48v horn makes my CA reboot

izeman

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Jun 21, 2011
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a little cryptic topic, so in more detail:

i have a 15s battery. from my controller the battery voltage goes to the CA. inside the CA is split the wire and connect it to a button switch that triggers the a 48v horn. as the horn is basically a coil which shorts out the supply power and draws lots of amps during "start phase" the voltage drops a lot (it's only awg20 wires). the CA detects a "low voltage cutoff" situation and turns off (-> reboots).

i guess i'd need some kind of inrush limiter. once the horn is "running" current is no big deal anymore. but the initial surge is too much.

any idea what i could do? there is only very little space inside the CA to install anything.



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izeman said:
...from my controller the battery voltage goes to the CA. inside the CA is split the wire and connect it to a button switch that triggers the a 48v horn. as the horn is basically a coil which shorts out the supply power and draws lots of amps during "start phase" the voltage drops a lot (it's only awg20 wires). the CA detects a "low voltage cutoff" situation and turns off (-> reboots).

First, hooking up relays to wiring shared with electronic goodies runs the risk of pounding the electronics with giant voltage transients as the inductor field collapses - this is why you see relays in these situations bridged with snubber diodes. This shorts out the spike so it doesn't propagate into the wiring and zap the electronic parts. A horn is much worse than a relay - it's just a huge inductor slamming on and off and bombarding the system with voltage spikes. So - if you want shared wiring, I would recommend putting a snubber directly across the horn terminals - the current is tiny, but the voltage can be high - use something like a 1N4004 or 1N4005. The diode goes on with reverse polarity so the little stripe is towards positive.

  • As a matter of best practice, I do not recommend sharing power wiring for both electronics and accessories. Give electronics and accessories two separate ground/power runs back to the battery and myriad problems cannot occur. Sharing power wiring may work by luck, probability, and circuit analysis, but separate wiring always works with no thought at all...

Second, you have a wire gauge issue separate from the transient issue. You could limit the horn inrush current with a PTC resistor, but this may prevent the horn from even starting. Those suckers are electromechanical and rely on momentum, etc to work. Dampen the inrush current and the diaphram may not mechanically displace rapidly enough to initially break the circuit to start oscillating ON/OFF.

  • For instance, you'll notice this type of horn frequently comes with a springy steel mounting strap. That trashy strap is actually part of the design and is needed to allow the horn body to vibrate freely - mount the horn solidly to the frame and it may not work at all or will sound muffled and have only a fraction of the volume - pretty much like pressing your hand onto a speaker cone. Both the horn body and the diaphagm must be free to resonate - the point being that there's a lot of non-obvious mechanical motion dynamics in play and a mushy start may not kick things into operation.
A very distant second to getting its own power might be to try to decouple the CA from the horn power. You can try adding a 10 ohm resistor in-line with the CA power lead where it enters the PCB. This will help decouple the CA from the voltage excursions on the common power line. Based on a modest 20ma draw for the CA and throttle, etc, the resistor will introduce a persistent 0.2V offset in measured battery pack voltage. If that is unsuccessful alone, you could try adding a large cap (15V and 100uf or more - as large as will fit) from the LED(-) pin to GND on the CA PCB (cap(+) to LED(-)). This will help to hold up the CA +5V supply while the horn bounces the voltage around on the other side of the resistor. I'm not enthusiastic about this decoupling approach with a horn, but if you have a custom harness already fabricated and want to engage in a little experimentation....





Or you could just run a couple of separate wires for the horn and lights and be done with it... :)
 
thanks tek. as always a very elaborated explanation and a helpful post. i hope i understood everything correctly and will try your solutions one by one.
i know the wiring is far from optimal, but i'm limited in wires running from the controller to the CA and this was the only way to do it.
 
hi tek. wanted to give some feedback.
i installed the 10ohm resistor (removed VCC from controller, and put it inline, connected horn), and i added a 100uF 16V cap to LED.
w/o the cap the CA still rebooted - not often, but it could happen.
now with the cap all is fine. also adding the resistor seemed to decouple the VCC somehow, as the CA doesn't see a sudden voltage drop which made it shutdown (and reboot).
on the other hand adding the diode was a DISASTER. i didn't have an 1N400[1,5] so i used the 1N4007 i had laying around. it just made the thin wire vaporise at the solder pad. i thought maybe i had it installed the wrong way, reversed it, and BANG, the switch just bursted in flames. hmmm. good that i had a replacement. removed the diode at all, now all is fine ;)
thanks again!!
 
Thanks for the feedback - appreciated!!! :D

izeman said:
hi tek. wanted to give some feedback.
i installed the 10ohm resistor (removed VCC from controller, and put it inline, connected horn), and i added a 100uF 16V cap to LED.
Great news!
I updated the illustration above with your verified part values. :)


izeman said:
on the other hand adding the diode was a DISASTER.
Hmmm - that's too bad - and strange. The diode you used should have worked fine. Soldered directly across the horn terminals it shouldn't have have any noticeable operating effect. I do recommend that you get a diode across the horn - maybe you can try another round and test it with jury-rigged wiring away from your usual harness and switches to avoid mishaps...
 
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