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Old 03-04-2011, 12:02 PM   #15
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As drawn M- and ground are tied together when heat is called and +12V is still applied to the PWM and that may cause MF1 and MF2 to self destruct.

May not, but it is something to think about using the other half of the DPDT relay to break the B+ to the PWM. If you do that you will have to get motor power from BLW during normal operation.
The source and drain on the MOSFETs are connected to M- and G respectively and there's no +12 connection except on the gate. Mosfets usually burn up with too much current through the channel and since his circuit shorts M- and G when the relay pulls down there will be no current. (The other possibility, overvoltage across the channel or between the gate and the channel, doesn't apply either, because there isn't more than 12 volts in the circuit)
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Old 03-04-2011, 12:07 PM   #16
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For what its worth I think what you are trying to do is a good idea plus you will not be using a lot of amps from the battery when the PWM is in control as the PWM is a current limiting device.

My only concern is having power on the PWM all the time the furnace in the on position.

Being the overkill kind of person I am I would cut power to the PWM with the second set of relay contacts when the thermostat is calling for heat.

How loud is the noise ???
Garry, I don't think it makes any difference. Power through the PWM is just a straight shot between B+ and M+. Obviously the PWM uses a little power taken off this "buss". The ground circuit is controlled by the PWM, so when the relay closes it just provides a constant ground while the PWM is pulsing between open and ground. The potential difference between the 2 grounds would be soooo small or non-existent as to not be an issue. (I think??) Power must pass directly between B+ and M+ to power the thermo commanded high speed.

The noise is loud enough to be irritating in the living area, not so bad in the BR.
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Old 03-04-2011, 12:26 PM   #17
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Two issues: 1) Sail switch, as Jammer suspected, is very sensitive. PWM and motor speed must be kept low for the furnace module not to throw the sail switch code, and consequently, not come on.
But, even at this low speed, an adequate amount of air is flowing.
There are a couple of choices all with tradeoffs.

You can physically modify the sail switch so that it requires more airflow to close. There's no safety problem with doing this but as the performance of the blower degrades with dust, bearing wear, and possibly low battery voltage, you may get nuisance trips.

You could add a circuit between the thermostat and the furnace so that, when the thermostat calls for heat, the blower shuts off, and then after a delay, the ignition module gets the call for heat. Not straightforward.

I suppose you could also add a circuit that fakes out the sail switch integrity check although that would undermine safety to some degree.

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2)During PWM operation, there is a buzzing noise, coming from the motor, that seems to correspond with the PWM frequency. This, I would like to find an answer to. Is it normal? Or do I have some sort of isolation issue not being addressed?
This is what someone (can't remember the name, has a banjo for an avatar) complained about with the Atwood two-stage furnace which also uses a pwm motor control.

If the PWM frequency is in the audible range there will always be some amount of hum or buzz from the motor. In the PWM circuit, R4 and C1 control the oscillator frequency. Decreasing the value of either will increase the frequency. You could experiment to see if there's a frequency low enough to provide good control but high enough that the hum isn't audible.
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Old 03-04-2011, 12:29 PM   #18
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In some cases it may work better to reduce the frequency to the point where the PWM noise is lost in the blower rumble. I would think you could get it down to 20 hz or so before the fan operation became noticeably jerky.

You can patch in a capacitor in parallel to C1, to lower the frequency, or a resistor in parallel with R4, with clips, to try this without having to unsolder anything.
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Old 03-04-2011, 12:36 PM   #19
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In some cases it may work better to reduce the frequency to the point where the PWM noise is lost in the blower rumble. I would think you could get it down to 20 hz or so before the fan operation became noticeably jerky.

You can patch in a capacitor in parallel to C1, to lower the frequency, or a resistor in parallel with R4, with clips, to try this without having to unsolder anything.
Could you give me more detail here. I think I understand, but, again, electronics is not my strongest suit.
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Old 03-04-2011, 12:51 PM   #20
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other than proving it's doable (the fan runny independently)

what's the 'potential' value with this?
Having gone through more than one odyssey on trailer heat I tend to agree although more experimentation can only improve our understanding of the possibilities.

