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Old Nov 07, 2012, 01:34 PM
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Letchworth, Great Britain (UK)
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Higher power replacement for 2N2222A?

I currently use 2N2222A NPN transistors to switch the loads in my PICAXE circuits. But I now have a potential requirement to swich a load greater than the 0.6A that this transistor is rated for. Any suggestions please for a cheap straight replacment (not surface-mounted) that will handle up to 1.0A? Voltages are low, usually only 5V, but let's allow for up to 20V.
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Old Nov 07, 2012, 02:40 PM
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2sd965 ???
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Old Nov 07, 2012, 04:16 PM
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Quote:
Originally Posted by abenn View Post
I currently use 2N2222A NPN transistors to switch the loads in my PICAXE circuits. But I now have a potential requirement to swich a load greater than the 0.6A that this transistor is rated for. Any suggestions please for a cheap straight replacment (not surface-mounted) that will handle up to 1.0A? Voltages are low, usually only 5V, but let's allow for up to 20V.
Abe,

The 2N2222A can handle 800 mA!

Cees
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Old Nov 07, 2012, 05:26 PM
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Quote:
The 2N2222A can handle 800 mA!
As in most things electronic, you can't just look at one spec. Some manufacturers may rate the 2n222a for 800ma, but at higher currents the HFE(current gain) could be as low as 30. With a gain of 30, you would have to drive 27ma into the base to get the 800ma out, and that is over the maximum allowable for a pic output. Also, some have a 1V saturation voltage so power disipation at 800ma would be 800mw and you would need a good heat sink to handle the wasted power.

The one Ace suggested with a minimum HFE of 150 and saturation voltage of 0.35v would be a good choice. Note that the middle pin is the collector so it has a different pinout than the 2n2222a. Here is one with better specs that has the same pinout as 2n2222a with the base in the middle. http://www.diodes.com/datasheets/ZTX1051A.pdf
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Old Nov 07, 2012, 06:37 PM
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If the base drive voltage of the base is high enough in a lot of cases it's possible to replace a bipolar transistor with a MOSFET. That way you can achieve a low voltage drop and you need next to no drive current out of the processor. It's a win-win.

A
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Old Nov 08, 2012, 02:06 AM
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Thanks for those suggestions. My usual supplier reports the ZTX1051A is discontinued, but it's still available at an alternative site. I can't find any UK source for 2SD965, so looks like the ZTX is what I'll go for. Same pinout is handy, for I won't have to change my circuit board layout.
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Old Nov 08, 2012, 08:48 AM
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Some versions may have been discontinued but as far as I know it is still in production and Farnell shows plenty in stock. There is also a ZTX1049A that has similar specs but is only rated to 25V and 4A. It may be a little cheaper and shows in stock at Future UK.

http://www.diodes.com/products/catal...p?item-id=5855
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Old Nov 08, 2012, 01:24 PM
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Thanks everyone. I'm learning some things here.

Farnell (UK) stocks the ZTX1051A at a reasonable price, so that's what I'm going for. They, and RS Components, don't seem to stock the 1049A on their UK sites.

The ZTX1051A has an HFE of 450 and a continuous collector current of 4A so, from what you taught me in post #4, I presume that means that my PICAXE only needs to pass a current of 9mA to fully trigger the transistor (not sure of terminology here)?
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Old Nov 08, 2012, 11:17 PM
"Simplify, then add lightness"
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Quote:
The ZTX1051A has an HFE of 450 and a continuous collector current of 4A so, from what you taught me in post #4, I presume that means that my PICAXE only needs to pass a current of 9mA to fully trigger the transistor (not sure of terminology here)?
Not really. The correct terminology is to drive the transistor into saturation, which is what you want to do to minimize the voltage drop and power loss and temperature rise. HFE is the current gain in the linear region where the voltage drop and power loss and heating would be greater. It takes higher base currents to drive the transistor into saturation.

You need to look at the spec that says "Collector-Emitter Saturation Voltage" where it says with 4A collector current, you need 100ma base current to get the voltage drop down to 210mV. Even then you would be dissipating 840mW(4A x 210mV), which is near or above the maximum for that package depending on the ambient temperature. I wouldn't count on safely handling too much more than the 1A you originally asked for. With 10mA base current and 1A out you would get a voltage drop of 110mV and 110mW dissipation which would be safe at any reasonable ambient temperature. You just need to select the base resistor to get 10mA of base current.
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Old Nov 09, 2012, 01:41 AM
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Thank you. The basic diagram in the PICAXE manual shows a base resistor of 1k, with 5v input. In calculating required base resistor value do I assume that PICAXE and transistor resistances are zero, so to get 10mA base current I simply need 5/0.01 = 500 ohms?
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Old Nov 09, 2012, 01:58 AM
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2n3055
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Old Nov 09, 2012, 09:48 AM
"Simplify, then add lightness"
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To be assured of enough base drive current, you would need to take into account the base-emitter voltage drop of the transistor and the output resistance of the Picaxe.

The spec you need to look for on the transistor is VBEsat. With 1A out of that transistor VBEsat is around 0.8V. That means the base resistance would need to be 5v-.0.8v/10ma or 420 ohms. Don't know what PIC they use for Picaxe, but output resistance when pulling up could be as high as 200 ohms, so a 220 ohm base resistor would assure that the transistor was driven into saturation.
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Old Nov 09, 2012, 02:06 PM
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Thank you jeffs555. My previous electronics experience was with valves and transformers, but I'm sure I'll get the hang of these transistor thingies before long! Thus far I've been lucky that my 2N2222s have worked okay without me doing the maths -- in fact in some circuits I forgot to install any base resistor, so the internal resistance of the PICAXE must be protecting it and the transistors from over-amping.

Thanks for your help, I'm beginning to understand now what all that transistor data means.
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Old Nov 09, 2012, 06:57 PM
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You should seriously consider using a logic level MOSFET like the IRLIZ44. Requires only a single resistor in the gate circuit, can be driven directly by the output of just about any CPU, very low on resistance = little heat dissipation and can handle much more current than just about any bipolar transistor.
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Old Nov 10, 2012, 02:21 AM
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At the moment I don't need really high power -- I'm just driving LEDs but wanted to be able to run two Crees in parallel at 500mA each, which is more than my 2N2222s could handle. So the ZTX1051A is ideal for my needs at the moment, and fits in the same circuit board without having to alter it.

In the future I'll certainly look at MOSFETs if I need greater load handling capability.
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