how to pick the correct transistor?
- Thread starter tyler
- Start date
May I suggest you look here. To give you an overview of transistor selection.
You need to answer for yourself the following: What is the motor voltage supply?, how much current does the motor draw at this voltage? Then select appropriately from there.
I generally tend to double the voltage rating of the trasnsistor and the collector current also.
PS please use the word PICAXE not 'Pickaxe'. It is written several times on the forum so it is difficult to understand your reason for spelling it as such.
You need to answer for yourself the following: What is the motor voltage supply?, how much current does the motor draw at this voltage? Then select appropriately from there.
I generally tend to double the voltage rating of the trasnsistor and the collector current also.
PS please use the word PICAXE not 'Pickaxe'. It is written several times on the forum so it is difficult to understand your reason for spelling it as such.
westaust55
Moderator
Some of the first things you need to consider are:
1. The Motor full load/nameplate current rating
2. The supply voltage
3. Are you switching on the high side (positive side) of the motor or the low (negative side)
4. If high side switching you will likely need an additional interface transistor if the motor voltage is greater than PICAXE supply voltage
5. If you intend to use BJT or FET type transistor.
If you intend to use a BJT type transistor then, in simple terms, you need to select a transistor capable of the motor current. This is usually given on the datasheets as Ic.
Then look at the transistor gain when in saturation Hfe(sat) and determine the minimum base current.
I recommend you operate the transistor in the saturated condition to reduce the collector to emitter voltage and also reduce heat.
Minimim Ib = Imotor / Hfe(sat) - - Hfe(sat) may be about a tenth of the linear region Hfe.
Then calculate the base resistor to allow sufficient current flow into the transistor base (Ib) and to protect the PICAXE output. Keep in mind the typical 0.7 Volt base to emitter voltage (Vbe).
If you are controlling more than a few tens of milliamps you need to consider the heat dissipation of the transistor and determine if a heat sink is required.
Others forum members will no doubt provide some guidelines for FET type transistors. Their pin and parameters names are different and instead of a current input for current control in simple terms are a voltage controlled (switching) device.
1. The Motor full load/nameplate current rating
2. The supply voltage
3. Are you switching on the high side (positive side) of the motor or the low (negative side)
4. If high side switching you will likely need an additional interface transistor if the motor voltage is greater than PICAXE supply voltage
5. If you intend to use BJT or FET type transistor.
If you intend to use a BJT type transistor then, in simple terms, you need to select a transistor capable of the motor current. This is usually given on the datasheets as Ic.
Then look at the transistor gain when in saturation Hfe(sat) and determine the minimum base current.
I recommend you operate the transistor in the saturated condition to reduce the collector to emitter voltage and also reduce heat.
Minimim Ib = Imotor / Hfe(sat) - - Hfe(sat) may be about a tenth of the linear region Hfe.
Then calculate the base resistor to allow sufficient current flow into the transistor base (Ib) and to protect the PICAXE output. Keep in mind the typical 0.7 Volt base to emitter voltage (Vbe).
If you are controlling more than a few tens of milliamps you need to consider the heat dissipation of the transistor and determine if a heat sink is required.
Others forum members will no doubt provide some guidelines for FET type transistors. Their pin and parameters names are different and instead of a current input for current control in simple terms are a voltage controlled (switching) device.
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Pickaxe
Pickaxe
Pickaxe
People get their panties in a knot for the smallest of things. Maybe we should get a grammar/spelling/? section
I want in on the speling sektion!
BTW, the word "PICAXE" is customarily shown in upper case letters, as per Rev. Ed.'s presentation format.
As for transistors, there are two basic types, BJT's, (the old style transistors,) and MOSFET's, (the new style.) Either will work for your needs if you select one which has sufficiently high rated parameters, (as mentioned above.)
Within the BJT style you will find two varieties, single transistors, and dual transistors (Darlington) that will provide higher gain specs than the single transistors, but will drop more voltage across the device. These are a good choice when a fairly high voltage is being switched, so the loss in the Darlington becomes insignificant relative to the voltage being switched.
Without more info on the motor it's hard to say which transistor type to use, but many of each type will likely work. The PICAXE itself is typically protected by placing a series resistor on the output pin that drives the transistor, limiting the absolute current that the PICAXE must produce, and the absolute voltage that the PICAXE output may see coming back from the motor via the transistor.
BTW, the word "PICAXE" is customarily shown in upper case letters, as per Rev. Ed.'s presentation format.
As for transistors, there are two basic types, BJT's, (the old style transistors,) and MOSFET's, (the new style.) Either will work for your needs if you select one which has sufficiently high rated parameters, (as mentioned above.)
