Resistance of a Transistor

Hi,

The 3055 Transistor is a hold horse. Has an emitter, collector and base.

You need to use current, to bring it in either a conductive amplifier state, or use it as switch.

I cannot see the diagram, but used as series regulator, count 0.7 volts loss beteen

collector and emittor. More comes when it happens. The resistance does not remain stable at all. That is why it can be used as an amplifier.

Gavilan, The horse was meant to be old instead of hold.

We want to help you. But need more input from you:

What is the load (amps or milliamps) of your led array?

What voltage you want it to work on?

Once you have these answers, we can go from there. Personally I would fix your problem with a stabilized series regulator, that can be made with a zener diode as reference, a small voltage regulator of the 78XX series, some resistors and capacitors, and when you want to adjust it, also a potentiometer.

This post turns out in a guess what game otherwise.

Importing your pic:

When you click on the camera, you need to upload a picture that you have in your computer or on a stick, card or a hard drive, connected to you computer.

When you refer to diagram you can copy it (slide it with the mouse) and paste into your text. Just follow the instructions in the forum window. You will need to use the non standard option. Goes for one paste at the time. (says "bypass custom menu")

regards.D

The MJE 3055 is an NPN Bipolar Junction Transistor (BJT). It is neither a MOSFET or a resistor. If your circuit is supposed to use very little current then a 10 ampere rated power transistor is the wrong choice. You imply that your circuit is a current gain, push pull, class B amplifier circuit. This is a common configuration for complimentary BJT transistors but without knowing the rest of the circuitry I cannot elaborate more. All transistors are non-linear devices. A resistor is a linear device.

A BJT increases current flowing through the collector. When a BJT operates linearly the base current multiplied by the transistor's beta (aka hfe) is the collector current. When a collector to BJT saturates the emitter voltage is a low fixed voltage.

The data sheet you have is more than sufficient information for designing a modest amplifier.

Using the junction to control the current passed thru the colector makes the 3055 APPEAR to behave as a resistor. Its a matter of semantics.

No, it is not a matter of semantics. A resistor has two leads, a BJT has three. A FET has a linear region where from drain to source it acts like a resistor. Current can flow in in either direction just like a resistor as long as VDS does not exceed a critical value. A pivotal reason for this is that effectively no current enters the drain. Current must enter the base of a BJT for the transistor to be considered ON. The current flows only into the collector of an NPN transistor.

A potentiometer has 3 leads as does a tapped power resistor. Semantics being the study of definitions, then please note I said the 3055 APPEARS to have the characteristics of a resistor. Therefore I feel the semantics of description apply. In any case, I have no argument on transistor theory, or anything else. For the record, a 3055 properly biased in a DC circuit can be made to have the same effect as a variable resistor wired in its place ( not pin for pin but emitter to collector ). I use the words APPEARS and EFFECT to point out that they are not interchangeable components, but create the same results in a properly executed circuit. No more, No less.

Thank you all for your input.

In answering the questions as best I can.

The above circuit is used to protect my DIY LED assemblies from the charging voltage; the LED assemblies are designed to operate at the battery voltage. If I operate them when the solar panel is charging the battery from bright sunlight it burns out the LED's. In the past I had just used the P Channel Mosfet to cut power when the photo-resistor was exposed to sunlight; but then I can't turn the lights on in the daylight. I have also used the voltage from the panel to forward bias the gate in stead of a photo-resistor but I prefer the first method.

I am trying to determine what the effective resistance is between the emitter and collector of the MJE 3055T as shown. I want to know this so I can adjust R2 so that the two sources of resistance = (Charging Voltage - Optimum Voltage) / I; but I cant do that unless I know the approximate value of resistance between the emitter to the collector.

When the photo-resistor is exposed to sunlight it switches the P Channel Mosfet off and turns the 3055 T on which switches the variable resistor R2 into series with the assemblies.

