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Is this a logical way to setup 2x LM317 in a driver?

Leodahsan

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Don't listen to anselm. If you want a simple trigger get a SCR. To untrigger, though, you will need a separate button (button 1 - on button 2 - off). I have 800v 55A rated SCRs here. ;)


btw you can use a transistor to trigger a load too..
 

rhd

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The SCR would be problematic in this scenario, because I'm looking to retain the momentary-on behavior of the button.

If I wanted to use a transistor to trigger the LM317 circuit, is there a way to do this that wouldn't require adjustment for different LM317 driver circuits? I'm still getting stuck on this notion of a transistor having a gain factor - which is really not what I want it for.

I'm really looking for as close to an IC incarnation of a relay, as possible.
 

rhd

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Ok - giving this a shot on my own. If I used these:
10 x TIP31C TIP31 TRANSISTOR NPN-SI 100 V3 A - eBay (item 260520413769 end time Mar-03-11 09:56:49 PST)

They have an HFE of 50.

Suppose I wanted to use them in 2x li-ion LM317 drivers. Then suppose that the highest current driver I'd ever be making was 1500 mA.

Then 30mA would be needed to trigger the circuit right?

So if I wired the transistor in series with a 250 ohm resistor, and my ~8V of batteries, that should produce 30mA of current in the base circuit. That would trigger the LM317 circuit (or at least an LM317 circuit I built that was under 1500mA of current)

Does that make sense?
And would it matter that if I used the same setup for an 750mA LM317 circuit, we'd have twice as much base current as actually needed to trigger it?
 

anselm

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As I told you before, you don't need a transistor, even though you can drive it as an
"all or nothing" kinda digital switch.
Simply put your switch in series with the resistor(s) that adjust the current on your LM317.
If that switch is open, no current at all is flowing.
In that branch of the circuit, very little current is flowing even when powering a 445m diode.

You don't need any extra transistor, that LM317 is already full of them, doing for you what you need done.:D
 
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rhd

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Simply put your switch in series with the resistor(s) that adjust the current on your LM317.
If that switch is open, no current at all is flowing.
In that branch of the circuit, very little current is flowing even when powering a 445m diode.
Awesome! This is a way better solution!

However, if that's true, why do we care about 1W vs 2W etc resistors for the current adjustment in LM317s?

Even the handy calculator I always use indicates the power rating necessary for those resistors. If not much current flows, why do we feel the need for a higher wattage resistor?
 

rhd

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Well... I was hoping to use a little momentary SPST switch. ~50mA current. :(
 

rhd

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Interesting ok - I think I follow that.

What transistor values would I be seeking?
 

jib77

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NPN type that can handle the amperage on VIN.

R2 only needs to be 220 Ohm not 10K, I chose 220 b/c:

1) 220Ohms is a very common resistor value.
2) 7.8V / 220Ohms = 0.035A (35mA) which should be safe for the switch and transistor base.
 
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jib77

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Yup, that should work. The important parameter is the max collector current. When used as a ttl switch they do not dissipate much power and their voltage ratings are usually already way above what we are working with.
 

Leodahsan

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^lol I was going to draw that but jib gone fast than me :(

@rhd
why you are buying transistors in ebay? :thinking:
 

rhd

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why you are buying transistors in ebay? :thinking:
Cheap. Easy to pay through paypal without giving yet another site my credit card details. Ditto for "without having to register yet a username on yet another site". Plus add to that list "No decent electronics stores locally" :)
 

rhd

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I guess because at 0.20 cents each, it's not worth my time + I don't have any e-junk laying around?!?
 




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