# Controlling Larger Voltage Signals

Joined Feb 16, 2010
112
I need to be able to control a signal that ranges from 0-~6V with a 5V micro output. It used to be controlled with a PNP but this is no longer feasible since the signal now will potentially exceed Vbe even with 5V micro output.

Relays aren't really an option since they are far too slow and expensive. I'm looking for a surface mount alternative, preferably analog. The switch is only on for 2 ms so relays turn on times of >0.5ms is far too long.

I have access to a 5V rail, a ~8.25V, and a line that is anywhere between 8-24V.

Help?

Thanks,
Bob

#### CraigHB

Joined Aug 12, 2011
127
Sounds like a job for a small signal MOSFET if you're just switching a light load, power MOSFET for a heavy load. They typically have gate to source voltage limits of 20V and drain to source limits of 30V, well above the voltages you're working with. Though, there are other factors to consider so selection depends on whatever requirements you have.

Joined Feb 16, 2010
112
The problem is that the load can be two different configurations. The PNP used to be a good solution because the emitter voltage ranged from zero to just less than 5V + Vbe. Since the clamp had to be increased to 6.2V to compensate for some LP filters added to pass BCI, the old clamp will no longer work. For different applications, the voltage on the collector will vary from 0-6V. This also means I won't be able to guarantee a correct Vgs across the entire range since the gate voltage will be a 0-5V signal.

#### CraigHB

Joined Aug 12, 2011
127
How about an opto-isolator?

Joined Feb 16, 2010
112
I found a photovoltaic FET relay that would probably do the trick but the price at quantity is still ~$1 which is far too high. I'm trying to find something for maybe half that or better. #### CraigHB Joined Aug 12, 2011 127 Try looking at the solid state relays here #### CraigHB Joined Aug 12, 2011 127 The transistor opto-couplers here are a lot cheaper if you don't need the higher loading. Thread Starter #### Robert.Adams Joined Feb 16, 2010 112 I was using Digikey but Mouser doesn't have much better of a selection. There are some good options for DC controlled AC loads but I'm looking for DC controlled DC loads at a cheap cost in a surface mount package with quick transition times. Thread Starter #### Robert.Adams Joined Feb 16, 2010 112 Ah, I see. Those optocouplers might work, looks like I have some datasheet perusing to do. Thread Starter #### Robert.Adams Joined Feb 16, 2010 112 Well, thanks. I've got the cost down to ~$0.30 but that is still nothing compared to the \$0.02+fractions of a penny per 2 channel's allowed with the PNPs.

It looks like I'd probably be better off redesigning the circuit instead of adapting the pass transistors to work with the modifications.

#### CraigHB

Joined Aug 12, 2011
127
Yea, sometimes it can be expensive to do things a certain way. I've run into that plenty of times myself. Hopefully you can come up with something inexpensive without too much trouble.

#### vrainom

Joined Sep 8, 2011
126
use an npn transistor to drive the pnp transistor

#### colinb

Joined Jun 15, 2011
351
use an npn transistor to drive the pnp transistor
That's kind of what I was thinking, but I'm confused about why the OP is making it sound more complex. Perhaps I don't understand the problem completely.

Robert.Adams: could you post a schematic or block diagram showing how the load is connected? Is the control circuit acting as a power source switch (i.e., high-side switch)? If so, then a npn -> pnp (or substitute one or both with a MOSFET) seems like a simple option.