modify a propane detector to turn on a blower

I was looking at the
SAFE-T-Alert propane detector which sounds an audible 85 db alarm.
model 40-442
http://www.safehomeproducts.com/shp...rown/12v-propane-detectors/160029/160029.aspx
manual
http://www.safehomeproducts.com/shp2/data/manuals/40-441.pdf

How could you create a circuit to use the alarm current to trigger the blower.
I was thinking a mosfet or switching transistor.
Any ideas?

 

Give it a time delay so you can abandon the boat and clear to a safe distance first.

 

A good idea! Any solid state switching device will do.
The blower itself must be mounted high enough to be out of harms way, otherwise the sparking brushes may spoil the fun.

 

A brushless inline blower will work fine, available in 3" and 4" diameters commonly.

 

blowers for gasoline boat bilges are sealed so no sparks, ignition and flame arrested.
It will have to be a solid state switching transistor-mosfet. the gate will be triggered by the alarm current. since we are dealing with low voltage DC then it should be a simple circuit.

think of a power mosfet as a solid state relay.
http://en.wikipedia.org/wiki/MOSFET

anyway before I go ahead with this, I was wondering if anyone had some particulars.

I imagine I need a heat sink, so thinking to use an old pentium CPU cooler or just a piece of flat aluminum plate to mount it.
I also have several old computer PS, are the large power transistors mosfets inside?
looks like a mosfet uses very little power to open the gate.
http://talkingelectronics.com/projects/MOSFET/MOSFET.html

 

Data sheet for a 55 volt 49 amp mosfet
Sounds like this would work fine, thoughts?
http://www.futurlec.com/Transistors/IRFZ44.shtml

N channel versus P channel here wont matter in my app since I am designing it myself?

 

Forget the heat sink if your blower draws only a few amps, the mosfet conducts as good as a solid copper wire.

 

I found this one and bought it.
Runs on 12 volts dc.
It allows a hookup to a blower relay etc...
This one is 12 volt DC and has a screw attachment for hooking up your 4 wire cable.
Similar to wiring a home thermostat.

http://www.ebay.com/itm/23075280153...X:IT&_trksid=p3984.m1438.l2649#ht_2630wt_1028

this other one is very similar but has four wires to attach power and the remote alarm relay, mosfet etc...
http://www.ebay.com/itm/LPG-Natural...Protection&hash=item4cfe3b4ae8#ht_3701wt_1168

then there is this type has a builtin AC recitifer and mentions hooking up alarm signal to another device

All three are basic same circuit and company design.

http://longsin.en.alibaba.com/product/377201684-200808080/High_tech_LPG_gas_alarm.html

So it should be fairly easy to do this I think. And likely tells how in the instructions.
Mosfet has gate, source, drain legs.
The 2 signal wires for this I assume one for the mosfet gate, the other for the source?
Will polarity matter on the 2 signal wires? I would think one wire is positive other is ground.
Will the mosfet gate leg to source leg circuit require a current limiting resistor ?

Mosfet drain leg goes out to blower and return wire is to source with battery power in between.
Should the power source for the blower be breakered? I think so.
I bought 4 of these IRFZ44N mosfets 49 amp, 55 volt.

Should I mount this detector in the lower bilge below the sole (floor), or would it detect just low in the lower cabin mounted on the side of a cabinet?
I have an upper salon and lower cabin and a bilge connects them both.

I think I have a good spot right behind my fridge which is in the lower cabin. Under the fridge I removed the floor covering board to expose air to help keep a heat rising convection flow to bilge and cabin air, so that ought to be a good spot. Except that I wont be able to see the lights if I put it there. But you could easily hear it, Oh well. compromises.

I imagine bilge air would be the first to be saturated with propane if there was a leak.

 

here is using mosfets as switches.
N channel mosfet goes in the negative ground side of the device.
The device has positive voltage always on, ground is switched off and on.
http://www.electronics-tutorials.ws/transistor/tran_7.html

Since I already have a blower motor and it is likely wired as ground always on and the manual switch supplies positive power to the motor, I will have to rewire so that the manual switch is grounding the return. And the blower motor has 12 volt power always on.
This will have the effect of likely confusing anyone who comes after me wondering what was going on since everything else with a manual switch is turned 12 volt positive open or closed.
Do I have this correct?

will the blower motor require this flywheel diode?

looks like it will


or not as according to datasheet for that mosfet, it already has a protection diode builtin.


http://www.stevenjohal.com/how-to-hook-up-an-n-channel-mosfet/

also likely needs a pull down resistor to close the gate

 

The IRFZ44N diagram shows the diode is already present. Use a 10-50K resistor between G and S and 100-500 Ohms between G and the switch or detector output.
The blower is wired between D and Battery +.

Good luck!

 

The big question is, what does the gas sensor give you as an output.
a) If it is a switch contact, you can take a P MOSFET and leave the blower as it is, i.e the casing is probably ground/earth
b) or, you have a small 12 Volt remote conntrol battery and have that switched to the gate of a N MOSfet type. You place the N type in your plus line and you don't have to modify the blower. You just have to be carefull that the resistor does not drain your small battery.
c) If it is electronic to plus 12 Volt, you have tho check whether your MOSFET can handle a voltage up to 15 Volt on the gate, and you have to rewire your blower , to have the casing floating.
d) If it is electronic to ground, you could use a P MoSfet and also use a MOSFet which can handle > 12 Volt Gate Voltages. You can overcome this problem by using two resistors, whereby the voltage cannot be greater than 5 Volt on the gate. The fiirst resistor can then act as drain resistor.

will the blower motor require this flywheel diode? Although the MOSfet has normally a diode build in , it cannot harm to have a bigger diode like a 1N4007 or a fast 5 Ampere placed over the MOSfet.

Can I post you per airmail an up to 20 Volt gate voltage Mosfet IRFP064N , 113 Ampere or a IRFP054N with compliments?. All what I need is a POSTE RESTANTE address ( your nearest Postoffice). I will use your forum name sdowney717 as name of the airmail letter.

Yes you need a pull down resistor. 100K is what often is used.

 

Sure, I will take all the help I can get, my address is at

925 Moyer Rd
Newport News VA 23608

The blower is a 2 wire design, so is not grounded by its case. It is a genuine PAR marine blower.
I was reading if you use a n channel on the high side, then you have to drive the gate voltage higher than the drain voltage. And gets trickier to do.
http://solutions-cubed.com/h-bridge-control-of-a-motorhigh-side-switch/

And I also read that P channel are getting better technology than used to be.

I rebuilt the blower and have a link to it here
http://www.cruisersforum.com/forums/f14/is-the-plastic-fan-for-blower-replaceable-69240.html

 

I will post you also a P type. But you must remember that a P type is higher in resistance of the IRPF054N (0.012 Ohm) or IRFP064N (0.008 Ohm). It depends thus what your current will be, whether you need a small heatsink.

The formula is easy. current in Ampere x current in Ampere x resistance of the MOSfet in Ohms. i.e if you blower is 2 Ampere, it means 2 x 2 x P type 20 millOhm = 80 milliwatt. The Mosfet will not get even warm. It is different with the currents I am working with.
I will post them tomorrow morning. I have no idea what it takes to get it to you. To Europe from here w with airmail, is about 7 - 10 days.

 

If it is too much, then I can likely get them here cheaper so it would not be worth your doing it. The four n channel I bought were $1 each and free shipping off ebay. But I do appreciate your kindness.

 

Not a problem, they have gone already.