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reginalStetson
05-18-2013, 02:34 PM
I'm looking for a transistor that is capable of acting as a switch to ground--and I need it to be fast.

My current design calls for an NPN transistor to act as an ON/OFF switch, controlled by a digital pin on the teensy using digitalWrite(HIGH/LOW).

I need this transistor to be as fast as possible--1 millisecond is the slowest it can be.

I will be using 21 of these transistors in my project, so an IC with individual Base controls would be great.

Any recommendations?

I'm looking at the TIP120.

PaulStoffregen
05-19-2013, 06:43 AM
How much collector current will conduct when it's?

How much voltage is present when it's off?

reginalStetson
05-19-2013, 01:22 PM
How much collector current will conduct when it's [on]?

How much voltage is present when it's off?

+3.3V. Nothing outside of what the Teensy provides, i.e., no external power. It's switching a digitalWrite(HIGH) load to ground.

I tested the TIP120 and the fastest I could get it to switch was around 20Ás.

jwmelvin
05-19-2013, 11:39 PM
Isn't the TIP120 a Darlington pair? Seems like you'd want a MOSFET for high-speed use. But you said you need 1ms and that the TIP120 is 20 us, so aren't you fine?

Jp3141
05-20-2013, 12:50 AM
Isn't the TIP120 a Darlington pair? Seems like you'd want a MOSFET for high-speed use. But you said you need 1ms and that the TIP120 is 20 us, so aren't you fine?

Although it may be difficult to get a MOSFET to turn on with 3.3 V, although 'logic level' FETs may work for you -- e.g. NTF3055L108 from On Semi. With a MOSFET you normally won't need a resistor between it and the Teensy.

What is your load ? If it is a relay, or even a resistive load but with a long wire, the inductance when turning off will generate a large spike that could damage the FET (or NPN). For a relay, you might be able to use a diode 'backwards' across the load. For a wire a zener (greater than the supply voltage to the load) to GND will work.

PaulStoffregen
05-20-2013, 07:19 AM
Darlingtons are usually slow. Mosfets can be fast, but only if you drive with enough current to charge the gate fast.

You didn't answer how much current you need, which is a big factor in choosing the right transistor. If you need only 100 mA or less, a 2n3904 might be a good choice. If you need more current, there are others, though the bandwidth decreases as the current handing goes up. Without knowing you needed load current, I ca't help much more.

reginalStetson
05-20-2013, 07:05 PM
You didn't answer how much current you need, which is a big factor in choosing the right transistor. [...] Without knowing you needed load current, I ca't help much more.

I'm not quite sure yet, all I know now is that they'll need to be fast. Here's the abstract for the design I'm working on: http://forum.pjrc.com/threads/23702-Delayed-switch-circuit

Donziboy2
05-21-2013, 12:32 PM
This might give you a good idea of timings.IRF9520 (https://plus.google.com/106613662443243081361/posts/CkfAffQRKQK) I did these tests not to long ago on a PNP MOSFET, if you look at the tests without a protection resistor there is a significant bounce (and a good current draw) when initially driving MOSFET's. So a protection resistor of some kind would be a good idea, also not all MOSFET's will actually drive anything at there VGS(on) rating, many manufacturers cheat and state VGS(on) at 250uA. You have to look at the graphs provided to find out how much Gate voltage is needed to drive the MOSFET on at a specific current rating. Most MOSFET's are pretty fast at switching, usually in the nS/uS range.

And here is some info on a few other MOSFET's I looked at. Mosfet data compared (https://plus.google.com/106613662443243081361/posts/PkNUBfUwgnj)

You can do a search on Digikey to find exactly what you are looking for in a MOSFET. They let you break it down pretty well.

cemsolak
09-03-2013, 03:20 PM
@ ReginalStetson, Why do you need a mosfet if you are not utilizing it for any high power function? Since you mentioned a high-level of 3.3V, my initial guess is that you're probably using a processor and you need it for a simple digital circuit. The function that you require is generally termed as a switching function and it is the most common function of a transistor. My advice it, use a simple NPN transistor, maybe a 2N222 and you'll be good to go. Hope this helps.