Mosfet Ac Relays

Discussion in 'General Electronics Chat' started by stroks, Aug 29, 2014.

  1. stroks

    Thread Starter New Member

    Aug 29, 2014
    Hi everybody,

    My project includes the designing a multi-channel current stimulator that will be used to deliver bipolar square pulses of 60V and 10 mA. In order to isolate this stimulator from the other devices in my project, I'm looking for ways to gate high-voltage low-amperage currents. It seems that MOSFET-based switches are quite nicely capable of doing this. Unfortunately all MOSFET-based relays have a switching on and off times on the order of 1 ms. This is not fast enough for my project. I noticed that MOSFETs themselves have on- and off-switching times of several micro- or even nanoseconds. So it seems that the slow operation of the MOSFET relays caused by the optical isolation mechanism which all relays of this type have. So I wonder, why there are no MOSFET relays without optical coupling, and will a pair of MOSFETs connected drain-to-drain do the job?
  2. crutschow


    Mar 14, 2008
    A relay typically has isolation between the input control signal and output being controlled so that's why SSRs use an optical isolator. If you don't need such isolation then you can just use a MOSFET controlled by a high-speed driver to switch the signal. Do the loads being switched need to be grounded or can they be floated above ground? That will determine whether you need a high-side switch or not.
  3. ian field

    Distinguished Member

    Oct 27, 2012
    There are versions of typical optocouplers that have a bidirectional pair of photo-FETs, the one's used in modems are typically rated about 230V.

    This might reveal some type numbers to use for further searches:
  4. stroks

    Thread Starter New Member

    Aug 29, 2014
    crutschow, thank you for your reply. I reconsidered the architecture of the functional nodes of my circuits, and now the requirement on the switch within one chip will be to carry without distrortion 60V in one direction and 10 mkV in the other direction.. I explored the topic more, and it seems that there are many technological considerations needed to successfully carry such different signals without distortion. But I found the analog switch MAX14802 that has no optical coupling, operates swftly and is capable of high-voltage loads. I think I'll just use this one instead of thinking thoroughly of the simplier hand-made circuit.

    ian feld, unfortunately they're all too slow - ~200 mks and more. Now I know that the one I've found were not an exception in terms of speed, thanks for the information.