Actually, I'll suggest an MPSA64. ON Semiconductor has discontinued the MPSA62, leaving the MPSA63 and MPSA64; the MPSA64 is a preferred device.

Mouser.com stocks Fairchild MPSA64's for $0.10/ea when you buy less than 10.

Here's a search on Mouser for low to medium voltage Darlingtons in a TO-92 case:
http://www.mouser.com/Search/Refine...0+1323038&Ns=P_qField|0||P_PRICE1|0||P_SField

 

Actually, I'll suggest an MPSA64. ON Semiconductor has discontinued the MPSA62, leaving the MPSA63 and MPSA64; the MPSA64 is a preferred device.

Mouser.com stocks Fairchild MPSA64's for $0.10/ea when you buy less than 10.

Here's a search on Mouser for low to medium voltage Darlingtons in a TO-92 case:
http://www.mouser.com/Search/Refine...0+1323038&Ns=P_qField|0||P_PRICE1|0||P_SField

The saturation voltage is about 0.9V, which may not be a problem. This is also a high side switch, which means he will need a level translator, although at considerably lower current than the one I posted.
You might want to post a schematic for luv2code.

 

Good points, Ron H.

The attached should work.

Note that V1 represents the output from the microcontroller, being turned on and off at a 1Hz rate (which you obviously wouldn't do, but this is just a simulation).

 

Good points, Ron H.

The attached should work.

Note that V1 represents the output from the microcontroller, being turned on and off at a 1Hz rate (which you obviously wouldn't do, but this is just a simulation).

That looks good. I wonder if his camera can tolerate the 0.9V saturation voltage. It may eat into his apparent battery time between charges.

 

You need a level-shifter transistor to drive the PNP darlington transistor or it will be turned on all the time.

 

You need a level-shifter transistor to drive the PNP darlington transistor or it will be turned on all the time.

Pay attention!

 

Pay attention!

This site keeps taking me to an old page instead of to the latest page.

 

Thanks for the diagram. I believe I understand it fine.

I don't understand the need for the level shifter.

I need a level shifter because the darlington array expects -12v between the base and emitter? And it's not possible to do this just with the output pin of the MC. So Q2 is an NPN transistor who's sole purpose is to shift my 5v high signal to a -12v "on" signal. Is that right?

It doesn't make sense because I thought the point of the darlington was that it has a built-in shifter. Would I still need the level shifter if I found an NPN darlington transistor? I might have ordered this one, which has an emitter-base voltage of 5v. It looks like I could use that to switch a 12v supply with a 5v signal.

 

I don't understand the need for the level shifter.

I need a level shifter because the Darlington expects -12v between the base and emitter? And it's not possible to do this just with the output pin of the MC. So Q2 is an NPN transistor who's sole purpose is to shift my 5v high signal to a -12v "on" signal. Is that right?

Not quite.
If the PNP's Vbe is near zero (equal to 12v measured from ground when Vee=12v), the transistor will be turned off. As Vbe increases to say, -2v (10v from ground), the transistor is ON and nearing saturation.

Since your uC is not capable of putting out a signal over 5v by itself, Q2 is used to pull the base of the Darlington away from Vee. The 220K resistor keeps Q1's base at Vee when Q2 is off.

It doesn't make sense because I thought the point of the Darlington was that it has a built-in shifter.

The Darlington is a very high gain pair of transistors in one package; it's more like a switch than a linear transistor. A very small amount of current on the base controls a great deal of current through the collector.

Would I still need the level shifter if I found an NPN darlington transistor?

No. The NPN Darlington would need to go between the camera and ground though.

I might have ordered this one, which has an emitter-base voltage of 5v. It looks like I could use that to switch a 12v supply with a 5v signal.

Yes, you could use that. You'd still need a 10k current limiting resistor from the uC to the base of the Darlington.

But if you used a logic-level MOSFET, you wouldn't lose the 0.9V across the collector-emitter junction when it's saturated.

BJT's (Bipolar Junction Transistors) have a minimum voltage drop across their collector-emitter junction. MOSFETS do not. They have a specification called Rds(on), which is the resistance of the connection from the drain to the source. Many MOSFETS nowadays have Rds(on) specifications that are small fractions of an Ohm. This means they run very cool, and there is little power loss or voltage drop.

A typical N-channel power MOSFET needs 10V on the gate to turn it on, and 0v to turn it off. However, there are logic-level power MOSFETS available that will work with 5v on the gate.

