Capacitor wiring advice needed.

[Hoohaa]

Hello. I'm in the process of redoing the wiring in my boat. My 12 volt fridge has a capacitor wide into the system.
Apologies about the crude diagram but I need to check just how it should be wired correctly.
Thanks for your help.

[JoeRobertJr]

Hi, Hoohaa,
The diagram looks correct to me for wiring the capacitor as a noise filter.
How physically large is the capacitor?

[EngineerRetired]

Is this a motor starting capacitor?

[rwidman]

Wire it the same way it was wired before you decided to rewire the boat.

[JoeRobertJr]

Originally Posted by rwidman

Wire it the same way it was wired before you decided to rewire the boat.

The OP is doing well by double-checking. Existing wiring is not always correct wiring.

[BillKny]

Originally Posted by EngineerRetired

Is this a motor starting capacitor?

Since this is a DC motor, it would be very unusual to have a starting capacitor.

It is for noise suppression, and his drawing is correct. Although, the capacitor for this application is usually wired directly across the motor terminals, or very close to it to avoid/reduce the radiating noise from the wires between the motor and the cap.

[s/v Jedi]

Also, it is recommended to add a 100nF ceramic or tantalum capacitor in parallel to the electrolytic capacitor. This adds RF noise suppression.

[captstu]

Originally Posted by EngineerRetired

Is this a motor starting capacitor?

DC motors do not have starting or running capacitors. This is a noise capacitor.

It should be wired + to + and - to -. Earth ground is not necessarily - on a boat.

Wire it as close to the fridge motor as is convenient - the wire between the motor and the capacitor is sort of an antenna radiating noise and may interfere with your electronics.

All wire used on moderate to heavy loads should be twisted to avoid electromagnetic interference that may your navigation compass.

[guyrj33]

Originally Posted by captstu

DC motors do not have starting or running capacitors. This is a noise capacitor.

It should be wired + to + and - to -. Earth ground is not necessarily - on a boat.

Wire it as close to the fridge motor as is convenient - the wire between the motor and the capacitor is sort of an antenna radiating noise and may interfere with your electronics.

All wire used on moderate to heavy loads should be twisted to avoid electromagnetic interference that may your navigation compass.

Stu is referring to the DC power wires positive and negative being twisted together in a 'twisted pair' to mitigate magnetic fields caused by the current flowing in the wire.

[RaymondR]

The capacitor could be either a small capacity capacitor installed to reduce RF, in which case it should be installed as near to the motor power supply inputs as possible.

However very high capacity capacitors are available and are used on the power inputs of those super high powered car sound systems bogans sometimes cruise the streets with, in order to supply sudden heavy current draws by the noise producing equipment. If this is the purpose the capacitor should also be wired in as close to the power input to the compressor as possible.

I have a portable fridge I sometimes use which has starting problems and will cycle on/off two or three times to get started when the house batteries are low which could probably benefit from this arrangement.

10,000 UF 40V-50V capacitors are readily available for this purpose.

[Hoohaa]

Sorry for not getting back sooner.
Here's a pic of the capacitor. I'm thinking the fuse should be on the other side of the capacitor?

[Hoohaa]

Originally Posted by JoeRobertJr

Hi, Hoohaa,

The diagram looks correct to me for wiring the capacitor as a noise filter.

How physically large is the capacitor?

It's starting my fridge?

[Hoohaa]

Originally Posted by EngineerRetired

Is this a motor starting capacitor?

Yes. It on my fridge circuit.

[Hoohaa]

Originally Posted by BillKny

Since this is a DC motor, it would be very unusual to have a starting capacitor.



It is for noise suppression, and his drawing is correct. Although, the capacitor for this application is usually wired directly across the motor terminals, or very close to it to avoid/reduce the radiating noise from the wires between the motor and the cap.

Noise suppression?
My fridge is happily running without it.
Could you explain why you would need noise suppression?

[RaymondR]

Yep, 47,000 uF is a little large for RFI suppression.