I have a ZooMed RT-10 Rheostat that controls 2 heating matts for an animal cage(s).
https://www.amazon.com/Zoo-Med-RT10-ReptiTemp-Rheostat/dp/B001OVD7Q8

One end of the device plugs into a standard 120V NA outlet......the other end of the device is a receptacle that the mats plug into. The rheostat is in between.
.....one lead from the source outlet passes through the Rheostat PCB and onto the matt receptacle
.... the other lead passes straight through, unbroken, from the source outlet to the matt receptacle

I want to protect this circuit with a (@1A) fuse....so that if the matt malfunctions....it doesn't pull excessive current and catch fire.
My assumption was to place an inline fuse on the wire with the rheostat on it (not sure if before or after the rheostat makes the most sense.)
But thinking about this more, what happens if someone unplugs the device from the source outlet, flips the plug and plugs it back in?

How best to protect this circuit so the fuse blows, and the power cutoff, if the load pulls too much (+1A) current?

Thanks
Jim

 

The max power from the controller is 150 watts so at 120 VAC a little over 1 amp max current, actually 1.25 amp max. I would fuse upstream of the controller at 1.0 amp and likely see how a 1.0 amp fast blow fuse works out.

Ron

 

And regardless of how the device is plugged into the wall...meaning the 2 orientations of the prongs on the plug....this fuse will properly protect against current overdraws with the design you propose?

 

One blade of the plug should be wider. The plug should only go into a North American mains wall socket one way.



Hard to see in the picture but I think the wider blade is towards the back. The wider blade should mate with the Neutral on a standard mains residential plug.



The plug should only mate with the outlet one way. You would fuse the Hot side or the smaller of the two blades (narrow blade).

Ron

 

I rather doubt that the device shown is actually a RHEOSTAT, because at the half power setting it would be getting quite hot, because of dissipating 75 watts as heat.
It is probably a simple TRIAC type of power control packaged to market to those folks who do not know what a triac controller would be. If the control package does not get quite hot in use, then certainly it would not be a rheostat.
IF the TS has a meter able to measure resistance then the verification will be simple.

 

I would think if this is used for heating pet cages, a thermal O/H sense device would also be necessary if not already fitted?

 

MisterBill2

You are correct. Looking at the components it's a pot and a JST16 series triac.


MaxHeadRoom

Can you please explain what a thermal O/H device is? Do you mean a thermostat? If so, that would be optimal since it would auto-detect the current temp and auto-adjust the wattage to get to the set temp. With this ZooMed controller you have to adjust temp manually.

 

I would guess same, it's just a simple triac controller type. Not much to them but they work for things like this.

Did you check the line plug for polarity (one blade slightly wider on the end than the other? Just curious. I would fuse it as suggested.

Ron

 

I'm volunteering at the local animal clinic and we had an idea to better manage the mess of wires associated with their animal cage heaters. I'd love it if one (or more!) of the experts here could evaluate this control box from a safety perspective. I haven't done any AC current projects like this so looking for some feedback. Thank you so much!

Oh..and its not completed yet, but I think you can get the jist for the document I've provided. Looking for an evaluation of the design not the actual box in its current state,

 

Just one comment so far. Make sure your crimps are all tight and secure. Loose crimp = arcing and arcing = heat and it becomes a down hill snowball. Make sure your wire gauges and switches are rated for the heater pad. currents. Only switch the Hot sides never neutral / ground.

Pilgrim and Charlotte thank you for volunteering.
Ron

 

What I do not see is a circuit schematic Certainly it loks good, though and it seems that the parts are a good quality. What I do not see is the maximum current for each load, and the maximum current for each supply line.I also do not see the rating for the fuse.
I NEVER follow links to amazon if I can avoid it, which I DID THIS TIME..
One puzzle is the single control and single fuse., as I thought that I read that each PCB had it's own power cord.
The one fuse for the single controller is reasonable, the one power control means that there will be one control for all 12 circuits.That is fine if they all need to be set the same.

 

It seems weird to me that the cord from the wall to the controller has its NEUTRAL (fat blade on the plug) going into the triac..and then the heating matt...........and the HOT wire (small blade) bypassing the triac and going directly to the heating matt. If you are trying to control the current to the heating matt (to control temp) why wouldn't the control be on the hot wire?

 

From a power control point of view it makes no difference on an AC circuit. For triggering the triac it makes more sense to have the triggering circuit closer to the neutral side of the line. It would also tend to reduce the possible effects of random electrical fields that might tend to cause undesired triac triggering.

 

But there was advice above to "fuse the Hot side"
So is it really a '6 & 1/2 dozen' kinda thing?

 

But there was advice above to "fuse the Hot side"
So is it really a '6 & 1/2 dozen' kinda thing?

There are electrical safety considerations (fuse Hot side vs bonded to ground neutral) and electrical circuit operational considerations (all current carrying wires are Hot for control).

 

There are electrical safety considerations (fuse Hot side vs bonded to ground neutral) and electrical circuit operational considerations (all current carrying wires are Hot for control).

Is my understanding correct then that for safety reasons fusing the HOT is best?

 

Is my understanding correct then that for safety reasons fusing the HOT is best?

Yes, What's called the HOT wire is the wire(s) not directly bonded to 'Earth' ground', so it has the utility supply AC potential IRT to 'Earth' ground. That doesn't mean the NEUTRAL wire is safe to play with.

 

But there was advice above to "fuse the Hot side"
So is it really a '6 & 1/2 dozen' kinda thing?

No, the advice is correct! "Fuse the Hot Side" because if there is an accidental fault that a conductor contacts a grounded object, a large fault current will flow, while if the neutral side accidentally contacts a grounded surface there should not be any current flow,. The reason is that in theory the neutral side of the line should be at ground potential, and usually that is true.

 

Is my understanding correct then that for safety reasons fusing the HOT is best?

Depending on the jurisdiction you are working in, there is specific regulations that should be followed.
If working in N.A., there is the NFPA70 - NEC/CEC & NFPA79 - Control Standards.

 

Usually fusing the hot wire makes the most sense, as I explained in post #18. Occasionally some folks think differently, perhaps they have valid reasons. Some folks have rules that discourage breaking the neutral side of the line, UNLESS you break ALL of the line circuits.