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24VAC to DC

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Zero999:

--- Quote from: joeyjoejoe on January 26, 2020, 09:18:41 pm ---I need to brush up on transistors/current, but I did find MUN5230DW1T1G with a 1k base resistor and 1k base-emitter.



--- End quote ---
That's more sensible. You need sufficient current to saturate the BJT.

joeyjoejoe:
So on the power side, I've found a more suitable component perhaps.

Power section with reference from TI Power Designer.


My question is on my inductor ( NRS4018T150MDGJ ) selection. It matches the impedance and inductance. On digikey, it lists the inductance frequency test at 100kHz. Does this mean it's not suitable for a 1.1MHz switching supply?

Other options -
SDEM25201B-150MS, 768mOhm @ 1Mhz
LQH3NPN150MMEL, 350mOhm @ 1MHz

(Since this is powered from a 24VAC transformer off mains, I don't care too much about efficiency.)

T3sl4co1l:
Test frequency is just what it's tested at.  They tested at 100kHz and found 15uH +/- whatever, and Q = whatever.

Doesn't mean anything about other frequencies.

Self resonant frequency is more relevant, but switching at SRF isn't a problem in itself, as that's the frequency where the inductor's impedance is maximal (parallel resonance).  It's the best filter it can be, eh?  Not for harmonics, however, and there's usually a second resonance somewhat above SRF which is series resonant (low impedance) which makes some harmonics much worse.  So you typically want SRF more than, say, 5 or 10 times Fsw.  Which is probably fine here.

Tim

joeyjoejoe:
Thanks! Indeed, I notice on their sheet a bit further down the inductance is fine into the MHz.

joeyjoejoe:

--- Quote from: Zero999 on January 26, 2020, 11:20:58 pm ---
--- Quote from: joeyjoejoe on January 26, 2020, 09:18:41 pm ---I need to brush up on transistors/current, but I did find MUN5230DW1T1G with a 1k base resistor and 1k base-emitter.



--- End quote ---
That's more sensible. You need sufficient current to saturate the BJT.

--- End quote ---

I'm trying to learn how to figure out base resistor selection. Using https://www.petervis.com/GCSE_Design_and_Technology_Electronic_Products/transistor_base_resistor_calculator/transistor_base_resistor_calculator.html, and populating an hFE of 3, 50mA load and 3.3Vcc/Vi, it spits out 200ohm.

So don't I need to find a BJT with a higher hFE?

Second, can I put an LED in series with the base resistor to light the LED when turned on as well as switch the relay? Would this be risky if I don't know the voltage drop of the LED (going the chinese route for LEDs), or there a good chance it falls in a standard range, and if there's enough overhead after the voltage drop we're good?

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