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.

I do not know if this makes a difference, but one of my switches is a dpdt that switches from cold to hot and so forth?

 

I do not know if this makes a difference, but one of my switches is a dpdt that switches from cold to hot and so forth?

So, you have a HEAT/COOL mode as opposed to something being able to select the mode automagically which is OK.

 

So am I just talking to myself here?

 

So am I just talking to myself here?

Supposedly in post #8 the attached file "IncreasingCurrent 555" is the work around to allow a current higher than 100mAh to be routed to your load while still using the 555 as the timer chip. The only problem with that is the transistor used would take away amperage which I don't want to do. At least the way my limited mind has worked it out lol.

 

Yes and in post 4 I mentioned using a common 5 volt rated relay instead of a transistor which would give you no voltage drop or current loss other than the tiny amount of current the relay coil requires to operate.

In other posts I also mentioned a simpler way to get your 4 - 5 second on and off cycle times with only one resistor and capacitor for the timing and even gave you the correct values for each as well when pointing out the errors in the values you chose. So have others hence the feeling like I am talking to myself.

 

Yes and in post 4 I mentioned using a common 5 volt rated relay instead of a transistor which would give you no voltage drop or current loss other than the tiny amount of current the relay coil requires to operate.

In other posts I also mentioned a simpler way to get your 4 - 5 second on and off cycle times with only one resistor and capacitor for the timing and even gave you the correct values for each as well when pointing out the errors in the values you chose. So have others hence the feeling like I am talking to myself.

Is there a particular relay model you suggest? Would I connect the relay to the same spot the transistor is on the circuit diagram?

 

researching things completely on my own without guidance of any kind.

you may find it beneficial to take a more systemic approach to learning.

In the end, you may get there faster.

 

What is the ultimate goal of the circuit:

a) Decrease the total power to the device by 50%. If so, your concerns about a small voltage drop across the switching component are irrelevant. If the switch component decreases the total power to the device by 10% (an excessive number for this example), the 555 circuit can be adjusted to decrease the power by only 40%, for a system total of 50%.

b) switch the device from hot to cold - 4 sec. hot, 4 sec. cold, repeat. Why?

c) other.

ak

 

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!!

130K (130,000) ohms and 47 microfarads is close to your needs.
If we tie the top of the resistor to the output we get a 50% duty cycle.

 

If you change your mind and want a thermostat, here's a 1 chip thermostat that's very reliable.
http://forum.allaboutcircuits.com/blog/lm723-as-a-thermostat.532/

 

Is there a particular relay model you suggest? Would I connect the relay to the same spot the transistor is on the circuit diagram?

Yes it connects to the output and the ground.

Most any small relay with a 5 - 6 volt coil and 3or more amp contact rating will work fine.

 

What is the ultimate goal of the circuit:

a) Decrease the total power to the device by 50%. If so, your concerns about a small voltage drop across the switching component are irrelevant. If the switch component decreases the total power to the device by 10% (an excessive number for this example), the 555 circuit can be adjusted to decrease the power by only 40%, for a system total of 50%.

b) switch the device from hot to cold - 4 sec. hot, 4 sec. cold, repeat. Why?

c) other.

ak

My ultimate goal is to turn the device on (cold), off, on (cold), off, cycle. Then once I flip the DPDT I want it to cycle on (hot), off, on (hot), off, cycle. I need the device to cycle on and off so that it is not allowed to get either too cold or too hot.

 

Yes it connects to the output and the ground.

Most any small relay with a 5 - 6 volt coil and 3or more amp contact rating will work fine.

I have seen this as a solution in lieu of the transistor. Just to make sure, I will need specifically a "SPST" (since I've seen SPDTs and DPDTs) 6V 3+A relay correct? All the ones that I have seen are for automotive which I assume is ok? Also, I have seen that there is a formula to determine if the 555 will have enough output to drive whichever specific relay I choose?

 

SPST or SPDT in series with the power supply to the device. For a SPDT, just ignore the normally closed (NC) contact.

ak

 

it is not allowed to get either too cold or too hot.

sounds like you want pwm control.

If you only want to cut the power in half, a 50% dc pulse train will work - a 555 timer.

if you want the ability to fine tune the power, the same 555 timer + two diodes + pot will work.

 

This question has come up several times - If all you want is to decrease the average power to the peltier device by 50%, why is the switching frequency so low? Electrolytic capacitors, like the 47 uF in your circuit, do not do well in timer circuits. They have very low accuracy, can leak enough current to affect the timing period, drift with temperature changes, etc. If you increase your switching frequency by 500x, the capacitor now is 0.1 uF, a nice, normal, ceramic capacitor that will sit there like a stone.

Note that switching the power on and off more rapidly will not affect how the peltier device works or how cold/warm it gets. 62 Hz still is very slow, and 50% is 50%.

ak

 

My ultimate goal is to turn the device on (cold), off, on (cold), off, cycle. Then once I flip the DPDT I want it to cycle on (hot), off, on (hot), off, cycle. I need the device to cycle on and off so that it is not allowed to get either too cold or too hot.

I see.
As others said, this timer concept is not the best for any form of temperature control even a crude one. What I would be looking at is at minimum using a very basic Op-Amp comparator IC in place of the 555 and having that uses a thermistor as a temperature sensor attached to the side of the Peltier that's attached to whatever it is you want to keep at a constant temperature.

Ideally however a proper temperature control IC and its related circuitry would be best.

Electrolytic capacitors, like the 47 uF in your circuit, do not do well in timer circuits. They have very low accuracy, can leak enough current to affect the timing period, drift with temperature changes,

Sort of true but unless the timing needs to be very precise and stable it's rarely an issue. Especially in a application where there is a +- 25+% tolerance for the timing cycle rates.

 

This question has come up several times - If all you want is to decrease the average power to the peltier device by 50%, why is the switching frequency so low? Electrolytic capacitors, like the 47 uF in your circuit, do not do well in timer circuits. They have very low accuracy, can leak enough current to affect the timing period, drift with temperature changes, etc. If you increase your switching frequency by 500x, the capacitor now is 0.1 uF, a nice, normal, ceramic capacitor that will sit there like a stone.

Note that switching the power on and off more rapidly will not affect how the peltier device works or how cold/warm it gets. 62 Hz still is very slow, and 50% is 50%.

ak

So I have seen suggestions about only using R2 and C1 for my timer. What values to you think I should use? I am unsure if I should use the online calculators as a reference anymore. Would you say 130K for R2 and 0.1uF for C1?

 

Would you say 130K for R2 and 0.1uF for C1?

that's a function of your application: your device's characteristics and your objective.

 

I am unsure if I should use the online calculators as a reference anymore.

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