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Help in choosing correct bjt transistor

mshh

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I am trying to do constant power load, the transistor switches the current source which ranges from 0-30v and the load is 6.8R50w, there is opamp power vcc 12v used to control the BJT base emitter voltage. Could you recommend BJT transistor that could draw 4-5amp with small heatsink
 

KlausST

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Hi,

* "constant current" sounds like linear regulation
* but then you talk about "switches the current" which sounds like ON/OFF, not linear.
--> you need to give complete informations: like schematic, maybe switching frequency...

I generally avoid to recommend BJTs. Because every BJT manufacturer and every distributor provides interactive selection guides.
This is much more faster if you do this than we do this. We just have limited informations ... means several requests of informations from you.
We don't know what sources, what device packages you prefer...

Also you talk about small heatsink. For linear regulation the BJT has no influence on dissipated power, thus the size of heatsink does not depend on the chosen BJT.

Klaus
 

mshh

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No ,I want linear not switch on/off. This is a constant power load If the voltage increased the current decreased and vice versa as attached. the current is ranging from 0-4amp and voltage from0-30v. I need BJT that can handle high-power with small heat sink
 

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BigBoss

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BJT will need a heat-sink but MOS may not..Consider a MOS Transistor that will operate in Linear region.
 

00kam

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Hello
don't waste your time the solution in your case will never be solved by use liner regulator .
because there is a power (50v-26.9volt)*(26.9volt/6.8ohm) watt on the" C .E" Junction of BJT is converted to heat energy and i am sure the heat can warm your finger if you touch the hate sink
now it is the power dissipation .can you tell me how you can get rid of this power by your circuit
the only solution and the one is the "switching regulator " with high frequency switching.


kamal
 

FvM

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BJT will need a heat-sink but MOS may not..Consider a MOS Transistor that will operate in Linear region.
Don't agree. The minimal heat sink size for a linear output stage is commanded by the application power dissipation and the acceptable heat sink overtemperature.
Maximal power disspation with resistor load occurs at Vout = Vsupply/2, for Vsupply = 50V and R = 6.8 ohm it's 92 W. You need a heat sink with Rth < 1 K/W which can be never small.
 

mshh

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What if the supply is30v ,will it need larg heatsink?
 

tyassin

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No ,I want linear not switch on/off. This is a constant power load If the voltage increased the current decreased and vice versa as attached. the current is ranging from 0-4amp and voltage from0-30v. I need BJT that can handle high-power with small heat sink

Just a question....how is the attached circuit able to be constant power? I see only constant current.
 

KlausST

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Hi,

Just a question....how is the attached circuit able to be constant power? I see only constant current.
For a fixed load of 6.8 Ohms..(that's what the OP wrote)
* constant current will give constant power
* constant voltage will give constant power

Usually a constant current circuit uses a shunt resistor to regulate the current. Here we just have one resistor ... the load.
So you may call it
* constant current
* constant voltage
* constant power circuit

Klaus
 

tyassin

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Hi,


For a fixed load of 6.8 Ohms..(that's what the OP wrote)
* constant current will give constant power
* constant voltage will give constant power

Usually a constant current circuit uses a shunt resistor to regulate the current. Here we just have one resistor ... the load.
So you may call it
* constant current
* constant voltage
* constant power circuit

Klaus

Hi
Yes i see.
But it semed like the OP wanted a circuit that could automatically adjust to keep the power constant, when changing the voltage upnor down.
 

KlausST

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Hi,

Change the input voltage ... the load voltage and current will be kept constant.

Klaus
 

mshh

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If the voltage changes the opam will compensate the difference so that the voltage is constant again.then the current is constant so the power is constant. I will set the reference power for the op-amp.
 

tyassin

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Hi,

Change the input voltage ... the load voltage and current will be kept constant.

Klaus

I have misread the initial question....sorry.
I thought it was to keep constant power from the power supply.
 

00kam

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Hello
"This is a constant power load If the voltage increased the current decreased and vice versa as attached. the current is ranging from 0-4amp and voltage from0-30v. " etc

The load is pure resistor(6.8ohm) it is a liner load. when the load voltage increases the current will increases
proportional to ohm's law .if the load is non liner the relation between voltage and current is opposite

kamal
 

danadakk

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Easy peasy

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The circuit acts to keep the same voltage across the 6R8 - hence constant current load

This is a bad way of trying to achieve it since the TIP122 can only dissipate 5 - 10 watts safely with a very large heatsink and fan

BJT's fail easily this way - if you must go this way you need an array of 150V 40A mosfets on a large heatsink with fan ....
 

Audioguru

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The 27V across the 6.8 ohms huge resistor produces 4A and the resistor produces 108W of heat. when the supply to the TIP122 darlington is 50V then it will burn up with 92W of heat even when its case is cooled to 25 degrees C with liquid nitrogen or something. Its datasheet shows a max allowed heating of 65W when its case is cooled to 25 degrees C.
 

dick_freebird

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Are you completely sure you want a
linear-topology E-load? Because a new
(-ish) thing in high power test is switching
type "regenerative loads" which take load
power and convert it back onto the main
power supply, DC-DC style.

With a current mode control IC you might
be able to get reasonable load current
control. The noisiness of it is a matter for
filtering, and considerations about whether
the element under test and its measurement
care at all about such things.
 

danielcharles

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Hi, I am Daniel

So below are some of the important parameters to keep in mind while selecting a transistor.
  1. Type number. ...
  2. Current Gain (β) ...
  3. Collector-Emitter Voltage (VCEO) ...
  4. Emitter-Base Voltage (VEBO) ...
  5. Collector-Base Voltage (VCBO) ...
  6. Collector current (IC) ...
  7. Total Power Dissipation (Ptot)
 

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