Within the BJT style you will find two varieties, single transistors, and dual transistors (Darlington) that will provide higher gain specs than the single transistors, but will drop more voltage across the device. These are a good choice when a fairly high voltage is being switched, so the loss in the Darlington becomes insignificant relative to the voltage being switched.
Without more info on the motor it's hard to say which transistor type to use, but many of each type will likely work. The PICAXE itself is typically protected by placing a series resistor on the output pin that drives the transistor, limiting the absolute current that the PICAXE must produce, and the absolute voltage that the PICAXE output may see coming back from the motor via the transistor.
westaust55
Moderator
spellin' it is then:
Obviously there are cases where spelling/grammar errors are understandable.
However, bad spelling/grammar/punctuation and (often) poor layout affects the readability for others.
When I see 20 cramped lines of text I struggle.
If I struggle or have to concentrate on 'translating' a pile of poor lazy prose I just switch off.
It's funny , we appreciate syntax/spelling standards in programming languages but often slip up when it comes to writing a clear explanation or question.
Sometimes it seems that some people can't be arsed to re-read their own typing.
On occasions it makes later searching (even for Ec
) difficult - if not impossible.
Back to transistors...
It's like most aspects of design; define the working parameters and then look around for bits that fit.
Westy has given a list of things to look at.
However, I fully appreciate for Newbies that much of this is like a foreign language.
So, if confused, clearly define what you want to do and people will help.
However, bad spelling/grammar/punctuation and (often) poor layout affects the readability for others.
When I see 20 cramped lines of text I struggle.
If I struggle or have to concentrate on 'translating' a pile of poor lazy prose I just switch off.
It's funny , we appreciate syntax/spelling standards in programming languages but often slip up when it comes to writing a clear explanation or question.
Sometimes it seems that some people can't be arsed to re-read their own typing.
On occasions it makes later searching (even for Ec
) difficult - if not impossible.Back to transistors...
It's like most aspects of design; define the working parameters and then look around for bits that fit.
Westy has given a list of things to look at.
However, I fully appreciate for Newbies that much of this is like a foreign language.
So, if confused, clearly define what you want to do and people will help.
WHITEKNUCKLES
New Member
Not fair John! Being dyslexic and not familiar with the word I spent some time finding that 'sektion' is a German word.
Later I had a wee laugh at myself, I had failed to see anthing wrong with 'speling'.
But nit picking is not helpful unless useful.
Dave
Sorry about my spelling of PICAXE i just wasent thinking as i typed it, as to the transistors so if i just pick one that is higher rated then i need then it will just give the voltage into the colector? what about the base tho will any transistors take to much current from the base and wreck the PICAXE? or is the PICAXE protected from this? thanks
Tyler
Tyler
mrburnette
Senior Member
Said another way, if you are driving a 1 Amp load via a 100 Hfe transistor, your base current is 1A / 100 = 10mAYes it is possible to still choose a transistor with the wrong ratings. What you need is one with a high Hfe (gain)figure. This effectively means that less Base current is required for the same Collector current.
Hope that makes sense to you.
The total transistor current is 1000mA + 10mA = 1.01A
westaust55
Moderator
please go back and read the steps in post 3.so if i just pick one that is higher rated then i need then it will just give the voltage into the colector?
what about the base tho will any transistors take to much current from the base and wreck the PICAXE? or is the PICAXE protected from this? thanks
Tyler
If you provide the motor data, we can help you select a transistor.
Otherwise the fundamental steps to determine yourself are give.
As mentioned in post 3 working with the transistor in the saturated mode will reduce Vce (and heat dissipation) but the gain Hfe(sat) is typically about a tenth of the Hfe in the linear region so need to check the datasheet for the intended transistor.
mrburnette
Senior Member
Yes, caveats exist and abound with every aspect of electronics and Post #3 is very well written for selection. I did a poor job on my late night write-up meant to impart a rule-of-thumb; it should have been prefaced with something such as, "For 3-terminal transistors, the sum of the the Collector current, Ic, and the Base current, Ib, represents the total current handled by the device, in D.C. mode. Operating Hfe (as opposed to published Hfe) is the ratio of Ic / Ib." Even this is not entirely precise, but should be adequate for the purpose at-hand.
For readers without extensive electronics training, this link should help in understanding the issues being discussed and provide a guide to calculating the required bias resistor values to drive the transistor outside the linear mode into saturation: http://www.electronics-tutorials.ws/transistor/tran_4.html
Credit for the JPG goes to the link above:
- Ray
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