I need to switch some GE #44 lamps from a TTL output. #44 lamps are 6.3 volts at 250 ma, so I'll need a transistor to do the real work. Unfortunately, it's just a hair too much power for one of the hundreds of surplus 2n2222's I keep around to do almost everything else here. Can someone suggest a small, cheap available transistor that would be appropriate for this use?
-- Lee K. Gleason N5ZMR Control-G Consultants snipped-for-privacy@comcast.net
Use a FET? The NDS351 comes to mind but only if it's driven from 5V TTL logic. Around $0.10 in large qties. If it's 3.3V logic you'll have to keep looking for one that has guaranteed Rdson data down there and is low enough in cost.
where did you get the idea the 2222's can't handle that? You need to saturate them,. They can handle 500 ma's so said this PDF..
The trick is to saturate the bias so that very little resistance effect is between the CE.
For switching DC voltage -- If the transistor is driven straight from a non-open collector IC -- I suggest using a high gain transistor such as MPSA13, MPSA14 or 2N6427 for switching one lamp. If switching two lamps -- MPSU45, CENU45, 2N6548 or
2N6549. None of these four are real popular, though. The CENU45's are still made - other three are tough to find.If the transistor is driven from an open collector IC with a stiff pullup resistor on the output then you can use something like a 2N6037 for switching one or more lamps.
If the transistor is pre-driven by a pre-amplfication transistor (i.e.
2N2222) -- then you can also use something like the 2N6037.As an alternative, you can also use logic level driven MOSFET's as well -- something like the IRL510 would drive lots of lamps.
If you're switching AC voltage than use something like the Bally pinball machines used -- 2N5064 for one lamp or MCR106 for more than one lamp.
-- Ed
volts
Yup, the 2222 is rated for a min beta of 40 at 500 mA. I'd just hook the TTL output directly to the base!
(Ducks incoming flames.)
John
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I was concerned about the total power rating of 500 milliwatts for the
2n2222. Power here would be around a watt and a half...-- Lee K. Gleason N5ZMR Control-G Consultants snipped-for-privacy@comcast.net
That would need to be a pretty high saturation voltage in the transistor to get 1.5W at 250mA.
The MPSA13 is rated at 625mW. With a saturation voltage of 1.5V and 400mA (typical GE44 is actually about 400mA) -- you're dissipating 0.6W... maxing out the MPSA13. Pinball people have been using MPSA13's and GE44 bulbs for years. The biggest problem happens when the bulb burns out. Unpredictable things happen within the bulb when they burn out -- sometimes the filliment will break, move and 'reweld' itself to a different place in the filliment -- giving a brighter bulb...and short lifespan on the transistor.
You can also go with a type 47 bulb. Same voltage but about 250mA. Less current so less power dissipated by transistor. Down side is the bulb is considerably dimmer.
-- Ed
Look here at figure 2:
The little detail is that "beta=10" thingamjig...that means if you drive the base with 25mA, the Vce at 250mA will be less than 0.2V. This makes the Pd = 0.25A x 0.2V= 0.125W. Now look at figure 3, Vbe,sat, at 250mA is about 0.9V. Use this to compute the voltage bucking your TTL drive. Then the base dissipation is 0.025A x 0.9V=0.0225W. The grand total is
0.0225+0.125=0.1475W. Now go up to the top and look at R theta,j-a, of 200oC/W for the TO-92 plastic, that gets you 200oC/W x 0.1475W=29.5oC junction rise above ambient. Now look at Tstg under max ratings, it says 150oC, this means you will be fine at ambients less than 150-30=120oC. recall this is 120x1.8+32=250oF. Which TTL part are you using to drive it? The 25mA base drive would suggest something called a *buffer*.
volts
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Couldn't he just make a Darlington pair from two of the cheap 2n2222s?
What about the 2222s in the can?
WTF...0.05W >
WTF...0.0725W
WTF...14.5oC
WTF...135oC
...275oF
The initial current surge (maybe 10x of normal) of the light bulb will likely take out the transistor unless some sort of "soft-start" is implemented.
...Jim Thompson
It's the same die you bloody MORON.
Graham
And what's the Vce(sat) of a darlington ?
Graham
Exactly- he needs to skip the SS stuff and go with a TIP31 grade medium power part, well within its SOA...they're cheap.
Idiot! It is a well known FACT that a canned transistor can dissipate more heat than an epoxy package because it can sink it better and faster.
Otherwise, your CPU would be in plastic, idiot!
I guarantee the canned device has different numbers.
I wouldn't expect a retard like you to understand, however.
I'm a sucker for the Zetex low saturation TO-92 transistors, but they are not very cheap and made by no one else. But drool over this data sheet:
But if I was going for dirt common and rugged, I would probably go with a 3 amp rated power tab transistor mounted standing straight up on the board.
Gain is more important than current rating, but look for something with its peak gain in the 400 mA area or above, for low base drive and saturation voltage. That puts you up in the 3 to 6 amp rated sort of thing.
The TIP 31 has higher gain at 400 mA than the larger TIP41 has, so it is probably better, given the limited base drive.
This one is pretty good at 400 mA, also.
But for my own stuff (not volume production design) I love the Zetex. Having 3 or 4 times the current gain really simplifies base drive and thermal problems.
Zetex makes the best stuff on the planet.
Bah. 2N2222 is the same thing as 2N4401, and I've got a 2N4401 in a one-cell-powered-LED-inverter (apparently called a "Joule thief") running as a blocking oscillator around 5MHz, with peak emitter current around
800mA. Saturation is under 0.3V. Okay, probably half of that is base current, but it's still more than low enough.Use a PNP (2907/4403) to take advantage of the TTL's current sinking capacity.
Tim
-- Deep Fryer: A very philosophical monk. Website @
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How about a ULN2001/2/3 darlington array. There will be 7 in a 16 dual in line package.
Sal
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