I have found that using e-cig/vape cartomisers combined with small diaphragm pumps from blood pressure cuffs make quite effective smoke generators for dioramas. They require a low voltage (5v for maximum life) however they have low resistance (2R - 1R5).
My question is what would be the best way to drive these heating coils at variable current, without creating a bunch of wasted heat with resistors?
I am thinking that the problem may be similar to high powered LED drivers.
I understand that PWM will vary the voltage. Can I use PWM as a function of Ohm’s Law to reduce the current?
If you have read this far, thank you! I appreciate any help I can get.
Yes. but the words "as a function of Ohm’s Law" should be struck out of that sentence, that does not make sense.
PWM switches the signal on and off. It is the ratio between on time and off time that controls the power in your circuit.
Do you need these smoke generators to operate on an on/off basis, or do you need a fully variable control?
If you drop the voltage they will not be at their proper operating temperature, which may cause issues.
PWM WILL vary the current. Look, suppose its a "1" for 0.1 sec and "0" for 0.9 sec.
the "1" (5V, 2ohm) gives 2.5A * 0.1 = 0.25 A sec
the "0" gives 0A * 0.9 = 0 A sec
I average = 0.25 + 0 / 1sec = 0.25A
My apologies. My formal training is in very high voltage AC distribution (110kV +) so I am a bit rusty on correct terminology.
I had planned on using one of the common 5A MOSFET modules. Do I supply it with the maximum voltage (24v in this case) or do I supply 5V (the voltage I need)?
Not at all. PWM varies the duty cycle - the proportion of time the heater is actually powered.
The effective power is directly proportional to the PWM duty cycle, but the power during the time the PWM switches on is proportional to the square of the voltage, so if you power the heater with twice the rated voltage, it will be four times the power and you will need a 25% duty cycle to equate to continuous operation at the rated voltage.
Why would you even think that?
I am literally curious in terms of "what (or how) were you thinking"? You clearly understand the consequences of applying excessive voltages.
An option to vary the current, without using a PWM or using some heat dissipative element (resistor or transistor) to limit, would be to use a switching power supply. This would typically be a module that you can buy a moderate to low cost.
You could choose a variable voltage output, say zero to 5V or a constant current output, such a Lithium battery charging module or an LED driver. I have seen such modules on Aliexpress.
It is likely that a buck converter based on a LM2596 could use the on/off control as a coarse PWM. There are links to discuss this, such as https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/711931/lm2596-how-fast-can-you-switch-input-pin-5-on-off.
When I have designed (my day job) using MOSFETs, I have usually targeted a gate drive of around 10V to ensure they are turned hard on; saturated.
You can use a single device with an NPN and PNP transistor in to provide level shifting. Or bias the FET ON and use a transistor, controlled by the uC output to turn off the MOSFET.
Be careful of what the circuit does if the uC loses control, e.g. at power on rest.
I have used used 'digital transistors' in many applications as a) if not biased they will turn OFF, b) no additional resistors needed so save cost (component, board space and placement cost) c) the are low cost and mutiple sourced.
thank you so much.
You clearly understand the consequences of applying excessive voltages.