Need some help Switching DC power sources

Thread Starter

BRP5544

Joined Aug 10, 2017
2
I need help switching between DC power sources.
I want to setup a light inside the house run on a solar panel with an automatic switch to mains power when the solar is insufficient (cloudy day / night).
The light is a 36VDC LED light, with an operational range of 36V to approximately 20V.
I want to have two power sources to the light (24 to 36VDC from the solar panel and 36VDC through an AC-DC inverter from the mains power). The solar is to be the primary power source with the mains being the secondary. I want the switching voltage from the solar to be below 24VDC.

I was thinking of using an electromagnetic relay with the NO contacts to solar, the NC contact to mains and the solar on the coil ie. the solar power supplies power while above 24VDC, once below, the relay drops out providing mains power to the light.

Sounds good in theory but Im not sure if it will work;
- Will there be issues with arcing (reducing the life of the relay) if the hold in voltage drops slowly?
- Im not sure if a relay will give a consistent drop out voltage (24VDC) with variations in temperature?
- I would like the drop out voltage to be adjustable in case 24VDC is insufficient for the light (brightness).

Are there any suggestions out there on how to do this?
 

AnalogKid

Joined Aug 1, 2013
8,534
Relying on the dropout voltage characteristics of a relay coil for DC level sensing is not, well, reliable. And yes, the contacts will chatter with a slowly decreasing coil voltage. Better to drive the relay with the output of a comparator circuit.

Since everything is DC and common-grounded, you can use power transistors, rather than a relay, for longer life.

What is the peak current being switched?

If you reduce the mains DC output to 25 V, then the entire switching and control system reduces to just two diodes.

ak
 

Thread Starter

BRP5544

Joined Aug 10, 2017
2
Thanks for the response AK,

Im mechanical so struggling a little with the electronic concepts.

The LED light has an optimal voltage of 36V and gets less bright as the voltage drops, so I need to keep the system voltage at 36V, with a tolerance on the drop out voltage of around 12V (maybe less).
Im thinking the comparator circuit might do the the trick, but can it work with a tolerance not just higher or lower, as in lower by 12V?

Is this an item / circuit I could purchase or something I would have to make?
 

AnalogKid

Joined Aug 1, 2013
8,534
When the load on the solar cell is switched tot he mains, the cell voltage will increase. This could cause the system to flutter back and forth rapidly as the cell voltage goes above and below the trip point. To solve this, the comparator circuit has hysteresis. Essentially, two separate trip points. Example - when the cell voltage drops below 25 V, the circuit switches to the mains. But now, the cell voltage has to be above 30 V before the circuit will switch back to it.

All of this happens with one comparator IC, often 1/2 of an LM393 because the part is universally available, cheap, and reliable. It can run directly off the 36 V mains. For transistor switching, the second comparator is used as the other transistor driver. 7 resistors and 2 power MOSFETs, or 9 resistors and 2 PNP transistors. Since the 36 V mains supply has a regulated output and the switch-over point is not super critical, you don't need a separate voltage reference for the comparator to compare against; 2 resistors is fine.

All of that probably raises a few (!) questions. Fire away.

Have you ever assembled a small circuit?
Have you ever worked with a solderless proto-board?
Where are you located?

ak
 
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