### replacing burden resistor on emontx

Just so I'm clear - I need to replace the burden resistor on the emontx. The steps are:

1) remove the existing surface mount resistor

2) clear the hole on each side of the existing resistor

3) use a through hole resistor, soldered in

I have the Magelab CT's with no burden resistor and need to attach the 3.5mm plugs. Once the existing burden resistor is removed, could I also mount the resistor inside the 3.5mm plug of the CT, if there's room?

The resistor I got have a body about 3 mm long and should fit. Or did I get the wrong resistor? They are 1/2 W, 1%

Thanks

### Re: replacing burden resistor on emontx

Yes, but the size of the wires on the Magnelab CTs might make it a bit difficult to do.

### Re: replacing burden resistor on emontx

If mounted inside a jack-plug, there wouldn't be much air-cooling.

### Re: replacing burden resistor on emontx

Also, if your calculated burden resistor value is less than the standard 22 ohm mounted on the board, you can just leave that surface mount resistor in place and parallel a proper value resistor soldered in through the 2 holes next to it.

Say you calculate you require a 15 ohm burden resistor then (22X47) / (22+47) = 14.99 ohms total so a 47 ohm resistor mounted in parallel should work.

### Re: replacing burden resistor on emontx

Without knowing what CT you have and the maximum current that you need to measure, it is anybody's guess as to whether you have the right resistor. The resistor you need is dictated by those considerations, not by the hole spacing. If your CT gives more than 450 mA secondary current, a ½ W resistor is not big enough.

You will be safer if you can get the resistor inside the plug body, but as Robin points out, it will need serious derating if you do. I wouldn't wish to estimate the derating factor that you'd need to apply. If your plug has a metal body and you can fill it with silicone grease to aid heat transfer to the outside world, then you won't need to derate quite as much.

### Re: replacing burden resistor on emontx

To answer some questions - I have the Magnelabs SCT-0750-000 CT. Based on info from the US page I'm using a 39 ohm, 1/2 W, 1% burden resistor. I'll put it on the board.

My house has a standard US 100 amp 2 phase service with a 20 KW propane back up generator which can produce up to 83 amps. I'm using about 1000 KWh a month which works out to about 1300 watts/hour or about 12 amps. While I haven't started to monitor yet, I suspect the biggest load is the heat pump - I think it's the only 220 V appliance. All others are 110V (including the propane clothes dryer and stove/oven and water heater)

### Re: replacing burden resistor on emontx

The SCT-0750-000  should give you a sniff over 11 mA at the max output of your generator, or 22.7 mA if you pass both legs of your supply through it (in opposite directions of course) at your supply's nominal rating. Let's say 50 mA worst case under a sustained overload. That current will be well in excess of what the emonTx can handle, but the dissipation in the burden resistor is still under 100 mW. I think your ½ W resistor, or even a ¼ W one, would be fine inside the plug body.

If you put the resistor on the board, you need to be aware that you will get a high voltage, quite possibly dangerously so, if you accidentally or deliberately unplug the CT while the circuit is carrying current. This is because the CT is a current source and it will generate whatever voltage is necessary to drive the current it wants to into its load. In theory, the voltage will go to infinity. In practice, the usual result is the insulation breaks down and the CT is destroyed. The unburdened YHDC CTs have a internal zener diodes to prevent the voltage rising to a harmful level.

### Re: replacing burden resistor on emontx

As Robert points out, you definitely do not want to unplug the CT while the monitored circuit is carrying current.

According to Magnelab's spec sheet,  the SCT-0750-000 does have zener diode protection, which should limit the output voltage to 22 volts.

### Re: replacing burden resistor on emontx

The zener diodes are obviously a recent addition, they're not mentioned on the (saved) data sheet I have. So I would still advise caution.

### Re: replacing burden resistor on emontx

I asked Jeff Lawson, Magnelab Sales and Support VP, if the Zener diodes in Magnelab's SCT
series CTs were a recent addition.

His response:

the Zener diodes (back-to-back) have always been installed in our non-burdened split-core CT versions.  If you suspect a unit(s) do not have this voltage protection, please send them to us for replacement/testing. A resistance check across the output leads should read approximately 500 ohms.

### Re: replacing burden resistor on emontx

Hmm,

A resistance check across the output leads should read approximately 500 ohms.

I guess that's how you get to be Sales & Support VP ....    ;-)

### Re: replacing burden resistor on emontx

500 Ω for 7,500 turns sounds a reasonable enough winding resistance to me - and unless you're using a Megger, you shouldn't even see the zeners!

### Re: replacing burden resistor on emontx

Bill

Your response from Magnelab about zener diodes in the split-core non-burdened CT’s got me wondering if the Wattcore WC1 100:0.1A I use also has them as an undocumented / unadvertised feature.

I had a spare one of these laying around so put it around the hot lead of a space heater pulling 12 Amps at 122V so the secondary current is 0.012 A  (1,000 turn ratio). Attached it across a 15 ohm resistor on a breadboard and measured about the expected 0.176 V and 0.508 V P-P.

Turned off the heater and replaced the resistor with a 2.2K ohm which should output about 26.4 volts RMS, so if it is going to clamp the output it should be apparent before going that high. The scope trace shows the output clipped (almost a square wave) and did it’s best to measure that waveform out at 6.4V RMS 14.4V P-P.

So apparently, this CT does have zener diode protection that clamps the output at about 6.4V RMS. Nice safety feature but as has been pointed out, do not plug / unplug any CT when the monitored circuit is carrying current.