Basic Transistor Circuit

[Thirtyniner]

Good afternoon!

I have spent many hours reading about transistor circuit theory but I am still confused so I have come here for some much-needed help!

I want to use a 12v, 20ma (max) source to switch on an array of 12v devices with a 12v supply and ground.

I figure I can achieve this with (let's say) a 100ohm relay, requiring 120ma to energise. Not wanting to load my source too much, would it be a safe bet to find a transistor that will saturate at 5ma, thus needing Hfe > 24?

Furthermore, will a high-side switching circuit be the best format for what I want?

And what value of resistor do I place between my source/input and the transistor base:
One that will limit it to 5ma?
One that will limit it to 20ma?
Or somewhere in between?

I would appreciate any and all input as this has been racking my brain for days and the more I read the more confused I become.

Thanks,

Oliver

 

[barry]

Rule-of-thumb: drive the base with 1/10 of the collector current to ensure saturation. So, you'd need 12 mA. 12V/12mA=> 1Kohm base resistor (neglecting Vbe drop).

High-side switching would require an PNP transistor, which are generally not as 'good' as NPNs. If there's no overriding reason to use a high-side drive (e.g., your load is ground-referenced) then I would just use a low-side NPN switch.