Help with transistor based switch

Thread Starter

cbennet1

Joined Jul 9, 2018
3
Hi All,

I purchased a small electronics kit for the expressed purpose of learning basic electronics. As fate would have it, the necessity has arisen ahead of the knowledge. :)

I replaced a home audio processor with a new one that only has a 12v 40mA trigger - vs the previous 250mA. I have a triggered outlet that needs 100mA or greater to turn on an amp and cooling fans.

I see a 2N2222 transistor in my aforementioned kit and played around with building a switch to close the circuit on a 12v 1A source (I was experimenting with a 12v 1A transformer but I planned on replacing with a 300mA model assuming that it would reduce heat) triggered by my 12v 40mA processor trigger but at the very least I'm clueless about the correct resistance values to use on the Base (and collector?).

I tried using a variable resistor on the collector and I could get ZERO volts and ~9v but it appears I could not get full saturation. I've perused the calculations for determining the correct voltage/currents to achieve the desired behavior but I'm fairly certain by the time I figure it out, my new processor will be obsolete! ;-)

I could go the relay route but a digital solution seems so much more elegant, reliable, and I'm sure simple to a knowledgeable person!

Any help would be appreciated.

Craig
 

crutschow

Joined Mar 14, 2008
25,247
You should be able to use the transistor as an emitter follower to increase the current.

Just connect the collector to the +12V supply, the base to the signal source, and the emitter to your triggered outlet, likely no added resistors needed.
 

Thread Starter

cbennet1

Joined Jul 9, 2018
3
You should be able to use the transistor as an emitter follower to increase the current.

Just connect the collector to the +12V supply, the base to the signal source, and the emitter to your triggered outlet, likely no added resistors needed.

Would this be irrespective of if I use a 1A or 300mA transformer on the collector?

Thank you!
 

AnalogKid

Joined Aug 1, 2013
8,534
Assuming "my 12v 40mA processor trigger" is in fact a +12 Vdc regulated output capable of supplying 40 mA, then the required 100 mA has to come from somewhere else. Continuing the assumptions, that should be a *regulated* +12 Vdc source capable of delivering greater than 100 mA.

Since the thing being "triggered" draws only the current it needs, there is no thermal advantage to a smaller transformer size. Note that for the same output power, a smaller transformer will get warmer; but this is due mostly to the size difference.

What is the kit you have, and what are the components? to build your own 12 Vdc power supply you will need

line cord
switch
fuse
transformer
diodes (2, 4, or bridge, depending on the transformer)
filter capacitor
regulator (suggest LM7812)
Output capacitor
output cable

It will be easier to get a generic switching 12 Vdc wall wart on ebay.

ak
 

Thread Starter

cbennet1

Joined Jul 9, 2018
3
Assuming "my 12v 40mA processor trigger" is in fact a +12 Vdc regulated output capable of supplying 40 mA, then the required 100 mA has to come from somewhere else. Continuing the assumptions, that should be a *regulated* +12 Vdc source capable of delivering greater than 100 mA.

Since the thing being "triggered" draws only the current it needs, there is no thermal advantage to a smaller transformer size. Note that for the same output power, a smaller transformer will get warmer; but this is due mostly to the size difference.

What is the kit you have, and what are the components? to build your own 12 Vdc power supply you will need

line cord
switch
fuse
transformer
diodes (2, 4, or bridge, depending on the transformer)
filter capacitor
regulator (suggest LM7812)
Output capacitor
output cable

It will be easier to get a generic switching 12 Vdc wall wart on ebay.

ak
I have a 12v 1A AC transformer to supply the 100mA minimum. What I need is a properly designed transistor based switch to "activate" it when the trigger is active.

If I'm reading crutschow's response correctly, it looks like trigger to Base, 12v 1A source to collector, Emitter to device to trigger, no resistors required. Transistors are cheap so I'll give it a try! :)

Thanks,
Craig
 

crutschow

Joined Mar 14, 2008
25,247
If I'm reading crutschow's response correctly, it looks like trigger to Base, 12v 1A source to collector, Emitter to device to trigger, no resistors required.
Yes.
The main negative of that simple configuration is that the output is one base-emitter drop below the input (about 0.7V) but that's unlikely to bother the device being triggered.
 

AnalogKid

Joined Aug 1, 2013
8,534
As an emitter follower, the 2N2222 will not be saturated. A lightly-loaded 12 V transformer can produce over 20 V after rectification and filtering. If there is no 12 V regulator, the 2222 could have to dissipate over 0.5 W. That's a lot for that small device.

ak
 

crutschow

Joined Mar 14, 2008
25,247
AK has a good point about the DC voltage from a 12Vrms ac transformer rectifier-capacitor being much more than 12Vdc (it will be near the peak AC voltage or 1.4 times the RMS voltage).
You should add a linear regulator such as an LM317 (adjustable) or an LM7812 (fixed 12V) to generate a stable 12Vdc from the rectified DC.
 
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