Each assembly draws about 20 mA of current. The optimal voltage for the 4 series LED's is 12.8 volts (3.2 V * 4) - not far from the full charge state of the battery.

When I tested the circuit, I tested it under subdued sunlight - it was cloudy. Also, the capacitor was not in the circuit.

I put a VOM in series with the source voltage and used another VOM to measure the no load voltage across the leads at the Assembly while I exposed the photo-resistor to darkness and bright light.

When the supply voltage is 12.99 I get the following readings from my VOM's. When I forward bias the transistor through a photo-resistor.

When the photo-resistor is exposed to bright light the no load voltage at the light assembly leads is 11.85 V. (V1) The current at the circuit fuse is .137mA. (I1) (In the 2m scale =.137)

When the photo-resistor is not exposed to light the no load voltage at the light assembly leads is 12.84 V (V2) and the current at the fuse is 053 mA.

R2 is set at 0 ohms.

C1 was not in the circuit as tested and is used to maintain a momentary bias on the G/B bus if the voltage to the bus encounters momentary interruption.

Will the resistance from E to C change significantly as I vary my load by adding or subtracting parallel LED assemblies?

Just out of curiosity, where did you get this circuit.

It doesn't make any sense when I draw it out. Having the Gate of the FET connected to the Base of the Bipolar (3055) doesn't make much sense.

Johny451:

I just built it and it seems to work fine. Its been a work in progress for quite some time.

The Gate of the MOSFET and the Base of the bipolar are on a common bus so that when the resistance of the photo-resistor drops due to exposure to sunlight both are forward biased. This causes the normally closed switch (P channel MOSFET) to open stopping current from flowing to the LED assemblies through that path; and causes the normally open switch (3055) to close rerouting the current through it and R2.

It seems to work quite well on the bench.

yeah it's followed by 1meg resistor 12µA
.model 2N3055 NPN(Bf=73 Br=2.66 Rb=.81 Rc=.0856 Re=.000856
+ CJC=1000P PC=.75 MC=.33 Tr=.5703U Is=2.37E-8
+ CJE=415P PE=.75 ME=.5 TF=99.52N NE=1.26 IK=1 Vceo=60 Icrating=10 mfg=STMicro)
x73=876µA (unless the 200µF thingy charges)
such making it very speciffic circuit
the collector resistance along with R_CE shunts the gate ... it's like regulated/adjustable intensity...
i don't see why the op needs the R_CE for (& i'm not interested)

Dear CI139;

Your efforts are greatly appreciated; but I failed to note that my level of understanding of this is somewhere between that of a great ape and a 5th grader. I THINK I understand basic principles - but often times I find that what I THOUGHT was not how things really are.

On the bench my little circuit SEEMS to work; and when I am done tiling my office I will take the time to put it to application in my little out building I use to experiment in. I have a small solar powered lighting system I can test it on.

If the simple circuit works (simple is good) then that is a good thing. If it doesn't then I will try something else; eventually I should be able to come up with something cheap, easy, and robust.

The idea is to come up with something that can be self assembled by folks in the third world that need some cheap and robust lighting - using components that can be delivered through the US mail interfacing with their local mail service.

It is to be operated at the most commonly available voltage - 12 volt car and motorcycle batteries - using the most commonly available and least expensive solar panels - those designed for charging 12 volt batteries.

Hi Gavillan,

The more complicated you go, the worse. If you want something with minimal parts that easily fits in mail packages, provide 3 things:

1. In full sunlight measure the voltage supplied by the panel.

2. Same measure the current that the panel supplies when shorted. Or you put an Ampere meter in series with the short, or you measure with a clamp- on DC ampere meter over the short wire.

3. Ah of the battery to apply.

When you come back with these 2 values, we can take care of:

1. A charging circuit for your battery.

2. A current regulator for your LED array.

In 1 simple diagram.

Thanks again DVM:

The idea is to have the simple board assembled by the kid whose building the lights for his village.

