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Old 03-03-2011, 05:10 PM   #1
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Electronics help please..

I'm pretty good with electrical, but a little weak in electronics.

What I am trying to accomplish: I want to be able to control my furnace blower speed to be able to recirculate air to the holding tanks in winter while utilizing electric heat (shore power) AND with intermittent furnace operation. So... fan running half speed, thermostat commands furnace on....speed automatically goes to full speed, and upon thermostat command "off", fan speed returns to half speed.

What I have done: Got a PWM motor controller (see pic #3). It, unfortunately controls the ground circuit. Problem is: In order to control motor variably, I must disconnect motor ground. (See pic #1) This prevents the fan from going to high speed when thermostat commands furnace on. Then I get no fan sail switch input and controller throws the sail switch code and, of course, no gas ignition.

I need to be able to give the fan motor an uninterrupted ground when the thermostat commands on.

Pic #2 shows a proposed fix, but I have questions before I start letting the smoke out of the electronics.

1)Can I use a relay with the primary circuit from the blue thermo wire (is that blue wire 12 volts and enough amperage?) to ground with the secondary from motor ground wire to ground?

2)Does this need to be a transistor instead of a diode protected relay? (this is where I am particularly weak) What do I walk into Radio Shack and ask for?

Hope this makes sense and I apologize for the pic quality!

Thanks!
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Old 03-03-2011, 06:16 PM   #2
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You want the relay to turn on from the BLW terminal on the controller, not from the thermostat. Not because of the current draw, but rather because that way you'll get proper operation of the post-purge feature which is important not only for the life of the heat exchanger (it prevents buildup of condensation) but also because if the sail switch doesn't behave the usual way the controller may lock out ignition.

So disconnect the red wire at the terminal marked BLW from the ignition controller, and tape it, and connect the +coil connection on the relay to the BLW terminal.

Everything else is fine as you've drawn it. One thing to be aware of is that the ignition controller might lock out if the sail switch doesn't open after the burn cycle, as a safety feature to lock out on a stuck sail switch. If you have the pwm turned down low enough it should still work ok.

Any 12v relay will work, diode, no diode, whatever. Be sure it's rated for the blower current, 10 amp contacts should be enough. Something like this plus a suitable socket would be ideal:

Amazon.com: Dayton 1EGW1 Relay, Octal, SPDT, 12VDC, Coil Volts: Industrial & Scientific

That particular one has an LED "on" indicator and a test button which makes troubleshooting a new circuit easier.
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Old 03-03-2011, 07:56 PM   #3
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Thanks, Jammer! I had completely overlooked the "afterblow" feature. Using the BLW circuit out of the board makes perfect sense!
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Old 03-03-2011, 08:09 PM   #4
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other than proving it's doable (the fan runny independently)

what's the 'potential' value with this?

the furnace is 35,000 ? btus...

a space heater 5,000 btus and 2 of those will essentially max out the electrical system.

the a/c heat pump is 5,000 btus or is it 10 ?

using 5-10,000 btus to warm a trailer this size is INadequate if it's really cold out.

and sending some of that heat down and out only reduces what is available to warm the living space.

tanks aren't at risk for freezing till the daytime HIGHS are ~30 or a bit lower.

if the average of high/low temps gets down to that level, a space heater or 2 isn't gonna do much.

it's a btu thing, right?

the outside temps at which small electric space heaters are USEFUL don't place the tanks at risk 4 freezing.
_______

additionally the furnace fan is running on 12v, even when plugged IN to shore power.

so if most of the AC juice from a 30 amp plug is being used by space heaters, there is the potential (perhaps)...

for the charger/converter to be taxed 2 the max and for the batteries to slowly discharge, even while plugged in.

in theory one might save a TINY bit of lp warming the interior this way,

but again the effectiveness of the electric heat will be diminished trying to 'circulate' to the tank spaces.
_______

cheers
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Old 03-03-2011, 08:41 PM   #5
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I understand, but 2 points:

1) a small airflow over an extended period of time in the tank area, even at a lower flow temp is more effective in pre-warming that mass than is periodic normal furnace function.
2) there is another 20 amp circuit right next to the 30 amp at the site. Not a problem to bring it in as well. Done it many times. I'm gettin' all I'm payin' for at the park.

I've been experimenting this week with only a 20 amp service out of my garage. No problem running 1700 watts of heater, the fridge on electric, converter, LED TV, LED lighting and furnace. No problem with batteries draining per the meter. I wouldn't want to try the heat pump....voltage a little low and not enough service. It's been in the mid low to mid 30's, cloudy, breezy. It's 63 degrees in the trailer w/o the gas furnace. Late last week it was in the 20s one night and I still had no problem with a 20 degree rise inside, over ambient with just the 1500 watter on. I think 20 degrees is worth a bit of propane.

