Micro Phototransistor datasheet.

[kal.a]

I decided to put it together on a bread board and play with it to get the most benefit. I'll do that tomorrow after IO pickup the transistors and caps. Meanwhile I have a couple of questions:

1- The zener diode I had across the led was to protect the led from over voltage. Anything over it's break-down voltage of 5.1 would not get through. Is that not how it would work?
2- The voltage at the collector is it going to be 16.39V?
3- What would happen if I remove the transistor from the circuit and connect the mosfet with its zener diode, cap and gate stopper wouldn't that work ? ( without regard to light of or dark on)

 

[spec]

I decided to put it together on a bread board and play with it to get the most benefit. I'll do that tomorrow after IO pickup the transistors and caps. Meanwhile I have a couple of questions:

1- The zener diode I had across the led was to protect the led from over voltage. Anything over it's break-down voltage of 5.1 would not get through. Is that not how it would work?

Afraid not. If the LED had 5.1V across it, it would either be taking an incredible amount of current or would be blown open circuit. You cannot define the forward voltage of a LED, or any diode for that matter (there is much more to this but this rule of thumb is correct to a first approximation). Instead all you can say is that at a certain current the VF will be in a certain range. You protect the LED by limiting the current. The Rolls Royce approach would be by a constant current generator. But a simple series resistor gives more than adequate protection in most cases. The other thing you can protect against is reverse voltage. You do that, if you have already got a current limiting device, by simply putting a reverse connected diode across the LED. A 1n4148 type small signal diode in normally adequate but a IN400x type diode can be used if you need to protect against higher reverse voltages. In practice you rarely need to protect against reverse voltage.

2- The voltage at the collector is it going to be 16.39V?

The voltage on the collector of the opto receiver transistor will be whatever the Zener diode voltage is. ie 15V

The voltage on the collector of Q20 will be around 0V when there is no obstuction in the opto slot. When there is an obstruction Q20 collector voltage will be the Zener diode voltage (15V). In the former case the MOSFET will be turned off and the motor will not be energised. In the latter case the MOSFET will be turned on and the motor will be energised.

3- What would happen if I remove the transistor from the circuit and connect the mosfet with its zener diode, cap and gate stopper wouldn't that work ? ( without regard to light of or dark on)

Nice thinking. That would probably work. But with light the opto recever transistor would be conducting possibly a minimum of 500uA so only 5V would be developed across R22 (1oK). This would not be sufficient gate voltage to turn the MOSFET fully on. If the particular opto sensor transistor produced over 1.5 mA that would result in 15V across the 10K resistor and the MOSFET would be fully turned on.

The only problem is that with 1.5 mA flowing through the opto receiver transistor there would be 0V ECV on the opto receiver transistor so you would be asking it to operate outside it's spec sheet recommended operating conditions. Given an average opto receiver transistor the circuit would probably work OK and generate about 14V on the gate of the MOSFET which would probably be enough to turn an average MOSFET on. It would not be a proper toleranced, worst case design though. In production you would get batches that would not work.

Incidentally, it would probably be better to increase the Zener voltage from 15V to 18V in any event. The limit is 20V defined by the maximum GS voltage of the MOSFET (see data sheet). This would give the MOSFET only design more chance of working.

 

[kal.a]

So I went shopping for transistors could only find BC547 but while searching I found an NTE equivalent transistor which I did not buy because I couldn't understand the datasheet
I see the absolute maximums but where does it say the On and Off voltages. How many volts to turn it on and how many volts for it to turn off?

In the datatsheet for the BC547 there's a "Base-Emitter On Voltage" is that it?

Thanks
Kal

 

[spec]

Hy kal,

You have my sympathy because there are literally thousands of NPN small signal silicon bipolar junction transistors (BJTs) available that all do a similar job. Why is that so. Mainly historical.

At https://www.electro-tech-online.com/threads/a-hard-to-resist-problem.146690/ post #15 I have tried to make a list of items that someone doing electronics might need. Two items are:
(1) Small signal NPN BJT= BC546
(2) Small signal PNP BJT= BC556

The only reason why I specified the BC546 is that it has a maximum VCE of 65V, whereas the other members of that family of transistors have a lower maximum maximum VCE. Thus the BC546/BC556 transistors are liable to suit the greater number of applications.

The other important point is that these two transistors are complimentary, which means that for all intents and purposes they have identical characteristics and the only difference is sex (PNP is considered to be female, but not officially).

There is nothing special about these two transistors. In fact, quite the opposite; they are as common as muck and dirt cheap, all reasons for choosing them as a default standard transistor

So you might buy a 100 of each to go in your electronics store and whenever you need a small signal transistor you simply use a BC546/BC556.

