Electronics experts

[alexanderfoti]

Hi all

On my motorbike I have a Facet fuel pump fited.

A facet fuel pump has a piston internally which is balanced against a spring. An electromagnet pulls it towards one side, where it then disconnects the electrical connection and the spring pushed it back the other way.

In normal operating it clicks at roughly 40hz. This pulsing load on my electrical system causes by dashboard LED's to flicker (as they are sensitive to voltage fluctuations).

Its not a wiring fault as it does the same when wired directly to the battery.

I have thought about putting a capacitor across the pumps power wires but cannot figure out if I need a massive capacitor to smooth the pulses?

Ideas?

 

[Craiglxviii]

I’d have thought you need a whole load of EMC choking...

 

[L John]

I have thought about putting a capacitor across the pumps power wires but cannot figure out if I need a massive capacitor to smooth the pulses?

Ideas?

Your battery is like a big capacitor and that's not enough to stop the problem.
Capacitors are good at smoothing circuits but just putting a capacitor across the load wont make much difference, not even a big one.

The simplest way is to put a capacitor in parallel across the LED's to use as a holding tank, the value of the capacitor will depend on how much load the LED's take but the higher the uF the more stable it will be.
The capacitor should be fed via a diode. That will allow the capacitor to top up 40 times per second but will not allow the capacitor to lose it's charge back to the battery when it's pulled down by the pump.
The down side is you will lose 0.6V across the diode.
You need to know what current the LED's take to get the right diode but any diode rated over the current needed will be fine.
The capacitor obviously needs to be rated higher than your battery so if it's a 12V system I'd use a 20V rated capacitor.
When you turn the LED's off they will dim slowly as the capacitor discharges through the LED's.

 

[alexanderfoti]

Thanks both.

Not ideal though, as there are many LED's throughout the circuit.

Headlight
Tail light
and dashboard

Which would mean having to put a large number of capacitor resistor cobo's across everything.

There must be an issue somewhere. I cant imagine that the pump causes this issue on all vehicles with small batteries?

 

[Jkh112]

I am wondering if your battery charge is low. Would a fully charged battery keep the voltage at a level which the leds can withstand with the fuel pump running?

 

[alexanderfoti]

I am wondering if your battery charge is low. Would a fully charged battery keep the voltage at a level which the leds can withstand with the fuel pump running?

Good shout, It does it even with the engine running and the voltage at 13+

 

[L John]

Thanks both.

Not ideal though, as there are many LED's throughout the circuit.

Headlight
Tail light
and dashboard

Which would mean having to put a large number of capacitor resistor cobo's across everything.

There must be an issue somewhere. I cant imagine that the pump causes this issue on all vehicles with small batteries?

You'd be surprised at the voltage fluctuations with even a small pulsed draw.
It's much easier to smooth the voltage to selected circuits than to stop a pulsed pump causing noise on the 12V line.

 

[alexanderfoti]

You'd be surprised at the voltage fluctuations with even a small pulsed draw.
It's much easier to smooth the voltage to selected circuits than to stop a pulsed pump causing noise on the 12V line.

Well, I wouldn't be that surprised considering I can now see it

Fair enough.So a diode and capacitor, wired in series, then placed in parallel across the load?

 

[L John]

Well, I wouldn't be that surprised considering I can now see it

Fair enough.So a diode and capacitor, wired in series, then placed in parallel across the load?

Yeah I was meaning even just turning car lights on lowers the car battery voltage a bit and that's with a big battery so pulsing load will be a problem.

This is basically what I'm suggesting but without the resistor. The output will still rise and fall but it will be significantly reduced to effectively stop the problem.

http://www.learningaboutelectronics...ectifier-circuit-with-smoothing-capacitor.jpg