### Author Topic: Power Multiplexer Circuit  (Read 264 times)

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#### eerocketman

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##### Power Multiplexer Circuit
« on: April 22, 2018, 05:57:24 am »
Hi, I am working an amateur college rocket. Currently, while our rocket is on the ground we are loading and preparing the rocket I would like to power my system via an off board battery however when the rocket is in flight I'd like to use the on board battery. I was wondering if a simple relay circuit would work as attached. Am I over complicating this (will two diodes in series suffice?) or under complicating it? The off board power source will be a 24V go cart battery while the on board power source is 2 1000mAh 4S batteries in series. The system current draw is around 2 amps max.

#### ogden

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##### Re: Power Multiplexer Circuit
« Reply #1 on: April 22, 2018, 06:43:57 am »
The off board power source will be a 24V go cart battery while the on board power source is 2 1000mAh 4S batteries in series.

So on-board around 32volts? Next time just tell voltage of both batteries so we don't have to guess chemistry and nominal charge %.

Answer is: you can live with just two diodes, no relay needed.

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#### Ian.M

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##### Re: Power Multiplexer Circuit
« Reply #2 on: April 22, 2018, 07:11:50 am »
Nope. You cant use *JUST* diodes because the on-board battery would supply the power as the off-board battery is at a lower voltage.

Also its desirable to minimise the weight of the circuit permanently attached to the rocket.  If your system can tolerate 34V on its incoming DC bus, the optimum solution would be to use a 3A, 50V rated Schottky diode in series with the supply from the on-board battery + use a boost converter with the go-cart battery to get a regulated 34V, which would be applied after the Schottky diode, and would keep it reverse biased so there is no load on the on-board battery while powering your system.  The boost converter's rectifier diode will prevent significant reverse conduction through it so you probably don't need an extra diode in series with its output.  Without a relay, you wont get a momentary loss of power as it switches over from external to internal power, and the only extra weight on the rocket is the Schottky diode in series with the on-board battery + the external power connector (+ associated wiring), as the boost converter stays on the ground with the 24V battery.

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#### ogden

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##### Re: Power Multiplexer Circuit
« Reply #3 on: April 22, 2018, 08:48:22 am »
Nope. You cant use *JUST* diodes because the on-board battery would supply the power as the off-board battery is at a lower voltage.

Yes, you are right. I forgot to mention that external supply shall be same (or slightly higher) voltage. Kind of obvious.
« Last Edit: April 22, 2018, 07:03:58 pm by ogden »

#### NiHaoMike

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##### Re: Power Multiplexer Circuit
« Reply #4 on: April 22, 2018, 09:11:04 am »
I have to admit I have never had the chance to work on a go cart, but 24V seems low. I'm under the impression they are generally 48V since that makes it easier to get a lot of power and still be within low voltage safety limits.
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#### mikerj

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##### Re: Power Multiplexer Circuit
« Reply #5 on: April 22, 2018, 08:12:10 pm »
Is an electromagnetic relay a good idea in something that would experience high levels of acceleration?

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