Capacitor wiring advice needed.

[AKA-None]

Also but not mentioned is that a cap of that size will store a lot of power that can discharge quickly

Eg do not get across those terminals

[Hoohaa]

Originally Posted by AKA-None

Also but not mentioned is that a cap of that size will store a lot of power that can discharge quickly

Eg do not get across those terminals

Thanks. It's currently in a plastic bag.
Once I remove it will it discharge on its own when disconnected from the power?

[SeanPatrick]

Originally Posted by Hoohaa

Once I remove it will it discharge on its own when disconnected from the power?

You can carefully place a resistor across the leads and let it discharge that way. Capacitors can store charges for long periods.


The videos I posted above go into more detail.

[rwidman]

Originally Posted by CatNewBee

Someone sought probably it is a good idea to put a large capacitor in front of the compressor to provide the power for the startup current of the compressor. This helps with crappy thin long wiring, that may cause a voltage sag and a low battery alarm at the fridge when starting.

If the wiring is incapable of supplying the correct voltage and current to the refrigerator, the proper solution is to replace the wiring with the correct size wires.

[RaymondR]

I would not throw it away just yet.

If it's wired into the power supply to the main breaker board it may have been installed to stop the drop out of navigation instruments when the engine is cranked. This occurs on my boat occasionally when the batteries are low and it's not a bad solution to have implemented provided the wiring is done properly and the capacitor securely fastened in place.

[rwidman]

Originally Posted by RaymondR

I would not throw it away just yet.

If it's wired into the power supply to the main breaker board it may have been installed to stop the drop out of navigation instruments when the engine is cranked. This occurs on my boat occasionally when the batteries are low and it's not a bad solution to have implemented provided the wiring is done properly and the capacitor securely fastened in place.

I would expect most cruising boats to have a separate battery for engine starting so there would be no voltage drop to the navigation instruments (unless of course, they were wired to the starting battery).

If that is the case, they should be rewired so they get power from the house battery bank.

[RaymondR]

Originally Posted by rwidman

I would expect most cruising boats to have a separate battery for engine starting so there would be no voltage drop to the navigation instruments (unless of course, they were wired to the starting battery).

If that is the case, they should be rewired so they get power from the house battery bank.

Or he could just put the capacitor back and then he wont have the problem whether or not the batteries are isolated. It,s a fairly benign device provided one does not short it out and will actually help protect everything on the house bus from voltage spokes.

I'm warming to one the more I have to argue for it's retention and will probably go buy one and install it the same way in my boat next week.

[Scattercat]

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.

Absolutely correct in every way. Another retired engineering type...�� ps: make that capacitor as big as you can afford in micro farrads and voltage rated to suit the battery in this case 12v..

[betwys1]

Originally Posted by 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.
Attachment 245396

Biggish electrolytic capacitors such as the one you use come in two flavors, AC and DC. The DC types will clearly show which is the positive terminal and you need to take care to maintain the polarity, else they explode. This will have been fitted to help with the starting current surge at the compressor. Fit it reasonably close to the compresssor and leave the fuse on the BATTERY side so if the cap or anything else shorts the wiring is protected. Sometimes low value capacitors are also fitted near a switch to cut the rf splash when a compressor starts.

Brian

[EngineerRetired]

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.

Depends on wiring... A starting cap is not unusual with high locked rotor amps and DC motors.

They are often used to absorb / block the voltage spike (counter emf) when power is disconnected and protects the switching relays or electronics. Yes, closer to the motor is better. Definitely between the motor and the switching system.

I would look closely at the old cap and see if there was a diode across leads.

[t588143]

This capacitor is a peak load shaving capacitor. Its purpose is to reduce the load during starting of the compressor. It should be wired as drawn.