The complaints about using a PWM scheme to reduce the heat are all off base. A resistive heating system is an integrating system, and so reducing the power by switching it off during some time cycle that is much shorter than the thermal time constant of the heated portion will simply lead to a reduced temperature due to the reduced power input to the heater. The only reduction in efficiency would be due to the power consumed by the PWM circuit, which could be very small with a careful design.

Why do folks want to make things so complicated? A variable duty cycle PWM controller can be as simple as a dual 555 timer IC, with the first section serving as a ramp-wave generator and the second section serving as a comparator to drive an appropriately rated FET device to switch the heating power off and on. That is about as simple as it can be, also cheap and using easy to find parts.

 

Hi MisterBill,

Yes i agree the buck is only needed if the heating element is extremely non linear or if driving an LED.
PWM will probably work, and should be the first try. Pseudo PWM even works on light bulbs to some extent.

The only other point would be that if the heat output had to be reduced by a lot then a different battery should be used.

 

I’m still keen on the diode solution. If the diodes are placed correctly, the heat produced by the diodes will contribute to warming whatever the heating element is heating. That means 100% efficiency for the battery. This solution is small and cheap.

The only reason I’d consider PWM is if a wider range of control is desired instead of a single setting. I’d buy a pre-assembled module off eBay. This solution will require some sort of enclosure (ie it’s bigger than the diode solution) and the efficiency will be 80-90% because the lost energy will not likely help warm the target.

Either is a perfectly good solution. The choice just depends on what you want.

 

I’m still keen on the diode solution. If the diodes are placed correctly, the heat produced by the diodes will contribute to warming whatever the heating element is heating. That means 100% efficiency for the battery. This solution is small and cheap.

The only reason I’d consider PWM is if a wider range of control is desired instead of a single setting. I’d buy a pre-assembled module off eBay. This solution will require some sort of enclosure (ie it’s bigger than the diode solution) and the efficiency will be 80-90% because the lost energy will not likely help warm the target.

Either is a perfectly good solution. The choice just depends on what you want.

Hi,

Yes that's a good idea. Even a second series resistor.
I would have given you two 'likes' but only one is allowed

See this is why it is good to discuss this stuff rather than jump to any conclusions right away.

 

Hi,

Yes that's a good idea. Even a second series resistor.
I would have given you two 'likes' but only one is allowed

See this is why it is good to discuss this stuff rather than jump to any conclusions right away.

Hi again,

Here is the derivation of the required series resistance, assuming this extra resistor will also participate in the heat.

The required voltage was given at 6 volts, and with a constant heater resistance of 9 ohms that leads to a power:
P1=6^2/9=4 watts.

Now to choose the value of the series resistor R2 we just need to note the battery voltage of 7.6 volts and the power in both resistors must equal 4 watts:
7.6^2/(R1+R2)=4

and since R1=9 ohms we get:
7.6^2/(9+R2)=4

and solving for R2 we get:
R2=5.44 Ohms.

Checking:
7.6^2/(14.44)=57.76/14.44=4 watts

so this works out and so the required resistance is 5.44 Ohms. The power level is:
(7.6/14.44)^2*5.44=1.51 watts

so the required power rating of this new resistor is about 3 watts, so a 5 watt unit would work well.

If two 1N4001 diodes in series are used, the voltage drop would be around the same with 300ma current level, so the two diodes should work also.

Now one last thing, note above the word 'constant' when referring to the 9 Ohm heater resistance. If this value goes lower because of the fact that it will be operated at a lower voltage about 80 percent of the original (which is significant for some devices) then the following formula is used:
v1^2/(R2+R1)=v2^2/R1

where R1 is the heater resistance, v1 is the original voltage, v2 is the target voltage, and R2 is the new series resistor value.
So solve that equation for R2.

 

If the diodes are placed correctly, the heat produced by the diodes will contribute to warming whatever the heating element is heating. That means 100% efficiency for the battery. This solution is small and cheap.