The circuit should be adaptable - without alteration - to ANY panel or charging system designed to charge common car or motorcycle batteries.

The circuit should be adaptable to any 12 volt car or motorcycle battery; regardless of amp hour rating. For instance; the kid using the 12 volt solder gun might want a larger Amp Hour battery than his brother who only uses it to light is doorway, dock, or fish cleaning house. One, simple circuit.

I appreciate all the information made available to me.

No fix necessary- the "resistance" of the transistor is adjusted automatically here.

It is a little difficult, but feasible:

12 Volts solar panels operate with up to 22 Volts open circuit (100 Watts types and more) For smaller solar applications not often a (more expensive) MPPT (maximum power point tracking) is used, but more a series regulator.

(The 3055 needs a big cooling plate for 100 watts) The transistor takes the voltage between e.g. 22 Volts and say 14 volts (and dissipates the heat = loss) A MPPT regulator doesn't have this kind of losses, but costs 300- 500% more. For small systems it is cheaper to work with a oversize panel that compensates for these.

The panels for car, boat, e.g also do not give the 22 volts, but more somewhat between 15 and 17 volts (open circuit)

The circuit I provided can take the solar panel and provide a charge to the battery. However the 12 volts output is not enough to charge the battery. Ideal would be a value between 13.6 and 13.8 Volts. Normally batteries are charged up to 14.4 volts, but keeping this value will overcharge the battery. (becomes hot and runs dry)

With the circuit, your battery will be charged at 13.6- 13.8 Volts. And when the capacity is used up and there is no charge, the voltage will drop down to 12 volts and lower. The 7812 is best replaced with a higher voltage type, or can be fixed with a zener diode or 2 or 3 diodes, to cheat the design voltage.

Sorry, but where I am there are no electronic components to choose from. (that makes you act surviving practical -)

The trick is to work the battery not too low in voltage also.

Your LED array, When you arrange the Leds in groups of 4 (If possible) you can feed these straight from the battery (with the resistor) otherwise you can use the posted circuit with the NPN transistor to get a constant current (provided by mjb) for this part.

Strictly when the LEDS can be rearranged this is not necessary.

Good luck

g.d. bloody trivia (i coudn't match your mosfet to any better fn.)

(nothing to do (muhahaa))

Dear ci139;

Thank you so much for your comments and time.

That's great news ? I think?

Gav

4.5mA idle/standby current being a grat news?
LT1716
LT1017...
_{otherwise it's aprogressive warmup/err.-red.}

Much better

(src Speeches of the President.txt)
this is your president speaking - zazazazazazazazazazazazaza - dolly holy mol-e - mo-chos cho-chos - moochoz craaszhias - thank-yoo ... thank you
(paste to "Text To Speach")

Hi Gavilan.

I am not getting further in this diagram, but to answer your question:

Measure the voltage between the - of your circuit (common) and the Emitter and also between the - and the collector.

Measure the current used by you LED array when on.

Subtract the lowest value of the voltage measurement from the highest and divide by the current drawn by the LED array,

Bring the voltages in Volts - e.g. Ve and Vc and put the current in Ampere (A)

The formula for your resistor equivalent is about = (Ve - Vc) /I led

Value in Ohms.

There will be a slight difference, since your circuit and transistor uses also a small current (neglectible)

You can also use a zener diode and not have to worry about too high voltage in the future. The circuit you have now is not very stable when the battery is charging.

The voltage changes will be noticeable in the LED current too.

This schematic will do the trick. It actually works, beginning at 13.1 volts. C1 is not critical at all (everything above 200 microFarad is OK) Don't know what solar panel you use and what the output is. I also see no battery charger in your circuit.

DVM:

Thank you; I will give this a try.

MJB:

Thank You for the considerable time it must have taken to formulate your reply.

