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

[w2aew]

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

We need more information. 
What type of "devices" are you powering up? 
Do they need to be grounded when off (thus need high side switching)?
What is going to drive or control the transistor?

[Thirtyniner]

Thanks for your reply. I shall divulge...

I have an Audison Bit One (12V Digital Signal Processor for automotive use) which outputs a 'trigger' for turning on multiple amplifiers, fans and a 12V DC-DC USB charger (down converts to regulated 5V for powering a Nexus 7, USB hub, powered hard drive).

Problem is, this trigger is limited to 20mA. I want the 'best' (read: easy and efficient) way of achieving this, basically.


Having reconsidered, there isn't any reason this circuit can't appear on the low side. I've been looking for relays with a higher coil resistance but the most common type that are easily accessible are around 80-90ohm, with capacity to switch 30-40A which is right on the money.

I hope that helps!

Thanks again.

[mrkev]

It looks like one power mosfet (like f.e. IRF530, AP23xx, IRLMxx) should be enough. You won't even need whole 20mA, and whole swich can be done with just that mosfet and one resistor between gate and ground to discharge... That is if you don't mind that in the off state, your DC/DC switch is connected just to +12V.

[IconicPCB]

Check out Omron G5V-2-12VDC relay. Coil resistance in high 200s.