Anti-spark techniques

[Tim Kearsley]

I'm about to build my first set up using 8 cells (actually two 4s batteries in series). I've used 6s LiPos on my Ready2Fly DH Venom and they give quite a crack when you connect up the power, so I'm thinking of using anti-spark techniques with the 8s setup. However..... I find references, particularly in relation to Castle Creations ESCs, that connecting up power initially through a resistor of, say, 33 Ohms can be damaging to the ESC and shorten its life.

Opinions on this seem to be divided, though the articles I was reading did date from 2008.

What's the current thinking (no pun....) on this - to spark or not?

[David Ashby - Moderator]

Tim,

I know Puffin sell those Jeti anti-sparking bullet connector plugs that seem to do the trick. Like you I seem to have a few 8S projects - one just complete and one in the pipeline - so it was these plugs I was probably going to use.

 

 

Edited By David Ashby - RCME on 05/12/2012 09:17:31

[Tim Kearsley]

I know the ones you mean David. I was going the cheap-skate way and making my own with 33 Ohm resistors, as per the article referred to by Tim Mackey in a post a year or two back. There was a long discussion on this topic on RCGroups back in 2008, and there was a technical description of why the application of Volts to the ESC through a resistor of more than an Ohm or two could cause damaging Voltage spikes in the ESC. Technology moves on at a fast pace though, which is why I wondered if the point is still relevant.

[Tim Hooper]

I've no idea concerning the 'slow death' theory when it comes to using these connectors. However, I've been using the Jeti items for some months on several of my 5S+ models, and I haven't noticed any ill effects so far.

tim

[Steve Hargreaves - Moderator]

It does seem an odd theory to me too.

In electrical terms all you are doing is allowing the input capacitors to charge up slowly via the resistor instead of applying the whole battery voltage & charging them up "instantly"......to me that can only be a good thing surely. In addition the DC arc (spark) caused in this way can & will pit & burn the connectors over time.....

I'm not a high voltage electric flyer but I would use the system outlined by His Timbo-ness if I was experiencing such arcing......

[Tim Mackey]

My cheapo HK 120A esc is still fine after many uses in the high voltage ( now 10s 2.5kW ) big seagull PC9 project - this is where I first used the aforementioned home made resisitive anti spark system.

Usual caveat... your mileage may vary

[John Privett]

I'm with Steve on this one. I'd be fascinated to see what the argument is for it harming the ESC - I'd have thought if anything it would prolong the ESC life.

[Tim Kearsley]

And I'm with all you guys in that preventing a fat spark seems the way to go to me. I was just reporting a thread I followed in RCGroups where this was debated. Also, it seems that Castle Creations declare that "the spark is your friend" and actively discourage the slow-start approach. Anyhow, I shall go ahead and use the technique, mainly because big fat sparks make me jump!!

[Former Member]

[This posting has been removed]

[Steve Hargreaves - Moderator]

I'd be interested to read the debate on RC Groups Tim...do you have a link?

I'm not convinced that a spark is anybodys friend so I wonder why CC say that......I notice they do in fact offer a Cap Pack for the input side to reduce any ripple voltage caused by long battery wires.....that will probably produce a nice fat arc when you connect the battery too.......

[Tim Kearsley]

Yep, certainly do Steve, you can follow the thread here It goes on for several pages and there is an explanation of why (perhaps) anti-spark precautions can damage the ESC. It suggests there is perhaps some experimentation necessary to establish the value of resistor is low enough to ensure auto-detection of the number of LiPo cells connected still works but high enough to effectively limit the spark.

[Steve Hargreaves - Moderator]

Thanks Tim......an interesting read.

If the guys at Castle Creations say the spark is OK then who am I to argue....I'm sure they know a heck of a lot more than I do.....

It seems there are two issues......

1) as you say Tim too high a resistor can lead to the ESC not recognising the cell count correctly as the voltage presented at the input stage might not be the full pack voltage.

2) When the ESC detects the battery it sends a series of pulses to the motor which will then emit beeps or tunes as appropriate..... These pulses can then apparently create high voltage spikes back into the ESC which are normally absorbed by the battery (which effectively acts as a large capacitor!!) If you've only connected a small resistor to charge up the input capacitors then the battery isn't really connected at all which means these spikes have nowhere to go & can damage the ESC.....which kind of makes sense.

But against that why would such a respected company as JETI produce their anti-spark connectors?

Interesting.....

Sorry Tim this probably isn't helping you much is it.....

[John Cole]

In electrical terms, 50 volts is not high voltage. I would not expect a spark as contact is made: a spark across an air gap requires pretty high potential gradient. What I suspect you are seeing is an arc. This would be caused by touching the contacts and momentarily separating them again before full contact is made. The 50 volts might well generate an instantaneous current charging the input capacitors of 1000 A. Plenty for an arc. But if you put the connectors together quickly and hard then I would not expect one.

The warnings about incorrect voltage-sensing make sense but I would suggest that the values of anti-spark resistor talked about are in most cases too high: I would be looking at 1 ohm maximum (charge current initially 50 A @ 50 V) and probably less.

[Tim Kearsley]

Point taken John and correct terminology noted. The practical problem with this is that you can't always get the connectors together quickly and hard. Up to now I've used Deans (actually not REAL Deans but one of the many cheaper clones) and it can take a degree of "fiddling" before they are firmly home and a solid connection is made. In this circumstance you get the arcing problem. I'm personally switching now to bullet connectors where this might not be such a problem.

The other point is about the value of resistor to use. If you use sub-Ohm values then surely you are still going to get a pretty hefty arc aren't you? To use your example, 50V and 50A equates to 2.5kWatts of power dissipated in that arc.

Amazing how what seems to be a simple question can get quite involved!

[John Cole]

The power is not the key factor in a short surge: it's the energy. When you make the connection the total initial energy passing through is just that required to charge the input-capacitors. The reason why CC say "the spark is your friend" (referred to above is simple: it shows the input capacitors are healthy! I'm not into high volts stuff myself, but I've seen somewhere that a CC ESC has one or more 330 micro-F capacitors. Assume 3 and thats about a milli-F.

A milli-F will take 1milli Couloumb (= 1 mA-second) to charge @ 1 volt. So at 50 volts it will just take 50 mA-seconds. That's a tiny amount of energy. But because it charges up quickly (milli second) you get a big but brief current. Hence the arc when you break it.