here that ask for what they want, not what they need. It is important to conceal the end use in order to avoid offers of a very simple method. One guy wanted about $60 worth of capacitors in a car stereo because he refused to use a relay. Some people have asked for impossible devices to do simple jobs. Some people guess at answers for them, some don't. I offered a thermostat. I don't expect he will use it. Meh. I cast my bread on the water. You can't force a horse to drink it.

 

The battery supply you mention will not supply the voltage and current you are after for any significant time. You will be lucky to get a few minutes out of it. For that kind of load you will need a more powerful battery, such as a 2s2000 lipo. (2 cells in series, 2000 mah) The lipo battery is rated for high discharge rates but you also must keep in mind that you must switch the battery off when the voltage decreases to 3 volts per cell to keep from damaging the battery. Use a rated charger for the lipo also. Even a 2000mah lipo at 3 amps will only give you about 30 to 40 minutes. You must determine your operating time before selecting the battery.

 

If you need to achieve a certain temperature, maybe look into using a pid controller. They can be bought cheaply from eBay nowadays and normally come with a k type thermocouple. This will give you an easy way off adjusting your target temperature to suit your needs.
The other advantage of this is the heat up/cool down times we'll be much quicker as the pid controller will automatically adjust the duty cycle depending on how far off target the temperature is.

If you really need a more manual approach then pwm is the way to go. Use your 555 to trigger a mosfet and a potentiometer to change the duty cycle and you have almost infinitely variable temperature control.

 

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.

Getting your first project to work is always a challenge, but extremely satisfying when it does flash the LED. The 555 timer is a very old but well proven chip and there must be 10's of thousands of circuits out there showing you how to use it. Yes, micro, milli, nano, pico all do matter and at multiplier factors of 1000, make a huge difference if you get them wrong.
The 555 is not a good choice for long long timing periods, say over several minutes, as the C and R values get too large. If you are new to all this and want to carry on with other projects, take a look at the CMOS series of devices (74xx, 4xxx etc.) as these might offer other ways of timing, plus an insight into the wonderful world of electronics.
Ultimately, the better choice for your sort of project, as mentioned by others, is to bite the bullet and go fully digital, i.e. a microcontroller like the hugely popular Arduino platform. This and others like BBC Microbit do timing out of the box and are the the choice in education for teaching digital electronics. Timing and inputs/outputs are what the underlying chips are all about and they are everywhere in use from heating boilers to washing machines. You can set times from microseconds to years, with digital accuracy, plus with the right devices on the output, you could turn the National Grid on and off.
AllAboutCircuits is one of the more helpful and civilised forums compared to others, but you do need to realise that most of these folk are very busy people and the better you explain the problem with a diagram, even if it hugely wrong, will get you help.
In your profession, electronics are the bedrock of diagnosis and treatment, and as you know, an ECG is vital, but a rudimentry one could be put together with just a few components.
Good luck.