Circuit & Component Check

I have several. I do not necessarily trust these. Note that some of them currently available on the Net (I just found one) specified Vce in units of amperes, for instance. In some cases, the units are mixed up. Thanks for the link, John. I do not have this particular one, and scanning through it, I do not see this error. Indeed, there appear to be some numbers I have not seen. On the other hand, note that it is not clearly stated that it is even an NPN at all. This appears further down, in the sheet, but it is almost inadvertant. Further, this spec sheet does not show any internal archecture and does not seem to state whether or not this is a single NPN or Darlington config. In the case of the 2N3055, there are so many newby's, this little bit of extra blurb would be useful to quite a few customers.

So what do you make of the problem of getting the 2N3055 to shut off? Could it be damaged?

Dominic

One thing that does not make sense is that Danny gets a varying low. I would think low should be right at ground potential for this circuit.

I think BC108 is not a resistor, but a small signal NPN, about 0.1 Ic and 300 Mhz with a large Hfe.

Dominic

It was bad code - it was only on part of the time, and my meter was averaging the high/lows!

I can't think that your 2N3055 is good, unless you have reversed the base and emitter pins or something similar. It has a lot more charge storage than the smaller transistor, so it is harder to turn it on and off quickly, but at this frequency, that should behave very similarly, except for leakage and rise and fall time. What you describe is not explained by the difference in the two types.

John (and others),

The circuit is 555 in classic astable mode. In one example circuit:

Timing cap, Ra and Rb give 1 kHz 7 volt square wave with about 65% duty cycle at pin 3. This is confirmed by oscillscope. Supply is 7 volts (Vcc1).

Pin 3 goes (series) to 100 Ohm resistor followed by 20 kOhm pot followed by NPN transistor base. The load is an 8 Ohm 0.25 Watt speaker in series with a 500 Ohm resistor and receives power from supply that is 20 volts (Vcc2) relative the 555 ground:

Pin3 ----> 100 Ohm ----> 20k pot ----> NPN base

Vcc2 ----> 500 Ohm ----> speaker ----> NPN collector

Emitter goes to ground

BF658 as transistor (small signal type) can give loud sound by tuning the pot. Oscilloscope shows current going on and off.

BF658 does this:

Swing is from Gnd to 16.2 V.

16.2 ___ ___ ___ | | | | | |

Never drops to ground potential. It can in some cases go to roughly

Dominic

Definitely a typo. Since Hfe is not an inherent property, but a result of a combination of factors, a meaningful Hfe spec always includes both a collector current (Ic) and a collector to emitter voltage (Vce).

By "normally" I assume you mean as a switch.

Higher beta implies that you need less drive current into the base to produce a given collector current. But you can usually provide excess base current to handle the high current case, without messing the low current case up, too much. The usual 'cost' is longer turn off time. Driving any switching transistor with a large voltage and large series resistor approximates a current source, which aggravates the turn on and turn off time problems. Slightly more complicated drive schemes (adding a base to emitter resistor to drain the stored charge out faster at turn off and paralleling the series resistor with a small capacitor to drive the transitions harder) can result in significant efficiency improvements.

Sounds about right.

if

If you run the 555 from a divider, it will have a hard time providing any significant base drive current without collapsing its supply. I would replace the divider with a 5 volt regulator.

That said, if you have a 100 ohm resister between a 5 volt powered 555 and the base of a grounded emitter NPN transistor, that leaves you with 5 volts minus pull up saturation drop of the 555 minus the base emitter drop of the transistor, across the 100 ohm resistor. Lets say that the pull up saturation voltage is about (based on the middle left graph on page 5 of

If the transistor has a saturated gain (remember, you get a lot less gain when the collector voltage gets near or below the base voltage) of something like 20 to 50 that allows a collector current of somewhere between .54 and 1.35 amperes. But the 555 supply has to be able to deliver the 555 consumption and the 27 milliamperes of output current while holding the supply steady at 5 volts.

Is all this making sense?

Thanks John!

I suspect it is bad, and I'll plan to try a new one. I do not think the spec sheets, combined with the info I got with the transistor when I bought it are very ambiguous, so wiring is certainly correct. The overall circuit is correct since it works with all other transistors, taking into account Ic and Ib, etc for each transistor.

Regarding the spec sheet you mentioned:

I could not help noticing that here too, DC Current gain (hFE* in Electrical Characteristics section) is specified as Vce = 4 Amps. Is it is ever correct to state Vce in amps or typo? This is confusing.

For the next level of complexity, I believe that the low beta of the

Dominic

I usually divert at least 1/10th of the base drive from the series resistor to the emitter, assuming about .6 volts Vbe. For example if you have 2.7 volts across your base resistor for a drive of 27 ma drive current, I would put a resistor no higher than .6V/.0027A=220 ohms base to emitter. THis will drain charge out of the base, even if the 555 output does not go all the way to zero volts (and it won't while current is passing back to it from the base).

The optimum capacitor value depends on several factors, including the base stored charge (smaller transistors generally have smaller stored charge, so using the biggest one you can find does not improve everything), the driving signal swing and rise and fall time, and the pulse width. You don't want this cap to drive the base so far into reverse bias that it breaks down the emitter junction, or it will slowly degrade the current gain if the transistor. If this happens, you can clamp the reverse voltage with a signal diode base to emitter.

And the series resistor and the paralleled speedup capacitor have to have a time constant less than the minimum on or off time so things get back near center before the next transition has to be handled. I usually look at the base voltage waveform with a scope and select a capacitor that causes Vbe to go at least to zero on turn off (all the base charge sucked out before the resistors take over to hold zero volts), after I have gotten the two resistors selected to provide the required saturation voltage, Vce, during the largest load current. If the driver swing is very large, compared to Vbe on, I sometimes put a low value resistor in series with the speed up cap to limit the peak driver current to a safe value during the transitions.

The whole idea of this network is a recognition that there are two current requirements for base drive. One is the DC requirement to force the transistor to maintain a low Vce during the on time, at the highest load current and the other current is required only at the transitions to drive stored charge into and back out of the base emitter junction to produce fast turn on and turn off for high efficiency switching.

OK. As you can already guess, I have a couple of batteries in series that yield 20 volts and I now have a voltage divider. My 555 now gets, and outputs about 5 volts, and the speaker, drawing from the top of the divider through collector gets consirably more. Even the 5 volt output exceeds the requirements for any of the transistors being considered here.

Dominic

I actually did have a plan to use an LM317 to regulate the 555 Vcc at 5 V. Another way I have run this circuit is with separate 9 volt (to 555 Vcc) and 20 volt or more (to coil) supplies. Point of exercise was to give a higher voltage to the primary winding of a step up transformer than the

At this point, I can get my transformers to work OK in the low kHz (1-2 kHz). I have also been thinking to try driving them via solid state or induction type relays. For higher frequency transformers (more than 30 kHz), a transistor would be needed because it would be difficult or impossible to use a relay. But at 1-2 kHz, most relay types look like another option.

I need some time on the rest. I think I have not been taking into account the pull up saturation voltage of the 555. Until now, I have not really understood this detail and have relied on a potentiometer at the NPN base and using this to tune the circuit for best output. I did note about 2.7 volts as you mention at the resistor and wondered how it came about. I need to read up on this.

Dominic

I have actually seen a couple circuits in which this was done, long ago. No hint was given about why these components were in this config or how to estimate values. Any idea where this is documented, and in particular, with some idea how to calculate the resistance and capacitance needed?

Dominic