1. Home
  2. Hobby Electronics Basics
  3. effect of reverse b/e current over time

effect of reverse b/e current over time

I'm looking at using an astable multivibrator, like the one in the diagram at this link

formatting link
running probably at a few tens of kiloherz to drive a mosfet in a forward converter. I'll probably run the multivibrator somewhere between 12 and 18 volts. That will put a reverse bias on the base-emitter junctions of the transistors in the multivibrator as the base voltage gets pulled below ground during oscillations. I've heard that reverse biasing the base emitter junction affects gain, but that the effect is of limited severity. I can live with the loss of a little bit of gain. But what about the long term effects -- will it damage the transistors, or will they operate reliably for something like their normal lifetime if they are not abused otherwise?

Reply to
kell
Loading thread data ...

I have seen such circuits fail, when the gain fell so low that oscillation stopped. It is a reliable circuit with a 5 volt supply, but one that damages the transistors when run from a 12 to 18 volt supply.

Reply to
John Popelish

I'll put a zener in series with each transistor's base.

Reply to
kell

I would imagine a resistor (comparable to the collector resistors) in series with the base and a diode across E-B to clamp the negative voltage to 0.6V.

I don't know if a series resistor would stop it from oscillating.

Tim

-- Deep Fryer: a very philosophical monk. Website:

formatting link

Reply to
Tim Williams

You could get around the issue of breakdown by using FETs instead - a VN2222LL (TO92 package for through hole) or 2n7002 (SOT-23 SMT) should work fine. They are both rated at Vgs of +/- 20V continuous, 40V non-repetitive pulse.

Cheers

PeteS

Reply to
PeteS

Beta degradation as a result of b-e zenering is a legend, but I've never seen a quantitative estimate of the effect. Somebody could do an experiment or two...

John

Reply to
John Larkin

"John Larkin" a écrit dans le message de news: snipped-for-privacy@4ax.com...

Not an urban legend, at least.

I just have made the experiment with a brand new BC547C.

New, @VCE=2V, IC=1mA, (beta ~ 370).

Carefully zener it (no cap discharge) for 15 seconds at 10mA (BE broke down at 10.1V)

Then under the same conditions, IC dropped to 965uA, a 3.5% beta drop. That's not insignificant at all.

What's more noticable is that as I'm writing this I can see the beta still sligthly dropping to 957uA (more than 4%). Perhaps the starting 965uA benefited from the 100mW heating from BE zenering.

I now have to get my little daugther to a painting exhibition.

More zenering this evening...

--
Thanks,
Fred.
Reply to
Fred Bartoli

Nice, first numbers I've seen on this. Looks like the beta degradation is falling off as the current-time product increases.

I'll have to try this myself one of these days.

I wonder if LEDs are degraded by reverse current.

Thanks.

John

Reply to
John Larkin

"Fred Bartoli" a écrit dans le message de news:43775c95$0$2169$ snipped-for-privacy@news.free.fr...

le

down

So, more zenering...

30s @ 10mA, IC dropped to 950uA -> 5% beta drop; 2min @ 10mA, IC dropped to 938uA -> over 6%.

Time to be a bit more nasty.

1 min @ 30mA, IC dropped to 922uA -> 8% beta drop.

2 hours @ 30mA, IC dropped to 843uA -> almost 16% beta drop.

One noticable thing is that the normal working VBE constantly dropped with the cumulative zenering, from 615mV down to 604mV.

It could also be interesting to see what happens with cap discharge like with a multivibrator but I've no more time now.

Maybe someone else?

--
Thanks,
Fred.
Reply to
Fred Bartoli

"John Larkin" a écrit dans le message de news: snipped-for-privacy@4ax.com...

dans

dans

an

still

with

I've become amuzed with this now and I'm emulating the 20V supply astable case with a 50ohm/20V generator @ 10kHz, and a 22nF cap. Ouch, about 160mA peak zenering current. Probably about what it'd be in such a circuit.

10 min at this regime and beta dropped further down to -18% 30 min later again brings us down to -20% and 603mV VBE.

I don't know which region of the BE junction is zenering first and what the degradation mechanism is. Maybe Jim has some knowledge about this.

When I've time I'll open a TO18 or a TO5 (if I can still find one) and try to have a look.

You'll have to be pretty persuasive with LEDs. The ones I've tried happily withstood 60V with under 10pA leakage IIRC.

--
Thanks,
Fred.
Reply to
Fred Bartoli

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.