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Transistors In Parallel

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Overclocked

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I need to increase output to drive an inductive load with a resistance of .6 Ohms, but I only have 1Amp Power Transistors. According to Multisim, I need atleast 5 Amps. I am using a 555 Timer to drive a Flyback from a TV (The flyback works btw) Can I connect transistors in parallel to increase current output? Ive seen it done on some audio amps
 

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Overclocked said:
So then what are the values of those resistors that you have placed? 12 Ohms for 1 Amp per transistor (12V/1A=12Ohms)?

Dont use 12 Ohms.. too big... you only need a single Ohm or less to share correctly at these current levels. I'd try 0.5 to start with..
 
How does Multisim figure a current of only 5A? Then the voltage across the 0.6 ohm resistor is only 3V! Where is the remaining 9V going to go???

I figure that the high level of the 555 is 10.6V, the 1k resistor won't have much voltage drop and the base-emitter drop of the darlington transistors drops the emitter voltage to 8.8V. Then without the equalizing resistors the load current is 14.7A and your darlington transistors will blow up!
 
Overclocked said:
I need to increase output to drive an inductive load with a resistance of .6 Ohms, but I only have 1Amp Power Transistors...

To increase current capability, transistors can be paralelled, but they need to be h21 matched. To lower the effect of not matching and to equalise currents flowing through transistors, you need to put a equalizing resistor series with emitters.

Choose resistor values as small as possible for lower wasted power.
To take effect, voltage drop across equalizing resistors should be comparable with the transistor's Vbe.

Resistors value depends on the current flowing through them, which depends by the number of the paralleled transistors.

I think, in the schematic you posted, you omitted to draw the inductive load which you reffer in text, and you drawed only a 0,6 ohm resistor. In my before reply, I added to your schematic the equalizing resistors, without correcting this issue.

I think that the completed schematic looks like in <555timer2.jpg> attached, or not?
 

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You also need a free wheeling doide to absorb the back emf generated by the coil when the magnetic feild surrounding it collapses.
 
Keep in mind using a current sharing scheme like this is always going to be more prone to failure (in direct proportion to the number of devices you use) than using a single device of the proper rateing (at least with bipolar transistors) If you really want to trust the device you're making bone up the money for a bigger transistor, it's not like they're particularly expensive. Mind you you can go down to radioshack right now and buy an IRF510 power fet capable of 4amps for like 2.50. And that's insanely high for price. mosfets also don't require extra circuitry if you want to parallel them. S'one of the reasons fets are used in high power circutis like this frequently.
 
em2006 said:
To increase current capability, transistors can be paralelled, but they need to be h21 matched. To lower the effect of not matching and to equalise currents flowing through transistors, you need to put a equalizing resistor series with emitters.

Choose resistor values as small as possible for lower wasted power.
To take effect, voltage drop across equalizing resistors should be comparable with the transistor's Vbe.

Resistors value depends on the current flowing through them, which depends by the number of the paralleled transistors.

I think, in the schematic you posted, you omitted to draw the inductive load which you reffer in text, and you drawed only a 0,6 ohm resistor. In my before reply, I added to your schematic the equalizing resistors, without correcting this issue.

I think that the completed schematic looks like in <555timer2.jpg> attached, or not?

Yea, thats the correct circuit, I used a Resistor to stimulate the load. I'll have to try an Inductive load on Multisim and see if its different. Its resistance Varies to some degree, It actually ranges from .6ohms to 1 Ohm. I used .6 ohms for worst case scenario. (I do have a Greater Amperage transistor, but I dont have a lot of them- If It were to blow somehow, I wouldnt have many to replace it)

Heres something Ive always Wondered, Why Are MostFets Better than Transistors? Are mosfets used in the same manner as transistors?
 
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I'll have to try an Inductive load on Multisim and see if its different. Its resistance Varies to some degree, It actually ranges from .6ohms to 1 Ohm. I used .6 ohms for worst case scenario.
You can't go by the DC resistance of the coil to determine the current draw. The inductive reactance of the coil will have a far greater effect on limiting the current through it than the DC resistance.
You should really put the coil in the collector circuit of the power transistors as depicted below:
 

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