# question

## What are “High” and “Low” terminals exactly? Pos. & Neg?

Can someone explain how the H/L terminals work, I’m having trouble with the translated definitions because it’s only 1 wire. What is high and what is low, I don’t get it. For example: How can I use them to protect lithium batteries from over discharge or over charging without the need for a BMS unit?

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In the Victron world these are used to switch the device on and off. Applying a positive voltage to h, for instance, turns the unit on. Removing the voltage turns it off. Can also be bridged for on.

Be st to check the documentation for the unit, but if you say which it is, we can explain further

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I must assume that the units own internal opposite pole is being used to close the circuit if only one wire is connected to the H or L terminal. So I assume the negative pole is obtained from within the device, ie an MPPT. Thanks for your help with this.
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This are TTL logic levels for digital inputs or outputs. A L- signal is ground up to 1.5V maximum, anything above is a H (high), as there are logic inputs/outputs on the other side, that operate at 3.3V, 5V or 12V.

To protect the I/O from overvoltage, there are usually Open Collector or Open Drain transistors at the outputs with pull-up resistors to the according voltage. The transistors, when conductive - pull down the voltage to ground, when open you see the Vcc voltage from the pull up resistor. You usually cannot drive relays with this signals directly, you need a transistor to amplify the signals, otherwise the current will destroy the interface transistor. A good design would use a optocoupler to isolate the interface between the devices and also drive the relay stage.

Google Digital outputs, Open Collector, Open Drain, TTL Logic, CMOS Logic, GPIO ports for further reference.

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Wow thanks for the information on Transistor to Transistor Logic (TTL), I’ve just read up on it more in Wikipedia, very good to know about, thanks again.

https://en.wikipedia.org/wiki/Transistor–transistor_logic

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