Re-visiting prior mentions, looks like a 2N7000 actually might work for you, if you put a couple of them in parallel. With 5v on the gate, Rds is about 1.5 Ohms. If you're running two in parallel, that works out to 0.75 Ohms. Your camera is rated for 200mA @ 12v, which translates to 60 Ohms. So, your camera would be getting 98.8% of it's rated voltage.

You would still need resistors to limit current from your uC's output pin to the gates of the MOSFETs. I don't know how much current your uC can sink or source per pin.

At Mouser, you can get two for a quarter and receive change back.
Link: http://www.mouser.com/Search/ProductDetail.aspx?qs=FOlmdCx%2bAA2o/KZOTk/3hg==
The one drawback to MOSFETS is their sensitivity to static electricity. One zap and they're dead. But if you observe proper ESD precautions when you're handling them, you should be OK.

 

All of the contributors to this thread have been very helpful. I really appreciate the extra time spent making diagrams and spelling things out. I feel like learned a lot.

It'll be a couple of weeks before I get all the parts in, and I will definitely update this thread with diagrams and pictures.

 

You would still need resistors to limit current from your uC's output pin to the gates of the MOSFETs. I don't know how much current your uC can sink or source per pin.

MOSFETs don't draw gate current, except to charge and discharge the gate capacitance when the input voltage changes. A small-value series resistor (10 ohms to 100 ohms) is generally added to prevent the possibility of oscillation.

 

Luv2code, you should be able to switch the power on the low side of the camera (between the camera and GND), as suggested by jpanhalt. Then you won't have to worry about a level translator.
Let me try to explain the need for a level translator when using a high-side switch, such as a PNP or a P-channel MOSFET, in case you didn't understand SgtWookie's explanation.
A PNP must have zero volts from base to emitter to turn it off. A P-channel MOSFET must have zero volts from gate to source to turn it off. Since your MC output will only go to +5V, and the emitter of your PNP, or the source of your P-channel MOSFET, is connected to +12V, you will not be able to turn either of them off without a level translator.
You need to decide which side of the camera you are going to switch, so we can focus on one solution, instead of flip-flopping between them as we have been doing.

 

Thanks for the explanation. It's pretty clear now.

I was after the best solution. It sounds like I should switch on the low side of the camera.

Jpanhalt's diagram was very simple, and that appeals to me greatly. Ron, you have capacitors in your diagram. In John's diagram, I would put them in basically the same place, between the MOSFET and ground, and also between the MOSFET and camera. I believe this is to reduce stress on the camera and MOSFET by preventing unwanted oscillation? John doesn't have them in his diagram; but it would be a good idea to put them in, correct?

 

Thanks for the explanation. It's pretty clear now.

I was after the best solution. It sounds like I should switch on the low side of the camera.

Jpanhalt's diagram was very simple, and that appeals to me greatly. Ron, you have capacitors in your diagram. In John's diagram, I would put them in basically the same place, between the MOSFET and ground, and also between the MOSFET and camera. I believe this is to reduce stress on the camera and MOSFET by preventing unwanted oscillation? John doesn't have them in his diagram; but it would be a good idea to put them in, correct?

That's an interesting question. I have never seen anyone use a capacitor when switching the ground side, but it might be a good idea to put one across the camera (+12V to the drain of the MOSFET), although there is probably already one internal to the camera. I would use a low Rds(on), logic-level MOSFET, because the camera output is referenced to ground, and I think it would be a good idea to minimize the resistance between camera/system GND to power GND.
The camera output brings up an interesting question. Here's a question for the other forum members:
Is it a good idea to switch off the GND connection on a device that has more than two terminals (such as a solenoid), considering that the other terminals are inputs and/or outputs?
I'm thinking the high side switch might be safer.

 

Wow, I didn't mean to be a party pooper!
Luv2code, if your camera's output is RF, you can switch either side. If it is a video cable, I think I would play it safe and switch the high side.

 

It's RF, and the base has an on/off switch. I could take it apart and see which side X10 put the switch. I'm guessing it doesn't matter.

 

It's RF, and the base has an on/off switch. I could take it apart and see which side X10 put the switch. I'm guessing it doesn't matter.

It shouldn't matter.

 

NDP4060L is a logic-level N-channel MOSFET that should work well as a low-side switch.

 

I like that part a lot. I may order a few of them for motor and other experiments with my uC. Thanks for finding it for me.

 

NDP4060L is a logic-level N-channel MOSFET that should work well as a low-side switch.

Nice find.
Too bad that Fairchild is accepting last-time buys on it, the replacement is HUF76407P3 which doesn't have as good specs (12A, 60V, 0.107 Ohms)
Link: http://www.fairchildsemi.com/sitese...itemap+id&ia=1&text=NDP4060L&as=1&render=1&w=