0
\$\begingroup\$

I am designing a switch for AC circuit. E.g. when I apply a small DC voltage, AC current is allowed to conduct, otherwise there is huge resitance stop the current from flowing.

I found 2 possible solutions on the web. TRIAC or 2 opposite MOSFET in series. But the problem is: the AC voltage is floating, it is not grounded. Its a AC wave produced from H bridge from another DC source. (which use the same ground as the small DC voltage to control the AC current). E.g. I cannot define the gate voltage correctly.

Does anyone know how to achieve the control? Thanks

enter image description here

enter image description here

\$\endgroup\$
10
  • \$\begingroup\$ Your 2nd circuit shows an opto-photo-voltaic device and this is usually resiliant in your sort of application. \$\endgroup\$
    – Andy aka
    Commented Jan 22, 2016 at 0:45
  • \$\begingroup\$ If you don't need a high-frequency switch, a conventional mechanical relay will work. \$\endgroup\$ Commented Jan 22, 2016 at 0:47
  • \$\begingroup\$ A Solid State Relay(SSR) will also work too if you want the opto-isolation. \$\endgroup\$
    – Axis
    Commented Jan 22, 2016 at 3:19
  • \$\begingroup\$ @andyaka Does that means I need to place a LED (to my DC voltage) and sensor(grounded to AC) in my circuit? \$\endgroup\$
    – yuhao
    Commented Jan 22, 2016 at 6:19
  • 1
    \$\begingroup\$ @yuhao - I notice that you say the AC is 200 kHz. This makes a massive difference to what technology can be used. Please edit your question to make it clear that your AC is 200 kHz. \$\endgroup\$
    – Andy aka
    Commented Jan 22, 2016 at 9:11

2 Answers 2

2
\$\begingroup\$

When I need to switch an AC load the first thing I do is look for an appropriate SSR. They are easy to use and pretty robust. They switch from a low voltage DC source and generally only take a few milliamps to turn on. Another bonus is that they come in a tons of packages ranging from tens of milliamps to hundreds of amps. Digikey has a wide selection of SSR to choose from. If you are looking for high current ones I suggest filtering the product search for chassis mount packages.

enter image description here

\$\endgroup\$
0
0
\$\begingroup\$

Why do you need another circuit to control your H-Bridge? Simply control each of the 4 transistors individually, and when you want to break the current flow, switch one of the conducting pair off.

Now if you don't have access to the H-Bridge because you're using someone else's magic, then I would suggest your second circuit with one addition. The circuit you have show has a great turn on time, but a horrible turn off time. Add an opto-coupled transistors like the diagram below to speed up turn off time.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
2
  • \$\begingroup\$ Hi, I do have control over H bridge. But The switch here has something to do with load modulation rather then turn the whole H bridge output down.May I ask what is necessary about Q1 Q2 on your diagram compare to the simple one I used as my 2nd diagram in the question? \$\endgroup\$
    – yuhao
    Commented Jan 22, 2016 at 6:31
  • \$\begingroup\$ Q1 and Q2 allow you to use 1 GPIO output to control the mosfets. The optocoupled transistor forced the charge stored on the mosfets back to ground (that center node) turning them off faster than if you just "stopped making current" through the photovoltaic. You could use 2 GPIO, and two less transistors, but then you'd need code to guarantee turn off speed. \$\endgroup\$
    – Dave
    Commented Jan 22, 2016 at 10:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.