electrical question: horn cuts my engine

B

bruno_mac_douglas

1 mW
Joined
Feb 20, 2020
Messages
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Hey guys!
I have a specific question on my bafang 750w. It runs well but I installed this horn:

https://fr.aliexpress.com/item/4000218317520.html?spm=a2g0s.9042311.0.0.27426c37MTynRr

it's directly connected to the output of battery, with a switch inbetween.

It works good except it cuts my engine sometimes, not always. I activate it and the LCD shuts down.
Sometimes the LCD becomes all white and the engine still works.

This morning this happens and then I had no speed indication after restart, it came back after a while.

I don't want to burn my lcd or engine because of the horn, what should I do ?

Thanks in advance
Cheers
 
Some things in your system are probably connected electrically to your ebike's frame. Insulate the horn and your switch from anything metal before trying the horn again.
 
Thanks, I tried keeping the horn not connected to frame and it's the same :-( I imagine it takes too much intensenty and stop the display. Still no Idea how to solve this, if anyone have a suggestion...
 
Maybe you’re getting some electrical noise? You could try putting a capacitor and a resistor across the horn terminals.
 
well it's a 48v, my battery too (well so it goes up to 54 but should not be a problem I guess?)
https://fr.aliexpress.com/item/4000218317520.html?spm=a2g0s.9042311.0.0.27426c37n7x0QE

Balmorhea which capacitor/resistor do u suggest ?

thanks
 
Balmorhea said:
Maybe you’re getting some electrical noise?
Click to expand...

That's the problem.

Balmorhea said:
You could try putting a capacitor and a resistor across the horn terminals.
Click to expand...

But that's not the solution.

bruno_mac_douglas said:
I don't want to burn my lcd or engine
Click to expand...

The engine will be fine, but the electronics are at risk. Especially not very well designed, and designed to a low price electronics.

You need a choke. They sell them for cars, and they are called alternator noise blocker or filter, or some combination of those words. Except you'll be blocking noise from the horn back to the battery. It works in both directions.

The horn is 48v @ 0.2A, which is pretty weak compared to a car horn, which would be 10-20A @ 12V. Any choke kit will handle the current just fine.

The internal components of a choke vary, so it's not 100% guaranteed to be good enough. But at that point you'd at least need an oscilloscope to troubleshoot further.
 
i can try that, where is it located in a car? I know on alternator you have like a 15vvoltage limiter, but I don't think that's it
 
bruno_mac_douglas said:
Balmorhea which capacitor/resistor do u suggest ?
Click to expand...

I'd start with a 100V, 100uF electrolytic capacitor, and a 5 kOhm resistor to discharge it.

A choke would probably work fine too, but I've personally only used smoothing caps and RC filters to reduce noise in DC systems.
 
Balmorhea said:
I'd start with a 100V, 100uF electrolytic capacitor, and a 5 kOhm resistor to discharge it.
Click to expand...

This is confusing. It does not sound like a 1st order low pass filter.

Can you draw a schematic or explain what goes where?
 
Comrade said:
Balmorhea said:
I'd start with a 100V, 100uF electrolytic capacitor, and a 5 kOhm resistor to discharge it.
Click to expand...

This is confusing. It does not sound like a 1st order low pass filter.

Can you draw a schematic or explain what goes where?
Click to expand...

What I'm recommending is not an RC filter. It's a smoothing/damping cap with a resistor to dump its charge relatively quickly.

Bridge the horn terminals with both components (observing correct polarity for the cap). That should snub voltage spikes coming from the horn.
 
The primary electrical "noise" with that horn would be high voltage reverse voltage spikes because that horn is essentially an inductor that energizes and de-energizes at the design frequency of the horn. Not sure what a cap in parallel would do, except maybe explode eventually. :wink:
 
Comrade said:
Not sure what a cap in parallel would do, except maybe explode eventually. :wink:
Click to expand...

I just told you what it will do in my above post.

