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Choosing a high gain transistor

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RFtinkerer

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I need to get .02 amp up to near 1.5amp. I am having trouble with calculating the right transistor. I assume I am going to need a Darlington pair to get such a gain. The lb = 18v .02 amp The Load resistance is very low., 7 ohms. The circuit does not need to be protected.
 
So you only need E=IR = 1.5*7 = 10.5V.

What is the power source for the 1.5A circuit?
18V?
 
2 , 9v batteries tied. But I am losing most of my amps to a device that switches a relay. I am trying to boost them back up. I could run the device off of a separate 9v batt. but I am trying to keep the project box small.
 
Look up the discharge rates for 9V Alkaline batteries. You are lucky to get ~0.05A (50mA) out of them. How long do you need the 1.5A pulse to last?
 
2 , 9v batteries tied.
The word "tied" is not an electronic term. Are they little PP3 9V alkaline batteries connected in parallel? Or are they connected in series? Are they brand new and never used before?

I hope they are not cheap carbon-zinc batteries from China that spent most of a year on a boat. In stores they are already leaking on the shelf.

The datasheet for an Energizer brand new 9V alkaline battery with a 300mA load shows its voltage dropping to only 6V in one hour.

A transistor used as a 1.5A switch needs a base current of 150mA for it to saturate with a fairly low voltage loss. You should use a Mosfet that uses NO gate current and has an extremely small voltage loss.
 

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Or go with a 9v nimh battery, they can give you more clout.
Be better with a bigger batt if the energy is high (as mike said longer pulse), if its a short pulse a cap might provide the ebergy needed.
 
I need the pulse to last a full second maybe 1.5 sec. max. When the coil energizes allowing the 18v to pass it heats up a small piece of nichrome wire for a model rocket launcher, so I need the full amperage that I had from the batteries before I connected the 5 volt device.
 
Sounds like a 9v batt isnt enough rather than a trans.
Tip122 or the likes would be a start.
 
Look at Model Rocket Launcher in Google Images. Most use six AA cells or C cells that are big and powerful. A little 9V battery or two in series will be overloaded.
 
Project complete. I decided to power the 5v receiver on it's own 9v battery and a regulator. I just added it to the outside of the project box. So now the 2 9v batteries in series on the inside of the box provide 18v and 2 amps of power directly through the relay, plenty of juice to light one of these homemade nitrocellulose igniters.
18v radio controlled launcher.jpg
 
I betcha the 18V from the two puny batteries in series drops down to maybe 5V when loaded with the 7 ohms load. Then the current is less than 1A and the batteries will not last long.
 
I have the same radio stuff controlling lights on my house, they are really good range wise.
 
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