# Can I 'top up' a dc circuit with another powersource?

#### Mark F

Joined Apr 1, 2010
2
Hi guys. I stumbled on this forum doing some research for a tinkerer's project my brain is immersed in at the moment. Looks like a friendly place to hang out! I'll have more questions to come, but before I reinvent the wheel I thought I should ask if there is an easy way to do the following:

- 'Top up' a circuit (48v DC, 800w load) with another DC power source. Load would be powered from batteries, but if needed (and only if needed) would seamlessly take power from another DC power source. Any point in the right direction would be appreciated. Thanks!

Mark.

#### Mark F

Joined Apr 1, 2010
2
Use a dual Schottky diode - like this:http://www.onsemi.com/pub_link/Collateral/MBR30H100CT-D.PDF for example, to mix two DC sources. Will automatically 'seamlessly' mix the two voltages.

Thanks for the reply! I'm a bit rusty on my electrical theory, so I need some clarity on a dual Schottky diode's operation. One of the design considerations is to use the available power from a solar panel or battery first, before 'topping up' from another source (grid, generator, batteries, etc) - using all the free/stored power before resorting to the second source (if even needed). For example (using a PV array) the primary power source under optimum/sunny conditions will provide enough energy to power the 800w load by itself, but when cloudy/dark (some/all) of the energy will need to be pulled from the second source. Will the circuit based on a dual Schottky diode just pull what it needs from either source, or can I dictate or manipulate which source has 'priority'.

Mark.

#### Wendy

Joined Mar 24, 2008
21,914
Schottky's are like any other diode, except they are very fast and drop a lot less voltage. Normal diodes would also work (but you would loose 0.6 to 1.0V across them as opposed to the Schottky's 0.2 to .5V). Basically it is a switch in this configuration, the highest voltage flows through. It is used for a lot of battery backups.

#### Wendy

Joined Mar 24, 2008
21,914
Schottky's are like any other diode, except they are very fast and drop a lot less voltage. Normal diodes would also work (but you would loose 0.6 to 1.0V across them as opposed to the Schottky's 0.2 to .5V). Basically it is a switch in this configuration, the highest voltage flows through. It is used for a lot of battery backups.