I think there is more advantage than you think BTU wise
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Old 03-03-2011, 09:00 PM   #6
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clearly using a small space heater can make the cabin comfy and cut the chill,

but if the temps are only down to the mid 30s or 20s, the tanks aren't at risk.

as u noticed "voltage a little low"...

i've seen this dozens, maybe 100s of nights in rv parks...

as it cools to the 40s folks start switching ON space heaters to get "all they're paying for" from the park electricity...

then use the microwaves and electric water heater elements...

and one can WATCH the voltage drop incrementally by the hour.

this is bad juju for that tele AND for the converter/charger and other stuff.

it's a quick way to COOK gadgets.

it's your show so play it however u like...

but running the furnace FAN isn't doing much, IF the tanks are not genuinely at risk.

and simply filling the fresh tank and NOT draining the wash water tank will protect them over night.

in truly cold temps the tanks will need the furnace heat.

cheers
2air'
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Old 03-03-2011, 10:52 PM   #7
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Yes, it is my show. Every component I have gives a minimum voltage requirement in the manual. "not tryin' never did anything!", Mr. Negativity.
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Old 03-03-2011, 11:40 PM   #8
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At least to me, the project sounds like it has potential value.

Assuming a 30 amp and 20 amp circuit, I can see at least 15K BTU of electric heat. This should be approx. equivalent to 20K BTU of propane heat due to the out the stack loss of the furnace. The 15K BTU heat should give a decent temperature rise in the trailer- perhaps 30-50 degrees if my trailer is any indication.

Pumping air of that temperature to the tanks would have to have some benefit. The tanks sit low and would naturally be below convection and be the coldest area in the trailer. Pumping air 40+ degrees above ambient would have a value. What value would have to be determined, but I can't fault the quest for knowledge here. But I'm absolutely certain the tanks would take longer to freeze with this circulation, all things being equal. Remember too, the blower would be running continuously vs. the furnace cycling. Perhaps the BTU applied to the tanks would be somewhat equal.

The furnace blower is approx. 5 amps and 68 watts. This grand amount of power will all go into heat.

The individual opening the thread appears to me to have a strong concept of operating the trailer systems. I'm sure if the voltage were to drop dangerously low, alternatives could be put in play, such as using the gas furnace.
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Old 03-04-2011, 01:03 AM   #9
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The problem with controlling motor speed by varying the voltage is the lower the voltage the higher current draw and in an attempt to maintain a constant speed the motor will burn itself out.

The PWM device (Pulse Width Modulation) is actually pulsing the motor XX ms on XX ms off and the variance in XX determines the motor speed. End result is a current limiting device.

The +12V does not vary the pulse (motor speed) is varied through M- and I think M- needs to be isolated from circuit ground.

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.
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Old 03-04-2011, 05:31 AM   #10
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This idea, IMHP goes toward an issue that stands out as something that could def. be improved upon.
I think that I have an opposite concern. I wouldn't want hot air to the tanks.

I was thinking that say if the temp was 35-49 degrees and one was using the furnace, would you really want the furnace pumping hot air into the tanks? It seems like a waste.
Down south here, it only occasionally gets cold enough to freeze water, but does get cold enough to warrant using the furnace. Why would I want to push hot air down to the tanks if there is no risk of them freezing if I didn't?
I was thinking that in a situation like that a damper to open and control the amount of heat from the furnace to the tanks would be a simple solution.
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Old 03-04-2011, 07:55 AM   #11
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Quote:
Originally Posted by garry View Post
The problem with controlling motor speed by varying the voltage is the lower the voltage the higher current draw and in an attempt to maintain a constant speed the motor will burn itself out.

The PWM device (Pulse Width Modulation) is actually pulsing the motor XX ms on XX ms off and the variance in XX determines the motor speed. End result is a current limiting device.

The +12V does not vary the pulse (motor speed) is varied through M- and I think M- needs to be isolated from circuit ground.

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.

You're losing me a little bit. When you say, "I think M- needs to be isolated from circuit ground.", are you saying a diode should be placed in the M- circuit? Before or after the relay?

I changed the circuit last night per Jammer's suggestion. BLW controls the control circuit in the relay. Everything else is the same.

Circuit works fine and I have normal furnace operation overriding the PWM as commanded by the t-stat., with afterblow working as originally designed.
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.

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?
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Old 03-04-2011, 08:07 AM   #12
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Quote:
Originally Posted by DanielB View Post
This idea, IMHP goes toward an issue that stands out as something that could def. be improved upon.
I think that I have an opposite concern. I wouldn't want hot air to the tanks.

I was thinking that say if the temp was 35-49 degrees and one was using the furnace, would you really want the furnace pumping hot air into the tanks? It seems like a waste.
Down south here, it only occasionally gets cold enough to freeze water, but does get cold enough to warrant using the furnace. Why would I want to push hot air down to the tanks if there is no risk of them freezing if I didn't?
I was thinking that in a situation like that a damper to open and control the amount of heat from the furnace to the tanks would be a simple solution.
If it's 35-40 degrees, the PWM would be OFF, and only normal heat pump/furnace operation is used. This circuit is only for upper 20s and below. To 2Air's point, I am not concerned with the tank becoming a block of ice overnight, in the 20s and daytime well into the 30s. What I am concerned with, is while utilizing electric heat, I may reduce furnace on time (and subsequent heat to tank area) that ice may start to form in the vulnerable and low points, ie. the waste valves. I want to prevent potential damage to the seals while opening and perhaps pushing ice into the seals while closing and tearing them. I think it would take just a few degrees of elevated temp. in all that mass to reduce the potential. If it's zero outside, of course more drastic intrevention would be required.
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Old 03-04-2011, 10:48 AM   #13
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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 ???
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Old 03-04-2011, 11:31 AM   #14
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I think an interesting companion project to this would be temperature sensors in or on the tank(s) or dump valve.
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