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Question about dew heater control.

Aisling Lightworks
 

I'd like to get Onstep to automatically switch my dew heaters on when it starts getting near the dew point. The programming is no problem and I'm going to use an extra pin on the bluepill, but I was wondering if someone could give me some pointers on the electrical engineering side, I'm assuming it's going to be something like this? What types of components will I need?

SteveS
 

This is the circuit I designed for my dew controller. I run my system at 12VDC
The FET  is a VN2106 and the Q6 is a AON6407 p channel  MOSFET   capable of 30V 32A. The AON6407 has very low on resistance so you dont typically have to worry about heat sinks or voltage loses.
Both those parts are surface mount but I am sure you can find similar parts in thru hole.
PWR_EN6 is the power enable line, which you can PWM with 5VDC to control your heater.
The PWR_SENSE6 line and corresponding resistors is for sensing if the fuse has blown.
The FUSE6 is a fuse holder. The connector CON6 is actually a set of thru hole anderson power connectors.

Your wire should be nichrome wire of sufficient resistance to get the max wattage you desire for your OTA.
P = V**2 / R, I typically use a simple formula of 1watt per inch of OTA circumference.

HTH
SteveS
--
Three stones observatory 40° 46' 42"N 88° 12' 30" W

Aisling Lightworks
 

Awesome, that's just what I needed. Thanks. Been meaning to learn all this stuff but there aren't enough hours in the day!

Dave Schwartz
 

On 2020-02-20 8:46 a.m., SteveS wrote:
This is the circuit I designed for my dew controller. I run my system at 12VDC
The FET  is a VN2106 and the Q6 is a AON6407 p channel  MOSFET   capable of 30V 32A. The AON6407 has very low on resistance so you dont typically have to worry about heat sinks or voltage loses.
Both those parts are surface mount but I am sure you can find similar parts in thru hole.
PWR_EN6 is the power enable line, which you can PWM with 5VDC to control your heater.
The PWR_SENSE6 line and corresponding resistors is for sensing if the fuse has blown.
The FUSE6 is a fuse holder. The connector CON6 is actually a set of thru hole anderson power connectors.

Your wire should be nichrome wire of sufficient resistance to get the max wattage you desire for your OTA.
P = V**2 / R, I typically use a simple formula of 1watt per inch of OTA circumference.

HTH
SteveS
--
Three stones observatory 40° 46' 42"N 88° 12' 30" W

Aisling Lightworks
 

That's awesome, Dave. Could you break down how I'd use that? Or point me in the right direction to learn it myself?


On Thu, Feb 20, 2020, 9:01 AM Dave Schwartz <Dave.Schwartz@...> wrote:
This
https://www.ebay.com/itm/TOP-MOSFET-Button-IRF520-MOSFET-Driver-Module-for-Arduino-ARM-Raspberry-pi/201145556765
is a really easy circuit to build. :-)

On 2020-02-20 8:46 a.m., SteveS wrote:
> This is the circuit I designed for my dew controller. I run my system
> at 12VDC
> The FET  is a VN2106 and the Q6 is a AON6407 p channel  MOSFET  
> capable of 30V 32A. The AON6407 has very low on resistance so you dont
> typically have to worry about heat sinks or voltage loses.
> Both those parts are surface mount but I am sure you can find similar
> parts in thru hole.
> PWR_EN6 is the power enable line, which you can PWM with 5VDC to
> control your heater.
> The PWR_SENSE6 line and corresponding resistors is for sensing if the
> fuse has blown.
> The FUSE6 is a fuse holder. The connector CON6 is actually a set of
> thru hole anderson power connectors.
>
> Your wire should be nichrome wire of sufficient resistance to get the
> max wattage you desire for your OTA.
> P = V**2 / R, I typically use a simple formula of 1watt per inch of
> OTA circumference.
>
> HTH
> SteveS
> --
> Three stones observatory 40° 46' 42"N 88° 12' 30" W
>



Howard Dutton
 

On Thu, Feb 20, 2020 at 06:01 AM, Dave Schwartz wrote:
This https://www.ebay.com/itm/TOP-MOSFET-Button-IRF520-MOSFET-Driver-Module-for-Arduino-ARM-Raspberry-pi/201145556765 is a really easy circuit to build. :-)
That's an n-channel MOSFET.  Turns on when a + voltage (above some design threshold that you must pay attention to when selecting a device) is applied to the gate.  Source to drain goes on the - (Gnd) side of the heater wire.

