Cake or death or emon pi burden resistor swap?

Getting Started
1

Hi all,
I’m in US, so bought two Magnelab SCT-0750-000 for emon Pi. I’ve gleaned this from the guides:

  • Magnelab SCT-0750-000 are 1:7500, needs a 39-ohm resistor, energy coefficient 192.31
  • “If you use an alternate CT, and emonTX on-board SMT burden resistor is not correct, then you must remove it, and (if and only if CT is unburdened), add a resistor in its place using the holes provided.”
  • Under the “Sentran” 3rd-party CT info, I learn that emonPi/emonTx onboard resistor is 22-ohm.

So, my CT needs a 39-ohm resistor, while onboard resistor is 22-ohm. Thus, it seems I must remove the 22-ohm, and purchase/install a 39-ohm resistor (E24 series, .025W, 1% tolerance). (or put a resistor in parallel with 22-ohms, such that it equals 39-ohm)

Is this correct?

  • If so, please recommend a source for E24-series resistors? (all I have on hand are 5 and 10% tolerance resistors)
    Thanks!
2

WRONG! If you put a resistor in parallel, you will REDUCE the total resistance.

[quote=“zakleeright, post:1, topic:1895”]- If so, please recommend a source for E24-series resistors? (all I have on hand are 5 and 10% tolerance resistors)
[/quote]

39 Ω is in the E12 series. You don’t mean to tell me there’s nowhere in the USA that sells 1% resistors, surely?
Try here: http://spiratronics.com/0.6w-metal-film-resistor-39r-pack-of-10.html
Unfortunately, it’ll cost you £6.99 to post airmail. About $9 in total.

3

Try amazon?

4

Thanks Mr. Wall…I wasn’t just wrong…I was WRONG!:grinning: (my Ohm’s law chops got rusty)
Also, thanks to you and chubbyhorse for answering my lame “source for resistors” question, which seems to have detracted from my main (still lame?) question…

Based on my scenario of using the Magnelab SCT0750-000 CT, must I unsolder the on-board SMT resistor and replace it? Apologies in advance if this question also seems silly…just hoping to know for sure before I whip out my soldering iron.
Cheers!

5

My buddy Bill Thomson verified that indeed I must replace the resistor, with 39-ohm resistor. Thanks much to all at the forum for the advice!

6

When you get to doing it, you’ll find a pair of holes waiting ready for the wire-ended resistor. So just flick the SMT resistor off with a hot iron and solder the wire-ended one in.