astable multivibrator semiconductors

For an astable multivibrator, would 2n7000 mosfets be more efficient than 2n3904 bjts?

Reply to
mrdarrett
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How long is a string?

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Reply to
Tim Wescott

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Reply to
mrdarrett

But what's the context? Most people wouldn't fuss over efficiency. In a handful of instances it might make sense.

Michael

Reply to
Michael Black

Without any more detail than that, no correct answer can be given, any more than one can answer "how long is a string?".

Post a circuit, and what you're trying to do with it, and we'll try to help out -- even if we have to start by asking for more detail yet.

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Tim Wescott 
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Reply to
Tim Wescott

A basic astable multivibrator:

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R1=R4=10k R2=R3=10k C1=C2=0.1uF Q1=Q2=2n3904 or 2n7000 (which is why I asked)

I'm tapping into the node between R4 and Q2's collector to run the gate of a MOSFET (irf530) to power a blower motor.

Michael

Reply to
mrdarrett

Oh, and +V is between 12v and 18v

Reply to
mrdarrett

Sorry, R2=R3=60k

Reply to
mrdarrett

If that circuit ever got into a situation where both transistors were turned on, they would saturate, both their collectors would be sitting at low voltages, and the base current delivered by R2 and R3 would be sufficient to keep them saturated.

Moreover, that circuit depends on the base current of a BJT to work: with a pair of FETs, the average voltage of the gates will rise to equal the supply voltage, they'll both end up turned on hard, and the circuit won't work.

You could come up with more elaborate biasing arrangements -- but why?

A 555 will work at 12V -- why not use a 555 and a 12V linear regulator? You should get plenty of drive to your MOSFET gate, and you'll have a circuit that won't fail 0.1% of the time, while leaving you unable to diagnose problems the other 99.9%.

To answer your original question: the base current of the BJT's is small compared to the power getting burnt in the resistors; the reduced power consumption (if any) would be lost in the noise of component variations from unit to unit.

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Tim Wescott 
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Reply to
Tim Wescott

Ah, ok. So, to summarize, did I understand this correctly:

1) the astable multivibrator circuit with discrete components requires BJTs to operate properly; the system will not oscillate with FETs 2) there is a 0.1% probability of the system failing to oscillate; use a 555 to avoid this 3) to reduce power consumption, increase R1 and R4

Why... I just get a kick out of using discrete transistors instead of an IC. That's why.

Thanks!

Michael

Reply to
mrdarrett

THAT circuit needs BJTs. You could make it work with FETs, at the cost of having to use more resistors.

Again, you could make the circuit more elaborate and stick with transistors -- but a 555 will just get 'er done.

I know. IC's are boring. But they're a "I haven't been screamed at by a customer for months now" kind of boring, which to my mind is a _good_ kind of boring.

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Tim Wescott 
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Reply to
Tim Wescott

Oh, alright. :) I was bored while on vacation a couple years ago and modified an astable multivibrator into a PWM motor controller. I thought it was kind of cute, just using two 2n3904s (and of course a power mosfet for the motor).

Noted. Failure to oscillate could be bad. For experimental use only :D

Michael

Reply to
mrdarrett

Most people would not fuss over efficiency? :D maybe I am just strange then.

Reply to
mrdarrett

With 12 volts or so Vcc, you'll be zenering the bipolar b-e junctions, which is A Bad Thing. Spice will typically not model b-e breakdown.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Oh! Ok, thanks.

Reply to
mrdarrett

It's actually mentioned in the Wikipedia article he cites. The solution is protection diodes so the B-E junctions never go more than one diode drop in reverse -- which means even more parts on the thing.

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Tim Wescott 
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Reply to
Tim Wescott

No, considerably less efficient. One cannot get as good power gain from a MOSFET with very low drain current as one can from a bipolar transistor. So, for any target transition slew rate, a power advantage will accrue to the bipolar design variant. You also get lower tolerances on turn-on threshold voltage (so can design for lower power supply voltages).

To play with these ideas, get LTspice or some similar simulator, and plug in some device models, and ... start evaluating designs in simulation. In particular, for power efficiency, crank down the applied voltage or raise all the resistor values, until something breaks.

Reply to
whit3rd

Thanks!

Reply to
mrdarrett

...

Ok, so I should stick with 6V max Vcc then?

Absolute max Vebo = 6V, from here.

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I'm surprised I didn't barbecue the 2n3904s then...

Thanks!

Reply to
mrdarrett

Which can be corrected in Ltspice :)

I found a couple of circuits in my junk folder that do not work as planed in the sim afterwards.

I inserted that correction (Spice Directiove). So I think it is something that should be in the models. oh well..

Jamie

Reply to
Maynard A. Philbrook Jr.

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