You should use a resistor to limit current into the gate... ringing due to gate capacitance otherwise.  Probably doesn't matter at low frequency, Steve knows better than I no doubt.

Howard Dutton
 

On Thu, Feb 20, 2020 at 05:14 AM, Aisling Lightworks wrote:
I'd like to get Onstep to automatically switch my dew heaters on when it starts getting near the dew point. The programming is no problem and I'm going to use an extra pin on the bluepill, but I was wondering if someone could give me some pointers on the electrical engineering side, I'm assuming it's going to be something like this? What types of components will I need?
Note, you show 5V in the image but the MCU is 3.3V

Howard Dutton
 
Edited

On Thu, Feb 20, 2020 at 06:20 AM, Howard Dutton wrote:
On Thu, Feb 20, 2020 at 06:01 AM, Dave Schwartz wrote:
This https://www.ebay.com/itm/TOP-MOSFET-Button-IRF520-MOSFET-Driver-Module-for-Arduino-ARM-Raspberry-pi/201145556765 is a really easy circuit to build. :-)
That's an n-channel MOSFET.  Turns on when a + voltage (above some design threshold that you must pay attention to when selecting a device) is applied to the gate.  Source to drain goes on the - (Gnd) side of the heater wire.

You should use a resistor to limit current into the gate... ringing due to gate capacitance otherwise.  Probably doesn't matter at low frequency, Steve knows better than I no doubt.
From what I see in the image their circuit pulls the gate to ground with a resistor (a good idea to keep the device from accidentally switching on before the MCU pulls the pin LOW.)
Then they have an LED with a current limiting resistor to show the user if the device is on/off.
What I don't see is the resistor limiting current into the gate.

Dave Schwartz
 

Hook your supply to VIN and GND, your load to V+ and V-. In this circuit, V+ is bridged to VIN and V- is switched (from the middle pin of the MOSFET - see the bottom view in the listing).

Then on the pin (logic) header hook your logic ground to GND, 5V or 3.3V (whichever the signal supplied by the MCU will be) to VCC and your selected MCU pin to SIG. This module will work with either 3.3V or 5V signals... that's why it gets VCC on the logic pins, so it knows how to compare high and low states to decide its trigger level. You'll have to experiment to see if SIG is 'active high' or 'active low' so you know what your 'completely off' state needs to be (that info is probably on the web for the module but I haven't had my coffee yet). Then you can control the duty cycle (i.e. heat level) with PWM.

On 2020-02-20 9:06 a.m., Aisling Lightworks wrote:
That's awesome, Dave. Could you break down how I'd use that? Or point me in the right direction to learn it myself?

On Thu, Feb 20, 2020, 9:01 AM Dave Schwartz <Dave.Schwartz@... <mailto:Dave.Schwartz@...>> wrote:

This
https://www.ebay.com/itm/TOP-MOSFET-Button-IRF520-MOSFET-Driver-Module-for-Arduino-ARM-Raspberry-pi/201145556765

is a really easy circuit to build. :-)

On 2020-02-20 8:46 a.m., SteveS wrote:
> This is the circuit I designed for my dew controller. I run my
system
> at 12VDC
> The FET  is a VN2106 and the Q6 is a AON6407 p channel MOSFET
> capable of 30V 32A. The AON6407 has very low on resistance so
you dont
> typically have to worry about heat sinks or voltage loses.
> Both those parts are surface mount but I am sure you can find
similar
> parts in thru hole.
> PWR_EN6 is the power enable line, which you can PWM with 5VDC to
> control your heater.
> The PWR_SENSE6 line and corresponding resistors is for sensing
if the
> fuse has blown.
> The FUSE6 is a fuse holder. The connector CON6 is actually a set of
> thru hole anderson power connectors.
>
> Your wire should be nichrome wire of sufficient resistance to
get the
> max wattage you desire for your OTA.
> P = V**2 / R, I typically use a simple formula of 1watt per inch of
> OTA circumference.
>
> HTH
> SteveS
> --
> Three stones observatory 40° 46' 42"N 88° 12' 30" W
>



Howard Dutton
 
Edited

On Thu, Feb 20, 2020 at 06:33 AM, Dave Schwartz wrote:
that's why it gets VCC on the logic pins, so it knows how to compare high and low states to decide its trigger level.
VCC isn't needed and from the images in the link isn't routed anywhere on the PCB.

