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Topic: SSR vs High Voltage Transistor. (Read 841 times) previous topic - next topic

cjdelphi

Maybe i'm just dense... but i'm sure i've seen voltage ratings on certain transistors over 250volts, if so, could you not use (not going to TRY!, just in theory here..) 240vdc (rectified) and feed 0v/240v through the collector/emittor same as a solid state relay?  or what's the differences exactly? I have no plans to try this so don't post a million warnings...

Quote



IRF710 10A 400V N Channel Power MOSFET Transistor TO-220
Please note that we are selling factory direct products. All our products are 100% brand new in manufacturer's packaging.

Specification:
Product Name   MOSFET
Model No.   IRF710
Channel Type   N Channel
Drain Source Voltage   400V
Drain Current   10A
Mounting Hole Diameter   3.5mm/0.14''
Pin Size   14 x 2.5mm/0.6'' x 0.1'' ( L* Pitch)
Total Size   29 x 10 x 4mm/1.1'' x 0.4'' x 0.2'' (L*W*H)
Weight   2g
Package   1 x N Channel MOSFET

Descritpion:

    Dynamic dV/dt Rating.
    Repetitive Avalanche Rated.
    Fast Switching.
    Ease of Paralleling.
    Simple Drive Requirements.
    TO-220 Package.



i'm happy to power 12v's but, if this is capable of switching 400V, then could one not use this instead of a Solid State Relay? (not going to try.. trust me...)

winner10920

Yes you could use this similiar to useing a mosfet with lower voltages, he only difference would be you need to be more careful about spacing
In a current project of mine im actually using 5x nchannel mosfets rated to 600v and I have plans to actually switch up to 500v with that,
You could probably make your own ssr with the right mosfets, that's probably what's inside most ssr's anyway

majenko

The main thing to watch with switching high voltages with transistors is that the high voltage portion of your circuit and the low voltage portion are physically coupled.  If the transistor should develop a fault there is a good chance the high voltage would be pumped right through the rest of your circuit, and *BANG* goes everything.

An SSR is isolated by incorporating an opto-isolator, so if there is a fault there is no electrical path between the two parts of the circuit, so there will be no bang.

There are IGBTs (Isolated Gate Bipolar Transistors) which are basically a MOSFET driving a BJT, which provides a better level of isolation than a plain BJT, and are used when very high voltages are being switch (thousands of volts), but still aren't completely isolated.

I'd stick to SSRs personally for high voltage work.  Less risky.

cjdelphi

For what i need, a plain old relay would do the job fine, i was purely interested in the differences between a mosfet and an SSR, I think i just got the answer, not much except there's more protection via the optocoupler :) - cheers, I prefer the theory over practical any day using anything more than 15v :) cheers.

Grumpy_Mike

When you rectify 240V AC you get about 340V DC.
While a transistor can stand a high forward voltage it is rubbish on reverse voltage. A lot of mains stuff will not work on DC.
Also DC at this voltage is more dangerous than AC because when you get a schock it paralyses the muscles and you "stick" and can't remove yourself from the circuit.

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