"BJT (Bipolar Junction Transistor) is really a dependent current source. Trying to use it like a resistor ---- "

My intent was not to use the BJT as a resistor; but as a switch to route the power through a resistor when the Gate/Base bus is forward biased. (I think that is the right expression.) But the transistor does contribute resistance to the circuit. I am just trying to determine how much resistance is contributed by the BJT and whether that resistance will remain somewhat stable as I add or remove LED assemblies from the circuit.

A bipolar transistor like the 3055 does not have a fixed resistance. This kind of transistor is a current controlled current source. A small current in the base-emitter circuit controls a much larger current in the collector-emitter circuit. The ratio of these two currents is a fixed value for the transistor called the beta or hfe. You have to use the right model to get meaningful results.

Rixter:

Thank you for taking the time to comment.

I am using these transistors as simple switches.

What type of transistor; when used as a simple on/off switch - has a fixed resistance?

Thank You.

"

What type of transistor; when used as a simple on/off switch - has a fixed resistance?

"

- Used as a simple on-off switch, any transistor would be operated in saturated mode; the voltage drop across the transistor would then be close to the spec sheet rating for saturation voltage drop. The " resistance " would then be determined by ohms law R=E/I. ( simplistically ). For purposes of argument, this resistance would be repeatable for a given voltage applied across the device. Unless the device characteristic would be degraded by loading or heat or damage due to abuse, the "resistance " should be repeatable, or to use your term inaccurately, "fixed ".

A Field Effect Transistor in saturation would approximate a fixed resistance. For example, see the Rds parameter in table 5 in this data sheet.

http://www.nxp.com/documents/application_note/AN11158.pdf

Rixter;

Thank you very much for this. It is going to help me understand transistor data sheets much better. I'm going to download it to my Kindle so I can spend some quality time with it.

Do most data sheets use the same parameters for similar devices?

Thanks again for your time.

GA from me.

This is a strange looking device.

Why does it have 3 source leads?

Also; in the MJE 3055 t data sheet the channels are labeled Base - collector - emitter.

I am using these devices as on/off switches

In my P channel MOSFET they are labled Gate - Source - Drain.

I understand that I switch on and off by putting bias to the Gate or Base.

I understand that in the P Channel MOSFET I am pulling the power off of the Drain; but I am pulling my power off of the collector with the MJE 3055 t.

Do I have it hooked up backwards? It seems to be working.

I think the MJE 3055 T is a Bipolar Transistor and is current controlled - is that correct?

In this application should I swap it with a MOSFET?

I'd suggest finding a book on transistors. The book can explain it a lot better than I can.

Very briefly, the way a Field Effect Transistor (FET) works is that there is a channel made up of a semiconductor. The conduction (inverse of resistance) of this channel is determined by how many current carriers are present (free electrons in n type, vacancies in p type). Carriers are attracted into or repelled from the channel by a voltage applied to the gate, and thus the resistance is controlled.

A bipolar transistor is controlled by current through the base-emitter circuit. A small amount of current in this circuit allows a much larger current to flow through the collector-emitter circuit.

As you can see, they are both called transistors, but operate in a different manner.

Here's another simple, but precision current limit circuit. Current limit not only protects the LED's, but can also extend run time from battery. You should add a 10 ohm resistor to balance current in each series string, as mjb showed. I am unable to post images using Chrome, but Firefox works fine.

I think your question about resistance vs. transistor was answered by previous posts.

It seems the question wasn't fully answered. The (BJT) transistor, when totally on will conduct like a diode, with an almost fixed voltage drop. The forward resistance of a diode is, by Ohm's law, the voltage across the diode divided by the current through the diode. Note that, since diodes are non linear, the effective resistance is not. The resistance of a Mosfet, when fully on is fixed.

I want to thank all of you for taking time from your busy schedules to assist me in this matter.

It was greatly appreciated.

I believe I can now - measure and calculate the resistance from emitter to collector.

AND that I might want to try a N Channel MOSET in place of my MJE 3055 T in an attempt to "fix" the resistance of the transistor when the transistor is fully on.

I just used what I had available at the time.