Control Heat Gun via PWM

[jbrewster]

I have been using a 10A light dimmer to control heat guns. I have not split the fan and heating element to run them seperately. Initially that was my intention but the roasts I achieved were great and I was not convinced that I was going to improve the results by doing it.

That's essentially how cheap "variable temperature" heat guns work.

[jbrewster]

I guess what I am trying to ask is,
1. Can you effectively control a heat gun's output via PWM? (without splitting heater/fan)
At low power levels, is the unit likely to overheat due to lack of air blown over the element or any other problems from dropping power to the combined heater/fan?
2. If splitting the heater/fan's circuits, is it best to only vary the heating element's output leaving the fan to run constantly?

1. Yes. But you'll have a lower limit after which the fan will no longer run (which is a problem, refer below).
2. Yes. I doubt varying the fan speed would do much for you in any case.

Any remotely sensible heatgun will have a thermal fuse or similar in the heating element to prevent overheating, and the elements are designed for use where there is continuous air flow, so even if there wasn't a thermal fuse chances are the nichrome wire would melt eventually without air flow (fortunately this is less dangerous with heatguns than say a hairdryer as the element is enclosed so less likely to set the casing on fire).

It's probably worth digging up Andrew Tridgell's thread on Linux powered coffee roasting (he also did a talk at LinuxConf.au on it), he figured out the lower limit for his HG and also has some details around caveats.

I'm about 90% done with a project which sounds a lot like what you're trying to do here, I'll be posting some details once it's done.

[Beanz.]

I have been using a 10A light dimmer to control heat guns. I have not split the fan and heating element to run them seperately. Initially that was my intention but the roasts I achieved were great and I was not convinced that I was going to improve the results by doing it. The only thing I have been careful of is not stalling or stopping the fan which would probably result in burning out the element and fan. I achieve a good balance of an acceptable fan speed versus required roasting temperature and RoR at the low end of the roast and as such have not needed to complicate matters with modifications. I have run this system on a couple of hundred roasts using both a low cost Aldi heat gun and a cheap Arlec heat gun with no problems.

[fg1972]

Thanks for the replies everyone,

I'm aware of smart heat guns that allow greater control from the gun itself but I'm only interested in controlling the heat output externally via hardware/software closed loop control so none of the controls on the gun will be utilized.
Splitting the heater/fan circuits shouldn't be a problem if the blower being powered directly from mains or via step down AC or DC off the heating element.

My original question was probably a bit vague,
I guess what I am trying to ask is,
1. Can you effectively control a heat gun's output via PWM? (without splitting heater/fan)
At low power levels, is the unit likely to overheat due to lack of air blown over the element or any other problems from dropping power to the combined heater/fan?
2. If splitting the heater/fan's circuits, is it best to only vary the heating element's output leaving the fan to run constantly?

[Steve82]

If you want to splash out on even more control there is the professional series as well, Bosch GHG 660 LCD Professional, which runs 2300W and as well as having the same kind of temp control as PHG630DCE, it has fan control in steps of 10 as well, giving as much variation one could possibly need. Very happy with my PHG630DCE though.

[jbrewster]

It is possible but it's not a simple task. Also requires mains wiring so unless you're experienced in that I wouldn't recommend you try.

If, however, you are experienced with mains wiring, there's an Application Note on the Freescale website covering a novel topology which is the simplest solution I've found for this, the App Note is aimed at PWM control of AC Induction Motors but will work for a resistive load (and can probably be simplified too as there's no need to manage flyback).

I'm intending to do this myself but I've not had time to implement it as yet.

It would be best if you can separate the heating element and the blower motor, however this is not a simple task in many designs as they use the heating element as a dropping resistance to reduce the voltage to the motor (which is typically a cheap permanent magnet motor), but I suppose you could just supply the motor from a dedicated supply (i.e. not directly from mains).

Yelta's solution is the easiest though

[Yelta]

Why not simply buy a Bosch 2000W Heat Gun PHG630DCE, 3 fan speeds, LED readout, minimum temp 50C max 630C continuously variable, available for $99 from Masters.
Been using one for years, gives great temp control over the roast and very reliable.

[fg1972]

I've got a few ideas floating around in my head to build a coretto style roaster and looking at controlling gun temp via external circuitry.
Has anyone had any experience driving a gun via PWM signal?
Is it best to split the fan/element circuit and vary the element only?

Cheers