There's a gate threshold voltage... and you want to be solidly above the maximum in the range quoted for that in the datasheet.  This is so the device isn't "just barely switched on" where it only has very limited current carrying capability relative to its spec's.  Normally a chart is included that shows how much current it can carry at a given VGS.

No idea which exact device they are using but they say it's ok at 3.3V (there's also a maximum gate voltage too but it's safe to say that's well above 5V.)

Aisling Lightworks
 

Thanks, guys. That's plenty to get me going. I was planning on just using the reticle pin to switch it on and off. I don't think that's PWM capable, but I'll never need the focuser so I might use one of those pins instead. PWM seems really attractive. Going to try to figure out how to use it.

Howard Dutton
 
Edited

I've also used similar to these for switching DC:
https://www.mouser.com/ProductDetail/Omron-Electronics/G3VM-61AR1?qs=sGAEpiMZZMsG1k5vdNM%2FcyYReKO95l5I99K6JwF4csw%3D

Just needs a resistor to limit the current into its internal LED that switches it on.

Howard Dutton
 

On Thu, Feb 20, 2020 at 06:52 AM, Aisling Lightworks wrote:
Thanks, guys. That's plenty to get me going. I was planning on just using the reticle pin to switch it on and off. I don't think that's PWM capable, but I'll never need the focuser so I might use one of those pins instead. PWM seems really attractive. Going to try to figure out how to use it.
OnStep has PWM commands for tasks like this built-in.  A slight change or addition to the STM32 pinmap might be necessary but no big deal.
OnStep also calculates dew point (BME280) and has an secondary temperature input "telescope temperature" via the DS18B20.

The WiFi Addon can already use those OnStep commands to adjust the PWM levels manually for a dew heater, you just need to extend this to be automatic.

Aisling Lightworks
 

Right, I know you've done a lot of the ground work already for this and I'll just have to tweak a few things to automate it on the STM32 build. Should be pretty slick.

Aisling Lightworks
 

This is the main reason I installed the BME280. The refraction compensation seems more accurate with it (really haven't done enough testing) but it would be great to not run the heaters all night, not have to babysit them to turn them on, and control the wattage. Might be the difference between getting 2 nights out of a charge and 1 night.

Howard Dutton
 
Edited

On Thu, Feb 20, 2020 at 07:15 AM, Aisling Lightworks wrote:
refraction compensation seems more accurate with it (really haven't done enough testing)
I doubt it (was playing a key role,) more likely other things were improved either by luck or plan.

Aisling Lightworks
 
Edited

On Thu, Feb 20, 2020 at 07:22 AM, Howard Dutton wrote:
I doubt it (was playing a key role,) more likely other things were improved either by luck or plan.
Yeah, I only had it out one night, could've been as simple as better polar alignment, but the drift analysis looked vastly different than I'd ever seen it.

Jerry
 

You can just use DC rated solid state relays on one of the IO pins, kind of a larger version of what Howard posted:


On Thu, Feb 20, 2020 at 7:30 AM Aisling Lightworks <stercust@...> wrote:
On Thu, Feb 20, 2020 at 07:22 AM, Howard Dutton wrote:
I doubt it (was playing a key role,) more likely other things were improved either by luck or plan.

 

 

Yeah, I only had it out one night, could've been as simple as better polar alignment, but the drift analysis looked vastly different than I'd ever seen it.
 

Howard Dutton
 

On Thu, Feb 20, 2020 at 06:58 AM, Howard Dutton wrote:
I've also used similar to these for switching DC:
https://www.mouser.com/ProductDetail/Omron-Electronics/G3VM-61AR1?qs=sGAEpiMZZMsG1k5vdNM%2FcyYReKO95l5I99K6JwF4csw%3D

Just needs a resistor to limit the current into its internal LED that switches it on.
There's a "gotcha" with these I just noticed though... they are too slow (or right on the edge of being fast enough depending on the case) for normal Arduino PWM.

Aisling Lightworks
 

On Thu, Feb 20, 2020 at 08:21 AM, Jerry wrote:
You can just use DC rated solid state relays on one of the IO pins, kind of a larger version of what Howard posted:
 
Howard just mentioned that the ones he suggested aren't really fast enough for PWM, are these fast enough?