Incidentally, although my circuit shows a BC547, in the ERRATA I say that a BC546 should have been specified. Why? Purely because the BC547 was a typing error and I meant to type BC546, but in the opto sensor circuits the maximum voltage is 24V so the BC547 is equally suitable.

Tomorrow though, you may be working on an design, say an audio amplifier, that has 60V supply lines. Then the only transistors that would be suitable would be the BC546/BC556.

So I went shopping for transistors could only find BC547 but while searching I found an NTE equivalent transistor which I did not buy because I couldn't understand the datasheet

That is not surprising because transistor data sheets are either packed with every parameter imaginable, each parameter having an impenetrable abbreviation, or the data sheet is sparse and lacking. In theory, you cannot use a transistor that has a deficient data sheet because you have no idea how the transistor will perform. In practice, you can make an educated guess.

The good news is that, for most applications, you only need to worry about a few fundamental transistor parameters to check if a particular transistor is suitable for your application.

I see the absolute maximums

Good; that is half the battle won!

but where does it say the On and Off voltages. How many volts to turn it on and how many volts for it to turn off?

BJTs are current controlled devices. Once again, this is a rule of thumb that is not strictly true, but unless you get into some advanced designs, it is a good maxim. Field effect devices (JFETs, MOSFETS), on the other hand, are voltage controlled devices (this is absolutely true).

A BJT has current gain (hfe, hFE, Beta, B) which simply means that the collector current will be the base current multiplied by hFE. A typical small signal NPN BJT may have an hFE of 100, so if you arranged for a current of 50uA to flow into the base, 5mA would flow into the collector. If you had no base current, there would be no collector current and the BJT would be turned off.

(hFE is the current gain at DC. hfe is the current gain at some specified frequency. A transistor's current gain drops of as the frequency increases. For a typical silicon small signal transistor the current gain drops to 1 at around 200MHz. 'h' just stands for hybrid characteristic, a fairly advanced method of modeling a transistor's performance which you need not worry about)

In the data sheet for the BC547 there's a "Base-Emitter On Voltage" is that it?

Another rule of thumb that every electronics engineer has tattooed on his brain is that when a silicon diode is conducting a small amount of current, it's forward voltage will be close to 600mV. So, for example, a 1N4148 diode conducting 50 uA may have a forward voltage (Vf) of 600mV. The same diode conducting 500uA would also have a Vf of 600mV. At 5mA the Vf would be ... you guessed it, 600mV. In other words when a silicon diode is conducting it's Vf is always 600mV. Once again this is not strictly true, but it is another useful rule of thumb.

So far I have been talking about diodes, well the base emitter junction of a BJT is nothing more than a diode.

The characteristic of a silicon diode are that if you fed it with a voltage source of say 500mV no current would flow. If you slowly increased the voltage to 599.999.. mV no current would flow. If you then increased the voltage to 600mV there would be an explosion because an infinite current would flow and blow up the diode. Of course, this is absolute nonsense but it is a good concept to keep in mind.

In practice the diode would not start conducting so suddenly, but it would still be fairly sudden. The message is that you do not generally put a voltage across a diode. Instead you put a current through it and a Vf results. This may not appear to be the case for many circuits, a rectifier diode for example, but it is true. To give you a clue, a rectifier diode has a special construction so that, say a rectifier diode capable of conducting an average current of 1A, may have a peak current rating of 100A.

To finally answer your question about the data sheet. Yes, the base emitter on voltage is the voltage that will exist when the transistor is conducting a specified current. The voltage is not the same for all transistors of a given type due essentially to manufacturing tolerances. A good data sheet will give a maximum value and an ideal data sheet will give a maximum and minimum value.

 

[kal.a]

I put it together on a bread board and it worked fantastic. I used a relay for a load instead of the PLC in case I wired it incorrectly and it worked well. I made one minor modification because the photo transistor is soldered to a circuit board and the anode of the LED and the collector of the photo transistor are connected together and I wasn't sure what that would do the the LED so I relocated the 10k resistor between the 1K resistor and the collector of the photo transistor and connected the base of the BC547 to that point (collector side of the photo transistor) and it worked fine. Of course I can always cut the trace on the circuit board to separate the anode form the emitter.

Thanks spec

Cheers
Kal


Edit: I was incorrect. The anode and Collector are separate but the Cathode and Emitter are joined.

 

[spec]

Hy kal,

Well done. I am just finishing off the reply above and will then have a look at your mod.

Cheers

spec

 

[spec]

Hi again,

Just had a look at your mod- I think you know more about electronics than you are letting on: pretty neat mod.