Those voltage spikes get stuffed largely into the cap instead of the bus, and released at lower voltage after the spike subsides. It's like using a spring to provide shock absorption. Since it's on the switched part of the horn circuit, it will only do its thing when the horn button is pressed. I doubt that would impose a duty cycle problem.

If it does get subjected to reverse polarity, that would represent a problem. My impression is that an inductor makes spikes of increased voltage but with the same polarity it has been charged with.

Anyway, it's a very cheap and easy thing to try. The worst that could happen is it doesn't fix the problem.
 
Balmorhea said:
Those voltage spikes get stuffed largely into the cap instead of the bus
Click to expand...

The cap has an internal resistance at least an order of magnitude higher than the bus.

Why would the spike get stuffed into the cap instead of taking the path of (literally) least resistance down the bus? :mrgreen:
 
Comrade said:
Balmorhea said:
Those voltage spikes get stuffed largely into the cap instead of the bus
Click to expand...

The cap has an internal resistance at least an order of magnitude higher than the bus.

Why would the spike get stuffed into the cap instead of taking the path of (literally) least resistance down the bus? :mrgreen:
Click to expand...

I haven't scoped it, so I don't know. But the cap has a way to absorb the transients, the other things connected to the bus maybe or maybe not. Parallel capacitances add up, so wherever there's a cap, it will absorb the voltage transients and flatten them to the limit of its ability. It could be anywhere on the bus, but in this case it should be on the switched part of the horn circuit so it only interacts when the horn is making noise. Certainly there's less series resistance in a cap than in whatever logic circuit is acting fruity for the OP.

I usually use big capacitors to turn rectified wild AC or rectified line AC into low ripple DC for LED lighting or audio. They work well for that, and I don't see why they wouldn't also smooth out line noise from a beeper.

Like I said before, it costs almost nothing and the worst that could happen is it doesn't fix the problem. If a choke works better or is easier, then that's a better way to go.
 
As long as you size the capacitor right that should be fine (just make sure that the coil inductance and the buffer capacitance don't resonate at the horn frequency)
 
Comrade said:
Balmorhea said:
Those voltage spikes get stuffed largely into the cap instead of the bus
Click to expand...

The cap has an internal resistance at least an order of magnitude higher than the bus.

Why would the spike get stuffed into the cap instead of taking the path of (literally) least resistance down the bus? :mrgreen:
Click to expand...
I thought high capacitance with high internal resistance is best for controlling voltage transients. Maybe I'm recalling that wrong. Can't be electrolytic though.
 
You probably want fairly low ESR so that the spike goes into the cap and not the bus; try a couple of these in parallel: https://www.mouser.com/ProductDetail/Panasonic/ECQ-E1106KFZ?qs=KRpiEsko4Hmyt%252BrPXyLDiw%3D%3D
 
E-HP said:
I thought high capacitance with high internal resistance is best for controlling voltage transients. Maybe I'm recalling that wrong. Can't be electrolytic though.
Click to expand...

This is unrelated to this specific case: but current always takes the path of least resistance. If you have a capacitor with high internal resistance, much higher than the bus, transients will just go down the bus. If you look at a diagram of an RC low pass filter, the resistor creates that resistance on the bus to make the filter work. Without the resistor, the cap will not do much.

The case of the horn mentioned above: it is mechanical device where a coil is energized, pushes a spring and diaphragm, which mechanically de-energize the coil. Then the spring pulls the diaphragm back, energizes the coil, and repeat.

When the coil is de-energized a large reverse voltage spike is created. The solution to that spike is a: https://en.wikipedia.org/wiki/Flyback_diode

But just a flyback diode might not be enough to completely resolve the issue. That's where an inductor enters the picture.
 
Guys, thanks a lot for that brainstorming here concerning this electrical noise from my horn :)
I was currently moving house, new job, so didn't have time to see what was going on here and reply.

I will see what I have and make some test, probably try resistor and capacitor if I find correct ones in my toolbox, then try to find a choke if it doesn't fix my problem.

I'll make sure to come back here to inform what was the solution, if I reach it!
 
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