Is there something like a transistor that works with current in both directions? - Page 1
Forum section: electronics / beginners

author: dsegel
author_rank: Contributor
timestamp: August 18, 2017, 06:14:02 pm
content: I want to switch current flow on and off on a path that changes from -10V to +10V relative to ground.Should I use something like a solid state relay instead?

author: Zero999
author_rank: Super Contributor
timestamp: August 18, 2017, 06:26:02 pm
content: Yes, a relay would do.How much current is flowing? If it's very low, you need a relay with gold plated contacts.Do you have both positive and negative DC power supplies?If it's very low current and you have a bipolar power supply, then an analogue switch would do. It's just an IC with small MOSFETs connected in reverse parallel, to switch the current and some level shifting to control the gates.

author: dsegel
author_rank: Contributor
timestamp: August 18, 2017, 06:36:36 pm
content: It's actually going to be the output from a dual H Bridge, so the voltage will technically be relative to the two outputs and not ground. Like the example configurations they show for controlling a motor in two directions. I want to be able to turn it on and off for individual devices (1 at a time, 6 devices total). The output will be tied to all 6 on one side, and I was hoping to switch them on and off individually on the other side.Current is about 1-2A total, but I could limit to all the way down to 300mA if necessary.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 18, 2017, 06:53:44 pm
content: There are ways to switch current in both directions, however it is kind of complicated and expensive if higher power is needed.So for the application describes I would more go for something like a separate H bridge for every motor or maybe one side in common for all motors and the other side (half bridge) separate for each motor.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 18, 2017, 08:18:08 pm
content: If that voltage is continuously alternating, you could use a Triac.Although the Triac losses would be excessive for such a low voltage.

author: Inverted18650
author_rank: Frequent Contributor
timestamp: August 18, 2017, 09:46:08 pm
content: Following too: I have an application where the max is about 500mA one way, 2A's the other.

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2017, 02:45:09 am
content: The easiest way is back to back n-channel power MOSFETs which may be driven by an optically isolated photovoltaic stack and that is how most solid state relays work now. Two MOSFETs in series are needed to isolate their body diodes in AC applications.CMOS analog multiplexers use the same idea but with parallel p-channel and n-channel MOSFETs in parallel with level shifters to provide gate control. When isolation is not required, then the same thing can be done with back to back n-channel power MOSFETs and level shifters to drive the gates.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2017, 06:20:39 am
content: Why not control the H-bridge itself? (Shades of x-y problem here...)Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2017, 09:26:28 am
content: Quote from: David Hess on August 19, 2017, 02:45:09 amIt could also be an oscillator driving a small transformer or tiny AC coupling capacitors, followed by a rectifier.Quote from: T3sl4co1l on August 19, 2017, 06:20:39 amThere are six devices and two h-bridges.Quote from: dsegel on August 18, 2017, 06:36:36 pmUse a separate h-bridge for each device? If that's too complicated, then you need a good old fashioned relay. The alternatives are just as complex as having multiple h-bridges.