Oxygen S-Cross CB -hunt for the shunt

[DBye]

I've been reading (always dangerous) about doing things to my shunt in my S-Cross CB.
I found the controller and had a look, but have failed to identify anything that looks like a shunt save for some really heavy solder tracks, but guess these could be for heat dissipation. Anyone have any ideas?

 

[Deleted member 4366]

There's three solid state ones on the back as far as I can see, 2 miliohms each. Assuming they're the only ones, a 10 miliohm one piggy-backed to each will change the resistance to 1.66, which should be about right. You need to get them the same size. An R006 would give slightly more power.

http://www.ebay.co.uk/sch/i.html?_odkw=r006+resistor&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xr010+resistor.TRS0&_nkw=r010+resistor&_sacat=0

 

[DBye]

@d8veh - Thanks, I did take a good look around the PCB, but didn't look too closely under the capacitor. There is a rather large resistor adjacent to it (see attached rubbish photo). When I have a chance I'll take another closer look.

 

[Woosh]

Can you tell us the markings for the circled components?
They look like R002

 

[DBye]

@Woosh they are R002 -please see slightly less rubbish photo attached.

Looking at the component side of the board there doesn't seem to be anything hidden under the capacitor save for an IC of some sort.

 

[Nealh]

As d8veh has said they are solid state flat shunts and not the U shaped found in conventional ali box controllers.

 

[Woosh]

They are labelled RSA, RSB and RSC, presumably RS stands for Resistor Sensor. They are current sensing resistors. R002 has a nominal resistance of 0.002 Ohm. Basically, they produce an inductance to limit the current to a preset value.

 

[Deleted member 4366]

Get the size code from here:
http://www.resistorguide.com/resistor-sizes-and-packages/

You need one thats 1W or more, probably this one if it's the right size:

http://uk.farnell.com/webapp/wcs/stores/servlet/Search?pageSize=25&st=0.01+ohms&catalogId=15001&categoryId=700000050528&langId=44&storeId=10151

http://www.ebay.co.uk/itm/BOURNS-CRF2512-JX-R010ELF-RESISTOR-THICK-FILM-0-01OHM-5-2512-/171138312808?hash=item27d8a36a68:g:NvAAAOxyrM5TIjOO

 

[DBye]

Thank you all for the very helpful replies. The R002's used seem on the large size so it looks like 2512 would be the closest fit.

As a sanity check - The three R002's created a total resistance of 0.666' mOhm.

If I piggy back R006's onto the R002's I'll reduce the total resistance (sum of all resistors in parallel) to 0.5 mOhm.

If I use a 10 mOhm resistor that would reduce the total resistance to ~0.57mOhm.

I'm tempted to go for the R006's (the maths was nicer!), so something like these:
https://tinyurl.com/l4prkjr

 

[Woosh]

DBye, I think each R002 is on one phase wire.
You need to buy 3, to piggy-back one on each R002.
You may think I am a doom merchant but there is always a risk to modify a controller.

 

[DBye]

@Woosh -Thanks for the clarification, that's what I was thinking, piggy back an R006 onto each of the three R002's.

I have a healthy fear of meddling with PCBs and electronics in general so I want to proceed with caution. I used to work in a clean room using 30kV power supplies (drawing only milliamps). We had a nasty shock when we realised the earth was insufficient and when using the power supply the taps, fridge, fume cupboards all became live!

 

[Deleted member 4366]

Don't worry, no harm will come as long as you're careful with your soldering. I've adjusted loads of controllers and never had a single failure.

R002+R010 = R0017 = 15% reduction or 15% more current
R002+ R006 = R0015 = 25% more current

I normally go to about 30%, but it depends on the speed you go, your battery, cooling, etc. 15% will be very safe.

 

[Woosh]

those resistors go on the underside of the PCB.
Would they touch the casing if you solder them over the existing ones?

 

[DBye]

Thanks @d8veh -I'll take the plunge and put my Ebay order in. Will go for the R006's - If I'm invalidating my warranty I want to do it properly

@Woosh -I had wondered that too. See post 3 for a pic of the PCB in the case, there is a good few mm of clearance, so hopefully it will clear it. If I find it is a bit too tight I guess I can try varnishing the inside surface of the case or use electricians tape. Hopefully that won't be needed.

 

[DBye]

From theory to practice:
http://www.pedelecs.co.uk/forum/threads/oxygen-s-cross-cb-shunt-mod.27282/

Thought I'd best make a new thread in the S-Pedelecs section to keep the Mod Gods happy!

 

[DBye]

Should I ever wish to downgrade, it will probably easier to buy 3 new R002's (size 2152 again).

Would something like these be suitable that are rated to 3W?

 

[Deleted member 4366]

A rough calculation says 15 amps max is 5 amps per shunt. Heating is therefore 5 x 5 x .002 = .05W, so 3W seems like overkill, but won't do any harm and will never blow.