Higher current NPN transistor with same characteristics as a BC337

Hi all, as title really. I have used lots of BC337s in the past (T092) case which are good for 800mA. I need something similar but to be able to PWM 1A+ LED's but I'm not sure what would do it?

A good MOSFET would do.
http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422

If you want to stick to BJTs then take a look at ZTX851. However, I generally use mosfets when switching more than 500mA.

Hi guys thanks for that.

I'm not really sure of the differences between the bc337 and a mosfet?

How do they compare in switching speeds?

Just noticed yesterday when ordering a few bits from RS that they now charge for delivery under £20 :(

MOSFET = Metal Oxide Semiconductor Field Effect Transistor BJT = Bipolar Junction Transistor

Basically, a MOSFET works by using an electric field to control current through a channel. When you apply sufficient voltage on the gate relative to the source (Vgs(th) in the datasheet), a low resistance connection between drain and source is created. The current that can flow through the channel is proportional to the voltage applied to the gate.

A BJT, such as the NPN of which you speek uses a current flowing into the base to control a current flowing into the collector. To turn a BJT on you need about 0.7v on the Base relative to the Emitter. When that is applied, the current flowing from collector to emitter is proportional to the current flowing into the base.

In summary: BJT = Current controlled load current. MOSFET = Voltage controlled load current.

With a MOSFET this mean that for any load current, very little current is required at the gate making them very microcontroller friendly. With a BJT you need either a transistor with very high current gain to switch a large load with a microcontroller as the Arduino for example can only source 20mA per pin. MOSFETs are also much faster at switching the currents, so waste less power when using in systems where high frequency switching (such as PWM) is used.

Well that’s only really true if the MOSFET gate is driven hard enough - larger MOSFETs have
a lot of capacitance of the gate that needs charging to switch - drive this with too little
current and it can take several microseconds to switch. Larger devices need maybe
an amp or so to switch at full speed (<100ns). Since the Arduino pins can safely provide
upto 40mA this can be an issue if fast PWM is required and lots of current needs
switching - in this case a MOSFET driver chip is the simple solution (they can level-shift
to 12V to drive non-logic-level MOSFETs too).

Try the BD13x family.

Thanks all. That is some really valuable info there.

Just got some of these coming from eBay http://www.ebay.co.uk/itm/251116903522? http://www.sunsphere.co.uk/datasheet/bc327_337.pdf looks like they will handle 800mA and 1A max. I might be OK with them, I will be switching either 3w or 5w at 12V so I make that 400mA max. Emitter base voltage is stated at 5V - does that mean the gate will be fully open 5V and maybe not fully open at anything less?

The fets seem a lot more expensive, but I will need to get some to control some RGB LED strips also :)

Thanks

"Max Power dissipation: 625 mW"

In otherwords, when 400mA is flowing through the load, if any more than 1.5 volts is across the transistor, it will likely overheat and be destroyed.

You should be looking for a transistor in at least a TO-220 package for high power loads like 1A+ LEDs.

dtokez: Emitter base voltage is stated at 5V - does that mean the gate will be fully open 5V and maybe not fully open at anything less?

Wrong. That is the ABSOLUTE MAXIMUM. You want Vbe(on)=0.7V. As I mentioned before, NPN transistors use current to control current. You ensure that there is 0.7V at the base with respect to the emitter, and then use a resistor to control the current flowing into the base from the Arduino.

If your heart is set on a BJT, you could look towards the TIP120/TIP121/TIP122 or similar (http://www.adafruit.com/datasheets/TIP120.pdf). They are darlington pairs so have a very high current gain allowing you to control large loads with low currents. They do have a downside that the Vbe(on) is higher (~2.5v)

Alternatively, if you want to try out MOSFETs which are somewhat easier to use for high power loads (complete isolation of gate and channel offers some protection for the arduino), you could look for IRF510s (http://www.irf.com/product-info/datasheets/data/irf510.pdf) or IRF640s (http://www.vishay.com/docs/91036/91036.pdf).

You want Vbe(on)=0.7V.

For switching, you want to maximize the base current - forget about Vbe. Simply inject as much current into the base as you can to saturate the switch as much as you can.

dhenry:

You want Vbe(on)=0.7V.

For switching, you want to maximize the base current - forget about Vbe. Simply inject as much current into the base as you can to saturate the switch as much as you can.

If you stick 5v on the base you will be running the transistor at its absolute maximum ratings which will most likely damage it in the long run. Furthermore if you do nothing to limit the base current you will damage the microcontroller from drawing too much current from its port drivers.

[quote author=Tom Carpenter link=topic=138597.msg1042171#msg1042171 date=1356257841] "Max Power dissipation: 625 mW"

In otherwords, when 400mA is flowing through the load, if any more than 1.5 volts is across the transistor, it will likely overheat and be destroyed.

You should be looking for a transistor in at least a TO-220 package for high power loads like 1A+ LEDs.

dtokez: Emitter base voltage is stated at 5V - does that mean the gate will be fully open 5V and maybe not fully open at anything less?

Wrong. That is the ABSOLUTE MAXIMUM. You want Vbe(on)=0.7V. As I mentioned before, NPN transistors use current to control current. You ensure that there is 0.7V at the base with respect to the emitter, and then use a resistor to control the current flowing into the base from the Arduino.

If your heart is set on a BJT, you could look towards the TIP120/TIP121/TIP122 or similar (http://www.adafruit.com/datasheets/TIP120.pdf). They are darlington pairs so have a very high current gain allowing you to control large loads with low currents. They do have a downside that the Vbe(on) is higher (~2.5v)

Alternatively, if you want to try out MOSFETs which are somewhat easier to use for high power loads (complete isolation of gate and channel offers some protection for the arduino), you could look for IRF510s (http://www.irf.com/product-info/datasheets/data/irf510.pdf) or IRF640s (http://www.vishay.com/docs/91036/91036.pdf). [/quote]

hi thanks for the help! I can't get my head around the power dissipation thing being only 600mW. At 12V that means they can only switch 50mA? I was testing a couple of these http://www.fairchildsemi.com/ds/BC/BC337.pdf 12V @350mA doing some slow cycling for about 1 hour and they didn't even get warm?

Ah, but you will not have 12V across the transistor when it is on, you more than likely had a saturated voltage of Vce at 0.7V, and current of .35A , which means power dissipated in the part is P=IV = 0.7V x .35A = 245mW

Hi crossroads thanks for clearing that up, I was getting pretty confused for a minute there :) I have always used a 1k resistor for current limiting from the arduino pin too the base - is this the optimal value I should be using!

Probably not. When the base is driven, you will see ~0.7V Vbe, about 1 diode drop. The other side of the resistor at 5V, so base current is: (5V - 0.7)/1000 = 4.3mA. You can usually go higher, like 20mA: (5V-0.7)/20mA = ~220 ohm, to make sure the transistor is turned on good & strong with Vce = 0.7V. Then whatever resistance is in the collector path will control the current flow, and not develop a higher voltage across the transistor, which can lead to higher power dissipation.

Bob, thanks for the help that all seems pretty logical now after your explanation :) I will have a fiddle.

In short though, I think I will continue to use the BC337s to switch the 350mA load, and wait till the 5W bulb turns up to see if it is actually 5W. Doesn't seem worth paying the extra for Mosfets, I was thinking to build a controller future proofed for 1A + bulbs but I'm not sure now.

Buy the way I'm putting 12 down lighters with LED MR16 bulbs in my lounge ceiling and controlling them individually over ethernet, should be cool if I can get it all running nicely.