AC WITH DC BACKUP

[bobhenry]

Previously posted in my build thread......

Bob's caboose build
by bobhenry » Wed Feb 27, 2013 8:01 am

Need a little electrical advice........

I have installed four 9 volt LED desk lamps as long necked wall sconces over the galley counter. The flexible neck will allow me to direct the soft white light at will where needed. I would like to power them with both the AC when available and the DC when off grid. They are equiped with 120v AC to 9 volt DC plug in wall wart transformers. I have a 9 volt DC tap on the solar setup I can utilize to get the needed 9 volt DC without the transformers. I would like to tap into the lamp cord between the lamp and the transformer and supply the 9 volt directly from solar to the lamp. My concern is back feed thru the transformer backwards. Conversely what happens when the ac converted 9 volt DC is fed into the solar panel controller ? Short of adding a double pole double throw switch to select the now converted ac or the solar dc power what are the problems of simply tapping in as described above and allowing the backfeed. Is there a better way to allow either power source without having to add yet more switching . Perhaps a diode or other type of check valve to isolate the currents automatically ?

[48Rob]

Bob,

On my trailer, I wanted to run the television on shore power when available, and battery power when not.
The television is a 12 volt model.

The solution that the electrical experts here helped me come up with was a relay.

It switches back and forth between battery and shore power as needed, by itself.



It cost me about $5 I think.
I've been very, very happy with it as it does just what I wanted without me ever worrying about it.

Rob

[Shadow Catcher]

Bob, what you need is a DC buck converter to get the 12, 14 what ever volts (it varies depending on what converter, batteries, solar are doing). I found a nice one to power our Android tablet that was very stable in output, as long as it lasted, Chinese junk, and I don't mean the ship. There are lots of others out there.
It had a digital volt/ammeter that was quite accurate. For the price about $9 you could buy a couple of them.

[bobhenry]

Bob,

On my trailer, I wanted to run the television on shore power when available, and battery power when not.
The television is a 12 volt model.

The solution that the electrical experts here helped me come up with was a relay.

It switches back and forth between battery and shore power as needed, by itself.



It cost me about $5 I think.


I've been very, very happy with it as it does just what I wanted without me ever worrying about it.

Rob

duh ! The Bosch relay ! Hell if I look real hard I can find the one I bought and never used. It's somewhere in the house ! On further thought those are 12 volt relays I wonder if 9 volts will trip them.

[Dale M.]

I would just run them on 9 volt DC tap and forget about any conversions, relays or transformers.... Let solar array and battery do the powering of them.... Current (amperage) load will be very small, and the will probably run for weeks without battery charge.... Beside any time you have shore power, battery will be charged..... You are making this more complicated than necessary....

Dale

[bobhenry]

I would just run them on 9 volt DC tap and forget about any conversions, relays or transformers.... Let solar array and battery do the powering of them.... Current (amperage) load will be very small, and the will probably run for weeks without battery charge.... Beside any time you have shore power, battery will be charged..... You are making this more complicated than necessary....

Dale

Well yes and no !

I am still in the building stage so the panels are not yet operational . I am relying on the 120 v ac for the converted 9 volt dc power to these lights for illumination during the dark winter build days. I tried powering one at 12 volts. It is still working but kinda like that flashlight you find with the very tired batteries. I suppose a small inexpensive 9 volt power supply might be the short term answer. I really wish they had been 12 volt but NO I couldn't get that lucky. Most every RV has all 12 volt dc lighting only and they simply convert the ac to dc. I have two of those converters so perhaps finding a way to drop the 12 volt output to 9 volts might be my best bet. So I guess a resistor of some size is in order to do that ?

[Shadow Catcher]

You need to reduce the voltage, hence the buck converter. The diagram will get you 12V which you have already found does not work. I did by the way re check the converter I have and I would assume I had a poor electrical connection and it now works fine. I also checked stability setting the voltage at 8 V and hooking up to a some what depleted 12 V battery, and putting it on a charger which starts at 14.5 V the output measured with a VOM showed it made no difference.

[bdosborn]

Don't forget the snubber diode or you might fry your TV.

Relay Basics

Quenching Diodes:
Anytime that a relay coil is driven by a circuit that is not specifically designed to drive a relay, you should use a quenching/suppression diode connected in parallel with the relay coil. The diagram below will show the connection of the diode. Initially, you may think the diode serves no purpose because the voltage applied to the relay cannot pass through the diode. This is true when the relay is energized. The diode comes into play when the power source is removed from the relay coil. When power is applied to the relay coil, a magnetic field is created and energy is stored in the coil. When power is removed, the magnetic field collapses causing a reverse voltage to be generated (it's called inductive kickback or back EMF). The back EMF can easily reach 200 volts. The diode will absorb the reverse voltage spike. This voltage, if not absorbed by the diode, will cause premature failure of switch contacts and may cause the failure of power switching transistors. You can use virtually any type of rectifier or switching diode (i.e. 1N4001, 1N4002, 1N400x... or Radio Shack part #s 276-1101, 276-1102, 276-1103, 276-1104).



Bruce

[48Rob]

Don't forget the snubber diode or you might fry your TV.

Bruce,

I'm guessing you were directing this to me?

If so, thanks!

I've had this setup for 7 years now...
I can understand how it could cause a problem, I wonder though, If I've gotten by without issue because of my inline switch.
I have a shut off switch between the television and power source (relay) that I use to prevent battery reserve loss from the television being in "ready" mode.
?

Rob

[bdosborn]

Rob,

Not to you specifically but I'm glad somebody read it. The switch would isolate any back EMF surges (when its open). The trailer is the first thing I've ever added a relay to and I was surprised that a diode should be added. I thought I'd throw it out there.
Bruce