Block Heater Current Monitor/tester




Living in Northern Ontario means frequently using a block heater to start your car on a -35C morning. For those of you lucky enough to live in a climate where you've never heard of a block heater, here's the wikipedia entry:

One of the fears of winter driving is getting in your car at 7:00 in the morning, turning the ignition and all you hear is "" then nothing. Maybe it was the wind or a stray dog that unplugged your cord or it could even be a bad cord or heater, either way you're calling a cab to get to work. There are quite a few manufacturers that include a little status light in the female end of an extension cord but as far as I'm concerned this is next to useless because it is just an indication of available power not a test of the actual load. What I wanted was some way to sense the current draw, kind of like a clamp-on ammeter.

After a bit of internet window shopping, I came across a device that did almost everything I wanted. For about $20 you get a little adapter that you either plug in between your cord and outlet or cord and block heater and an LED indicates current. Perfect...except that I sure don't want to venture out in the deep freeze, to check that little light, any more than I have to.

This then is my project: A current monitor that can be embedded in an outlet box inside your home so you can monitor the use of a block heater or roof heating cables etc.

Teacher Notes

Teachers! Did you use this instructable in your classroom?

Step 1: Warnings

Before you go any further here are a few caveats that you need to be aware of:

1. Although the final circuit is electrically isolated from household 120 volts, a line voltage lead needs to be disconnected from the switch, passed through the coil and reconnected.
2. You need to comply with any electrical or building codes in your area.
3. I am not responsible for any misadventure, this instructable is just to show you how I did it.

Before starting on my project I sent in a question to Ontario's ESA (Electrical Safety Authority).
Here was my question and their response:

"I am considering installing a current transformer in the interior switch outlet for my exterior outlet feeding my car's block heater. The transformer supplies 10mA to an LED installed in the switch plate. The transformer is a toroid that the switched load simply passes through so there is line voltage isolation. Are there any concerns with this?"

"To get a technical advice about your project, ESA highly recommends contacting a licensed electrical contractor. You may want to check the following link for a list of Licensed electrical contractors in your area:"

After asking a few local contractors it was clear that they had no idea of what I was talking about.
Oh well, I tried. On with the project.

Step 2: Tools and Materials

1. Current transformer 2000:1.  I found this one on Ebay for about $8.00 Cdn. with free shipping. It outputs 10 mA with a 20 amp load.
2. Red LED. Nothing special just your standard.
3. 2 pieces of heat shrink tubing about 1 in. long.
4. A bit of solder.

*Optional: If you're worried about the PIV of the LED you can put a regular diode in parallel but reversed polarity to shunt the reversed bias from your LED but I tested this setup on a fairly high inductive load and I found no issues. The current transformer is basically operating as a constant current source so the LED has no problems handling the 10 mA.

1. Soldering iron.
2. Drill and 3/16 in. drill bit or to fit LED. Check first in some scrap wood before drilling your switch plate.
3. Heat gun, hair dryer or something hot to shrink the tubing.

Step 3: Assembly

Assembly is quite simple.
1. Cut each lead of the LED to about 1/2 in.
2. Each leg of the LED is soldered to the transformer leads. Because it's AC, polarity doesn't matter.
Just remember to slide a 1 in. chunk of tubing up the transformer lead and out of the way before soldering!
3. Shrink the tubing with your heat gun as close to the LED as possible.

As mentioned before you can add further protection for the LED but the one I chose worked well under a heavy inductive load all by itself.

Pictured is the glowing LED with my 20 amp tablesaw as the load. It's a bit blurry but you get the picture.

Step 4: Testing

Before installing you might want to test it. Pictured is a test jig I use with my clamp-on for testing motors and appliances for current draw. All you really need is some kind of a load that you can unhook and separate one of the leads and pass it through the hole in the current transformer. Something like an old vacuum cleaner that you've replaced the cord cap on is ideal. Remember though that you can only pass one of the leads through the hole or the AC phases will cancel each other out. With the load running the LED should glow proportionally bright to the load you're testing and with a 60hz flicker.

Step 5: Installation

Installation is the hardest part of this project. I wish I had some pictures of my installation but they didn't turn out.
1. Turn off the breaker or fuse that services the switch box that you want to monitor.
 My exterior outlet is split and has two interior switches; one standard and one with a timer.
2. Decide where on the switch plate you would like to put the LED. Remember it needs clear access to an open area of the box for the leads.
3. Drill the hole in the switch plate and insert the LED. It should be a really tight press fit.
4. Unhook a load line from the switch that you would like to monitor, pass it through the transformer and reattach it. I chose a line that feeds both of my plugins so I can monitor each one.
5. Reassemble the switch and switch plate.
6. Enjoy the security that you can safely tell when your block heater or roof heating cable is working.

Pictured is my setup monitoring a heating cable that passes in a drain pipe under my driveway. This pipe routinely freezes up in the spring and if I don't unthaw it the overflow really ices up my driveway. The heating cable draws about 5 amps so the LED is at about 1/3 brightness.

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    5 Discussions


    5 years ago on Introduction

    I am looking to take this concept slightly further and wanted your thoughts..... I would like to install a device that will monitor the amount of current used while the light is on. You can purchase the "Kill-a-Watt" device that will do this for anything that device that plugs in but i can't seem to find a way to accurately measure the amount of current used in single light fixture in my house. You can take a guess as to how long but having something that would essentially do what the Kill-a-Watt device does would be very beneficial. I also live Canada and am always looking for ways to track exactly how may hours each light is on in each room. The 2 Gang faceplate you are using in this picture is what made me want to contact you. The one side appears to have a small LCD panel on it. I was thinking if you could make such a device it could be installed similar to your 1st image with a LCD readout indicating what the fixture has drawn for current. If you have any ideas or know of anything online i would certainly be interested.... (PS- I am just finishing my 1st year electrical apprenticeship as well which is where the recent obsession with conservation is coming from :)

    1 reply

    Reply 5 years ago on Introduction

    rmhome, there are some nice units that display a digital readout for the current as well for around $10. Do an Ebay search for "current transformer voltmeter". If I understand you correctly though you also want to log the power usage as well. If this is the case and you're a DIY type, I suggested investigating embedded micro-controllers such as the Arduino. The Arduino has tons of home automation projects and the learning curve is real easy. You can pick up an Arduino or Arduino clone for between $10 -$20. The Arduino has lots of 3rd party plugin sensor and output modules called "shields" that do everything from motion sensors to temperature probes. Just google "home automation arduino" and you'll get over a million hits. Hope this helps.


    6 years ago on Introduction

    Thanks for the comments. For some reason the Recaptcha spam filter won't let me post replies.
    For RushFan (Rush as in "Tom Sawyer"?): LOL It must seem funny when Canadian tourists drive to Florida, half of them with block heater plugs hanging out of their front grills. Probably looks like all we drive are electric cars! ;-)
    For Lectric Wizard: I probably should've titled my project "Heating cable current monitor". The more I think about it, the more possible uses I come up with. Anything like lighting that's clearly evident when it's on and working is not relevant but possibly some remote loads like a security light or an exhaust fan in your attic.

    This is a great idea. I live in a Manufactured home in Canada. One of these is going on my heater cable for the water pipes before next winter !! THANKS!


    6 years ago

    Excellent! I live in Florida, so I have to admit I had no idea WTH a block heater was, but now I do, and if I even need one, I am going to use your instructible!