Providing Vce on the datasheet, is greater than the maximum possible voltage between the transistor's emitter and collector in the circuit, it doesn't matter.

When your transistor is turned off the voltage betweent the collector and emitter will be 27V, so a transistor with a maximum Vce of >30V will do fine.

The beta doesn't have to be exact either, just as long as it's great enough to pass the required collector current at the expected base current. Normally you specify a minimum beta, for example with an Ic of 10mA the beta needs to be >20. Forget trying to find a transistor with an exact beta of 20 at 10mA - it just won't happen. You should design the circuit so it works well with different transistors with different betas.

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Thanks hero999,
I think i am more or less confused about selecting which transitor i need but i am getting somewhere now with little more help from hero999 or anyone i think i can select a transistor.

ic=1A, ib=25mA, Vce=23V
For Vce i don't have problem i will look for transistor that can support higher value than Vce
I have a microcontroller that send 25mA it hard to find a matching current base ?? I found ib =25mA but the ic=0.25A for it i want higher ic
when i find ib=25mA but this time ic =2A larger that what i want??

Question while looking for a transistor i want to know what to look for hfe? or P? or Vce? or ic? or ib?

For Vce i understand that higher value is no problem.
Little more guide i will be fine

Thanks in advance

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Assuming that you're using the transistor as a switch:

Ib will depend on the base resistor, not the maximum output capability of the microcontroller.

Ic will depend on the load resistance.

Generally to ensure the transistor saturates properly, the base current needs to be 1/10 of the collector current. Therefore, the maximum load you can drive with your microcontroller and a single transistor is 250mA.

Look at the load line and minimum hFE at different collector currents on the datasheet. It might be possible that the transistor could give you a much higher gain than 10 at the collector current you desire and still have an acceptable saturation voltage. If you want to switch really large loads, then use a darlington pair or an NPN driving a PNP if you can't stand the extra saturation voltage.

Generally for larger loads power MOFETs are a more sensible option but make sure you use a logic level device and that it can supply a high enough drain current for the gate voltage given by the MCU.

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Ya i am using transistor as a switch.
I want to use NPN becuase while i send a low signal no current flow in base and collector. when i send a high signal in the base current flow in the collector.

My Question is does the current is determine by load or by transistor?
because using darlington pair ic can reach up to 4A and i only need 1A.
but if i use a switch transistor i need 1A for ic but ic=0.25A does that have a side effect like burning the transistor.

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If the transistor doesn't have enough gain to provide the current required for the load at the collector, the transistor will have a large voltage drop across it and it will burn up if the power dissipation exceeds the maximum rating.

For example, if the load requires 500mA and Ib is 25mA but the transistor only has a gain of 10, it will limit the current to only 250mA. Of course the maximum collector current is limited solely be the load at the collector, if the load only takes 500mA then no more than 500mA will pass whatever Ib is.

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hi,
For the purpose of deciding the transistor, assume you have the ideal transistor.

Now show us on you circuit diagram where you intend connecting this ideal transistor.
Not a block diagram that you posted earlier, but a circuit diagram.

When we can see how the ideal transistor is connected and what it does, we can advise an actual transistor type number.

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Here is the diagram

I think i understand more about transistor thanks to hero999

I just want to find a database for transistor or place to search for it because google is not helping and i don't have exact value to search for therefore if you know a website that have a database for transistor i appericate it

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What does R2 represent?

Any chance you know how much current it draws?

Just look through catalogues of component suppliers and find a transistor with a suitable hFE, Vce and Ic.

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it uses ic = 1.049A
R2 is just a resistor don't know why i must use it i saw NPN transistor always uses R1 and R2.

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Well you must know what R2 is as you know the collector current.

Assuming the MCU has a maximum output of 25mA you need a transistor with a gain of at least 40 at 1A (good luck) and the minimum IC needs ot be at least 1.2A.

My advice is to use a logic level MOSFET.

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For the load current of 1.05A you need a darlington transistor that will switch it when its base current is only 25mA or less. A TIP110 darlington transistor has a typical saturation voltage loss of 0.9V when its collector current is 1.05A and its base current is 10mA. Its max voltage loss might be 2V.

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Wow 5 red squares, at least the avatar is cute.

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hi uaefame,

The motor in the Energy convertor, does it have to drive in a clockwise and anticlockwise direction when its working.?

Also does the motor have to move/drive at different speeds.?

How many wires on the motor are free for you to connect too.?

Is one lead of the motor connected to 0V or +23V.?

Sorry about the number of questions.

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Hi ericgibbs,

Energy convetor is only a driver for the motor.
When a negative voltage is applied to the energy converter it rotate counterclock wise and when positve voltage is applied it rotate clockwise
(-23V & 23V).
there are 2 wires comming out of the motor which goes to the energy converter.
Nope the lead of the motor is connected to the energy converter. If you look at the specification of the motor it need 48V.

Hi everyone
I wanted to select a transistor i found a website http://www.qsl.net/yo5ofh/data_sheets/transist.txt
This website is easy to search for transistor but first it gives Voltage, current, power can some one clear to me what is these specification.
I mean is current =ic?? voltage =Vce?? and power =max power transistor can handle.

Hi audioguru

I see you selected a transistor for me but i have several question about what you wrote?

First what you mean buy saturation voltage loss of 0.9 when the ic=1.05A and base current is 10mA. is it the same as Vbe=0.9V??

and the max voltage loss is 2V?? what does that mean??

sorry for my silly question

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[quote=Hi ericgibbs,
Energy convetor is only a driver for the motor.
When a negative voltage is applied to the energy converter it rotate counterclock wise and when positve voltage is applied it rotate clockwise
(-23V & 23V).
there are 2 wires comming out of the motor which goes to the energy converter.
Nope the lead of the motor is connected to the energy converter. If you look at the specification of the motor it need 48V.
[/QUOTE]

hi,
This is the point thats been causing me concern, it appears that the motor requires to be able to be driven in either direction.
A single power transistor will not be able to drive a motor clockwise and anticlockwise.

Look up 'H bridge' on the web or this forum. This type of bridge will enable the motor to drive either way.

Hope this helps.

If you look at the specification of the motor it need 48V.
Please post the motor spec.

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