Linear voltage regulator datasheet question

[c_mitra]

I put the value at 6.8 -> 8.2 ohms for a 3A max transistor. Does that sound about right before I go and solder them in?

Major part of the current passes through the collection -emitter path (ignore the base for the time being as a first approximation)- 3A passing through 7 ohm will produce a voltage drop of 21V. This is therefore too high value. The potential at the emitter will be roughly 21V and the base need to be higher still...

You need a resistor in the range of 0.5 ohm (use a 2-3W capacity). Also see post #18.

 

[boylesg]

Major part of the current passes through the collection -emitter path (ignore the base for the time being as a first approximation)- 3A passing through 7 ohm will produce a voltage drop of 21V. This is therefore too high value. The potential at the emitter will be roughly 21V and the base need to be higher still...

You need a resistor in the range of 0.5 ohm (use a 2-3W capacity). Also see post #18.

Plain old V=IR huh. That other formula I found, involving 70% of the transistors current capacity, is garbage then!

 

[d123]

Hi,

Without being too sure of how much current has to pass through the regulator, an alternative to paralleled transistors (pcb space) is a high current linear regulator, or better yet an LDO, you can get 5A ones

e.g.
LM138/LM338 5-Amp Adjustable Regulators
LM1084 5A Low Dropout Positive Regulators
MIC29150/29300/29500/29750 High-Current Low-Dropout Regulators 1.5A to 7.5A

There are a lot more to choose from. Not sure if a switching regulator is adequate for that part of your circuit instead.

 

[c_mitra]

Without being too sure of how much current has to pass through the regulator, an alternative to paralleled transistors (pcb space) is a high current linear regulator, or better yet an LDO, you can get 5A ones

e.g.
LM138/LM338 5-Amp Adjustable Regulators
LM1084 5A Low Dropout Positive Regulators
MIC29150/29300/29500/29750 High-Current Low-Dropout Regulators 1.5A to 7.5A

There are a lot more to choose from. Not sure if a switching regulator is adequate for that part of your circuit instead.

I strongly recommend this solution.

 

[boylesg]

Hi,

Without being too sure of how much current has to pass through the regulator, an alternative to paralleled transistors (pcb space) is a high current linear regulator, or better yet an LDO, you can get 5A ones

e.g.
LM138/LM338 5-Amp Adjustable Regulators
LM1084 5A Low Dropout Positive Regulators
MIC29150/29300/29500/29750 High-Current Low-Dropout Regulators 1.5A to 7.5A

There are a lot more to choose from. Not sure if a switching regulator is adequate for that part of your circuit instead.

I thought about obtaining a high current linear device but they were around $15 each but I had a more common 1.5A device and a tonne of transistors to choose from. I figured I may as well just use learn to adapt what I already had.

 

[c_mitra]

I thought about obtaining a high current linear device but they were around $15 each but I had a more common 1.5A device and a tonne of transistors to choose from. I figured I may as well just use learn to adapt what I already had.

Of course you always use what you have. If this is for a one-off project, that is. Cost is always one important consideration for all projects (unless some one else is paying).

 

[d123]

Hi

I thought about obtaining a high current linear device but they were around $15 each but I had a more common 1.5A device and a tonne of transistors to choose from. I figured I may as well just use learn to adapt what I already had.

Aside from what we all have to do: adapt what components we have to the circuit... I don't know where you saw those prices, outrageous, that is three times the normal price for a fixed 5V/5A Micrel - and Micrel is slightly pricier than other brands, yet look at this electronics retailer's prices for the 5A adjustable TO-220 and TO-263 - 7GBP, VAT included...