That's a good solution if indeed, the diodes can contribute to the heating element heat.
If the oven temperature is too high for the didoes, then an additional series resistor placed to help the heaters should also work well.

 

That's a good solution if indeed, the diodes can contribute to the heating element heat.
If the oven temperature is too high for the didoes, then an additional series resistor place to help the heaters should also work well.

I was picturing something like heated gloves or a seat warmer. You're right, it could be something much hotter.

 

6 Watts is not even enough for heated gloves.

Bob

 

6 Watts is not even enough for heated gloves.

Bob

Maybe a vaping gizmo?

 

The original complaint, back in post #1, was that it was "a little bit too hot", which says that the power needed to be reduced a bit, and since the amount was not stated an adjustable amount would make the most sense. In addition, it makes sense to reduce the power dissipation and increase the run time. So an adjustable simple PWM system makes sense, and avoids having to deal with either hot resistors or hot diodes. And a diode flowing over half an amp will certainly be fairly hot.

 

PWM. My electric stove, the heating elements on the top were PWM. Much slower than modern PWM, but because of the thermal inertia of a heating element, you don't need to switch it on and off thousands of times per second. As mentioned, the electric stove has a heating element that is controlled by a knob. Inside that knob is a bimetallic strip and a tiny heating element. Depending on where you set the knob you control how much heat is reaching that tiny heater. It warms up then switches off the power. It cools off and switches power back on. It may operate depending on the setting, somewhere (guessing here) of 5 cycles per minute. I said "MINUTE".

You don't need to precisely control power to a heater you only need to turn it on until warm then off till slightly less than warm. I built an oven for curing epoxy samples using a 555 and switching a lightbulb (heat source) on and off every few seconds. The overall heat was pretty constant. Before that we tried a conventional toaster oven. The problem with that was that when temperature was reached the power would shut off. But because the heating element was still some 5,000 degrees F the temperature would continue to rise in the oven. Eventually it would cool and the temperature would start to drop back down. But when it was cool the power would come back on and it would take a minute or so to heat back up. The temperature profile was so wild that it was useless. I chose a lightbulb because it could be easily controlled and the runaway (thermal inertia) temperature could be much better controlled.

The TS wanted cheap. Diodes - if they can be counted on to drop enough voltage is probably the cheapest way to go. But if that doesn't work out - and I've tried to drop a few volts using diodes - - unsuccessfully - I think the best solution is PWM. And it can be built easy enough using a cheap 555. The heater can be on for 90 seconds then off for 30 seconds and you get 75% of the heating power without having to calculate the resistance of the heating element. Plus, the PWM will be far more adjustable. So if you want just a little change in heat you have it. Or a bigger change in heat - again, you have it.

I wouldn't go with an Arduino. You said you want a cheap and easy solution. Try the diodes first. If that doesn't work out then try a crude PWM (555 timer).

 

The original complaint, back in post #1, was that it was "a little bit too hot", which says that the power needed to be reduced a bit, and since the amount was not stated an adjustable amount would make the most sense. In addition, it makes sense to reduce the power dissipation and increase the run time. So an adjustable simple PWM system makes sense, and avoids having to deal with either hot resistors or hot diodes. And a diode flowing over half an amp will certainly be fairly hot.

Hi,

Actually around 300ma.

 

That's a good solution if indeed, the diodes can contribute to the heating element heat.
If the oven temperature is too high for the didoes, then an additional series resistor placed to help the heaters should also work well.

Hi,

All calculated in #25.

 

I would try the PWM method if it was my project.
With an Arduino, FET and a thermistor, you could control the temperature too.

Agreed, PWM is simple and easily controllable, either by tweaking a simple potentiometer or something more complex.
In the industrial world, most heaters are controlled using forms of PWM.

 

Hi,

Yes if it needs to be *adjusted* then obviously there has to be a way to adjust it on the fly. If not, it's easy to try a couple different resistor values.

There is also the thought of what happens as the battery runs down. Then we need not only PWM but also a control system with feedback. It all depends however on what kind of accuracy we need in the heating temperature.