PV1500 Fish hooks (by Gerard Larooy)

Component: PV1500

Category: Isolators

This unique opto coupler originally came from I.R. It produces a D.C. voltage on its output when current is put through the LED on the input. This, on the surface, looks like a great way of driving MOSFET's or rolling your own solid state relays. It's all true but there are a few fish hooks.

-The LED operates in the infrared range so its voltage is lower than a red LED. (Not a show stopper).

-Output current of PV1500 and all derivatives is in the uA range. Some later versions had half the output current of the original. If the pulldown resistor isn't changed accordingly the gate of the MOSFET will starve, making the FET run much hotter than it should.

-Some devices have an internal pulldown and some don't. Operation without internal or external pulldown will cause overheat and FET destruction if the FET is sized for switching service.

-Some devices have internal "fast" turn off, some don't. Turn on is always slow and since the device's introduction, the on resistance of FETs has decreased with a corrresponding effective input capacitance increase. It could be shown that under some conditions the switching losses could exceed the conduction losses at frequencies as low as 50Hz. Be careful when "updating" FETs. The "better" FET could run hotter when using a photovoltaic stack.

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Logic Level FETs
By Myles Douglas-Withers on 06 Jan 2015

The FET issues discussed above are exacerbated when using logic level FETs because the input capacitance for similarly rated devices may be substantially higher