Transistor

[uaefame]

Hello everyone,

I am trying to design a switch using a transistor. I am using transistor because it can handle two current at the same time low and high. I can use microchip to send low signal and transistor open a gate for high current flow. That all my knowledge about transistor.

I want to choose transistor that will do my job?
I have a microcontroller which i will connect to the base which will send 5V signal with 20mA current this will be connect to the base of the transistor.
VCC=27V that mean i use 3X9V battery
VC=23V the device i am using need 23V
Vb=5V from microcontroller
ib=20mA
ic?? not sure about it
R1&R2&transistor not sure about it??
I know that i need NPN transistor
hfe=??
Please tell me how can i select it, if still more information is needed tell me so i can find it out.
**broken link removed**

 

[crutschow]

What is the transistor output connected to and what output signal level do you need? What is the "high current flow" value? The answer to those questions will help determine how to design the circuit.

 

[ericgibbs]

I want to choose transistor that will do my job?
I have a microcontroller which i will connect to the base which will send 5V signal with 20mA current this will be connect to the base of the transistor.
VCC=27V that mean i use 3X9V battery
On load the batteries if they are PP3 will be about 8.4V/battery so, about 25V
Will the 23V device accept 25V.?

VC=23V the device i am using need 23V

Vb=5V from microcontroller
ib=20mA
ic?? not sure about it
The hfe of a particular transistor is depends upon the collector current.
example: let the collector current be 100mA and the hfe be 50, this would give a minimum base requirement
of 2mA... In order to fully saturate the transistor I would suggest a base current of 5mA.

R1&R2&transistor not sure about it??
For R1 the base resistor, assuming a +5v output from the PIC, (5 - 0.7Vbe)/0.005 =860R,, say 820R
If your battery was 27V and the device required 23V, you just cant drop 4V across an emitter resistor as the transistor would not switch correctly with a 5V base drive.

IF you must lose 4volts put a resistor in series with your device.
Example, if Ic thru the device was 100mA, the 4V/0.1 40R, say 39R [use a 1Watt resistor]
I know that i need NPN transistor
hfe=??
Please tell me how can i select it, if still more information is needed tell me so i can find it out.

hi,
Do you follow this.?

 

[Papabravo]

I don't think the circuit you have shown will be a very good switch. The current through R1 will be equal to the collector current. What ever that current is will depend on the charge on the batteries and the value of R1. As the batteries discharge things will change. You may not be able to keep Vc close to 23V. When the transistor is off Vc will be at Vcc or 27 V, and when it is on it will be at about 4.5V olts

Next I have some doubts that you will be able to source 20 mA into the base of the trasnistor. The emitter will be at 4.3V when the base is at 5V. In this condition the collector will be at Ve + 0.2V or about 4.5V.

I think you need to go back to the drawing board.

Edit: My comments refer to the original circuit, and not Eric's improved version.

 

[ericgibbs]

If you tell us what the device is and what you want it to do.

We could probably advise a better circuit.

 

[uaefame]

I have Energy converter III box with 2 sensor connect to it. I want to remove these sensor a replace it with microcontroller. The output of the sensor needs 23V to run the motor if 0V the motor won't run. While repacing the sensor with microcontroller it realize that the microcontroller only provide 5V and the output of the sensor require 23V therefore i came with an idea to use transistor as a switch.

The sensor and the energy converter III information is given in this pdf
**broken link removed**

 

[blueroomelectronics]

The DEGERenergie Solar tracking system appears to have some brains, why would you want to replace it?

 

[uaefame]

My instructor told me to design an open loop system for the PV solar tracking system. These sensor are closeloop. I am trying to replace it so i can design an openloop system.

Thanks everyone for replying

 

[Papabravo]

My instructor told me to design an open loop system for the PV solar tracking system. These sensor are closeloop. I am trying to replace it so i can design an openloop system.

Thanks everyone for replying

That is probably the most misbegotten assignment I ever heard of. The whole point of closed loop systems is to improve performance. I can't imagine why a sane instructor would make a request like this. I'd ask the school for my tuition money back if some clown asked me to do that.

 

[blueroomelectronics]

uaefame is fast becoming the new Walters.

 

[uaefame]

The close loop tracking system is so strange. It doesn't track the sun and sometimes it is off by 40degree. Therefore while contacting the company i was asked to do open loop tracking system.

 

[Papabravo]

OK, so in the process of trying to get the closed loop system to work the suggestion was made to try an open loop system. It is not clear that you can do this in all cases without rebuilding the system from the ground up.

For example, if the actuator is a DC servo motor with a rotary encoder for position feedback, then there is no practical way to to do this open loop. You can try energising the motor for a fixed time at fixed intervals but ultimately you will have no idea how to make the system perform. I don't know all the details of your system so I can't say weather it will work or not.

It seems to me that if the closed loop system was properly designed then you should be able to identify the problem and improve the situation.

 

[uaefame]

Back to the main topic open loop, i am a mechanical engineering i am trying to use the old system because its easy i am trying to control it using time I will give it a try.

I gathered all the data from the system now i need to select a transistor so anyone can guide me i am waiting for a reply.



Vc=23V
R1=22ohms
R2=148ohms
hfe=36
P=24W
Vb=5V
ib=0.029A
ic=1.049A

Use my above figure where i first post but change the R2 place to be with the base.

I gathered all data. Now,I only need to select transistor but i am confused how i use these data to select

Thanks in advance

 

[uaefame]

I guess finding tranistor is hell of a job. I spend 2hours just to find a good website but noting perfect yet.

Is there a website where you input ib or ic or other thing and it sort the transistor. If you know a transistor that is close to what i am looking i am looking forward to know its name. If you know a good website that helps too.

 

[Nigel Goodwin]

I guess finding tranistor is hell of a job. I spend 2hours just to find a good website but noting perfect yet.

More a question of no body understands what you're trying to do, your diagram makes no sense, and your random lists of specifications and values doesn't either.

 

[Hero999]

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.

 

[uaefame]

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

 

[Hero999]

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.

 

[uaefame]

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.

 

[Hero999]

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.