If the trailer is heated to 70 degrees using electric heat or some other means other than the furnace I think there really isn't any risk of plumbing freezing until the outdoor temperature drops below 20 degrees. From a thoroughly warmed trailer some heat will end up being conducted down to the tanks. The much-ballyhooed tank heating feature doesn't direct enough air to the tanks to amount to much of anything. I suspect that, in reality, in truly cold weather this system won't do much besides keep the the area in the vicinity of the tank outlets (fresh water outlet to the pump, wastewater outlet to the valves) thawed, leaving a cake of ice in the center of the tank with no one the wiser.

And in temps below 20 degrees you're going to need more than 4000 watts to maintain interior temperatures at 70, and that means 50 amp service and modified wiring in the trailer. Or, perhaps, as dznf0g suggests, a combination of 20a and 30a service.

I think if I were to try to run electric heat, only, in my trailer, I would utilize an external electric tank heating pad on each of the three tanks and on the valve area.
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Old 03-04-2011, 12:56 PM   #21
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In some cases it may work better to reduce the frequency to the point where the PWM noise is lost in the blower rumble. I would think you could get it down to 20 hz or so before the fan operation became noticeably jerky.

You can patch in a capacitor in parallel to C1, to lower the frequency, or a resistor in parallel with R4, with clips, to try this without having to unsolder anything.
Now I'm confused. Found this online:

"Depending on the motor used, there can be a hum from the motor at lower PWM%. If objectionable the frequency can be changed to a much higher frequency above our normal hearing level (>20,000Hz)"
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Old 03-04-2011, 12:58 PM   #22
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Could you give me more detail here. I think I understand, but, again, electronics is not my strongest suit.
Get a selection of capacitors and resistors from somewhere. 0.1, .2, .47, 1, 2, and 4.7 uF for the capacitors, 47k, 20k, 10k, and 4.7k for the resistors.

Then experiment. Try each of the capacitors in turn connecting it to the existing capacitor C1 so that they're both in the circuit together. That should make the sound lower in pitch, the larger the capacitor, the lower it will be. Then try the resistors, same way, but it should raise the pitch, with lower resistances raising the pitch more.

You can use two resistors at once, or two capacitors at once, too, to try to find the best spot.

If you find a combination you like you can make it permanent.

Be sure the capacitors are rated for at least 15 volts and that they are connected with the correct polarity.
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Old 03-04-2011, 12:59 PM   #23
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Now I'm confused. Found this online:

"Depending on the motor used, there can be a hum from the motor at lower PWM%. If objectionable the frequency can be changed to a much higher frequency above our normal hearing level (>20,000Hz)"
I think that matches what I'm saying although I am adding that it may be made less objectionable also by lowering it. Try both.
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Old 03-04-2011, 01:01 PM   #24
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Get a selection of capacitors and resistors from somewhere. 0.1, .2, .47, 1, 2, and 4.7 uF for the capacitors, 47k, 20k, 10k, and 4.7k for the resistors.

Then experiment. Try each of the capacitors in turn connecting it to the existing capacitor C1 so that they're both in the circuit together. That should make the sound lower in pitch, the larger the capacitor, the lower it will be. Then try the resistors, same way, but it should raise the pitch, with lower resistances raising the pitch more.

You can use two resistors at once, or two capacitors at once, too, to try to find the best spot.

If you find a combination you like you can make it permanent.

Be sure the capacitors are rated for at least 15 volts and that they are connected with the correct polarity.
I get it....all in parallel, right?
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Old 03-04-2011, 01:04 PM   #25
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Yep, to make it easier.

Connecting them in series would have the opposite effect in each case.
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Old 03-04-2011, 01:10 PM   #26
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Thanks, I'll try that. It'll be a week or so, as I have some personal and business travel. But I will report back.
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Old 03-12-2011, 02:46 PM   #27
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Ok, so I tried all the various resistors and capacitors. All they did was change the pitch of the noise, within the audible range. I did quite a bit of online research and found that you must have a PWM at a high enough frequency to be out of human hearing range. I found an 18kHz PWM and it works and is very quiet. The only thing I may change now is to put a weight on the sail switch so I can use a little more speed with the PWM and still have the furnace come on and off when commanded. Does anyone know where the sail switch is physically located on the Atwoods?
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Old 03-18-2011, 10:57 PM   #28
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I just stumbled upon another advantage of this mod. I'm out on my first spring trip and it's cool. About 40*, so the heat pump is the preferred heat method. The problem with the heat pump is somewhat stratified temps in the AS. Warm up high and cold at floor level. I turned the furnace blower on low with my PWM controller and can now mix that low level cold air for a more even heat from the pump throughout the whole trailer.
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