MOSFET as switch
MOSFET as switch
(OP)
I have some 7-segment LED displays which require ~9 volts to run. I want to control them using a TTL digital logic circuit which works perfectly on it's own when 5-volt displays are used. I need a way to power the 9 V displays from 5 V logic via a BCD-to-7seg decoder chip. I anticipate having to use a 9 or 12 V main supply, a voltage regulator to go down to 5 V for the logic, and a MOSFET to take the 5V signal and push the larger voltage through the LED. I can't seem to find a good MOSFET to use though, and if anyone has a suggestion as to what particular transistor to use or a better idea, I would appreciate it.





RE: MOSFET as switch
Personally i'd use standard transistors (bipolar) 2N3904 NPN
or 2N3906 PNPs.
But if you really want a FET look at the VQ1004J or the very popular VN2222L.
RE: MOSFET as switch
Why not simply use an open-collector buffer to drive the segments?
TTFN
RE: MOSFET as switch
thanks all
RE: MOSFET as switch
RE: MOSFET as switch
You should be using a pnp, unless you want to run two supplies. You could run a single supply with npn, but you'd need either a bootstrap capacitor or you'd need a zener clamp on the base drive
TTFN
RE: MOSFET as switch
The OC buffer is sounding like a better idea now that I've tried breadboarding it with no success. The breadboard setup is pretty clunky though since I don't have a single board large enough to fit the display. I'll look into it and see what I can come up with.
RE: MOSFET as switch
The VN10KM is a useful small MOSFET in a TO92 package. Costs a lot more than a small signal BJT, and doesn't offer much benefit other than slightly easier drive requirements. Have you considered the ULN200x series of driver ICs? Ideal for interfacing logic to heavier current. From memory I think the full PN of the one I'm thinking of is ULN2004, but I'm at home and my catalogues are at work. I'm pretty sure there are a few other ICs in the series.
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RE: MOSFET as switch
http://roc
RE: MOSFET as switch
If you use p-channel or pnp, then you can use active low to drive the transistor without needing a supply higher than the Ve or Vss of the transistor.
TTFN
RE: MOSFET as switch
Something else to check is the source/sink capability of the logic family used. TTL can sink more than it can source: about 16mA vs 2mA for 74LS, for example. If you are driving BJTs into saturation this could be significant.
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
RE: MOSFET as switch
If the displays are common Cathode you are correct he would need to follow your suggestion.
RE: MOSFET as switch
RE: MOSFET as switch
Dirty minds think alike... oops, that's not right... great minds think alike!
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
RE: MOSFET as switch
Looking at the datasheet for the ULN2004, it appears to hold alot of promise. It should be extremely convienient considering there are seven pairs on the chip and seven segments in a display.
Using a 12 V supply instead of a 9 V supply isn't a hardship for me, since it will be regulated to 5V for the TTL logic anyhow.
RE: MOSFET as switch
If you are running the display from a PIC you *could* run it directly from the PICs pins. (same total limit concerns tho)
RE: MOSFET as switch
RE: MOSFET as switch