Yes, it will work fine, but it is not, strictly speaking, a worse-case toleranced design. I will post a circuit shortly to cover that issue.

spec

 

[spec]

​

ERRATA
(1) R13 is a new component omitted from the other designs. It has no function except to limit the current through the opto transistor under fault conditions

 

[spec]

​

SELECTING A SMALL SIGNAL BIPOLAR JUNCTION TRANSISTOR (BJT)
If you are not familiar with transistor data sheets, using them can be daunting. Most data sheets are very good theses days, but because they are comprehensive, they present a myriad of data using seemingly arcane acronyms which can be completely overwhelming at first. At the other extreme some data sheets are deficient and do not provide enough data for you to use the transistor. If possible, it is best to avoid such transistors but, with experience, you can normally make informed guesses to arrive at working parameters.

The good news is that for most applications you only need to worry about a few parameters, and once you decode some of the acronyms, using a transistor data sheet is quite straight forward.

There are three broad areas that define a transistor:
(1) Mechanical Characteristics
(2) Absolute Maximum Ratings
(3) Electrical Characteristics

Mechanical Characteristics should not present you with any problems so only Absolute Maximum Ratings and Electrical Characteristics will be further covered.

Absolute Maximum Ratings
The maximum ratings give the values for various areas: temperature, voltage, current etc, that must not be exceeded under any operating modes, or the performance of the transistor will be permanently reduced or the transistor will be destroyed.

When selecting a small signal BJT only these absolute maximum characteristics need to be taken into account for the vast majority of applications:
(1) VCEo
(2) IC
(3) Ptot

VCEo means Voltage Collector Emitter with the third terminal, the base, open circuit.

IC is the maximum permissible current flowing from the collector to the emitter.

Ptot is the total power dissipated by the transistor and can be closely approximated by the product of VCE and IC. Ptot is normally specified at either a case temperature of 25 deg C or with the case in air at an ambient air temperature of 25 deg C. In practice, to check that the transistor is not over dissipating do the finger test. If you just cant quite keep your finger on the case that is around 70 deg C and the transistor will be fine. Note that, once again, there is much more to transistor power dissipation than this, but if you use the finger test the component will be safe.

Electrical Characteristics
The electrical characteristics specifies how the transistor will perform under defined operating conditions. For nearly all applications only the following electrical characteristics need be considered:
(1) hFE
(2) VBE
(3) VCEsat
(4) ft

hFE is the current gain of the BJT at DC. It is IC/IB. hfe is the same thing except at a defined frequency. h stands for hybrid characteristic which is a set of models for transistor analysis, which you do not need to worry about for the majority of applications.

VBE is the Voltage across the base emitter terminals that will arise under defined base and emitter or collector currents. A very good data sheet will give maximum and minimum values for VBE, while a good data sheet will just give maximum VBE.

VCEsat or sometimes just Vsat is the voltage across the collector and emitter when the collector current is so high that it drops the supply voltage across the collector load resistor. Vsat is specified under defined conditions. Vsat for a small signal transistor at an Ic of 1ma would typically be 100mV. Many newbees cannot accept this because the base voltage would be around 600mV so how can the collector volts be almost zero. Hard as it is to believe, it is fact.

ft stands for transition frequency. As frequency increases the transistor hfe falls until it reaches one. This point is the transition frequency (ft). (this is a gross simplification but is quite adequate for most work). In general, the further away from ft that your circuit operates the better. Take the BC546. The average ft is 300MHz, so it would be fine for an audio amplifier where the maximum frequency required would probably be around 10MHz. Note that the audio frequency range is 20Hz to 20KHz so you may think that 20Khz would be the maximum frequency required, but this is not the case because you need the transistor to operate beyond the signal pass band so that frequency stability can be ensured. In general, the transition frequency of the BC546 will be adequate for most general purpose applications.

On a really good transistor data sheet, additional information will be given. This often comes under the heading Typical Performance Characteristics. As you can see above, the BC546 data sheet has a comprehensive collection of data under this heading. This information is invaluable when applying the transistor.

The data sheet might have information describing any specialist areas associated with the transistor and finally the data sheet may give applications information, typically showing a range of circuit applications. Most application information though is normally available from the manufacturer as a separate application report or application note.

You may wonder why there is a family of transistor types on the datasheet: BC546 thru BC550 and you may think that each transistor is specially made to have a lower noise or a higher VCE for example. While that may be the case for certain specialist transistor families, more normally, the transistors from a particular family are all made the same, even on the same semiconductor crystal. But, because of the nature of fabrication, individual transistors will have different characteristics so the manufacturer tests and sorts the transistors into groups and allocates a group a number in the family range.

As a general approach in electronics design it is wise to choose a range of components that will suit most general applications. These include, small signal transistor, medium power transistor, high power transistor, and the same for MOSFETs. You might also choose one type of resistor and a couple of types of capacitor and finally, a couple of inductors, one for switch mode power supplies and one for small signal filtering. Collect data sheets and application data for all of these universal devices so that you can look up any information that you might need when you are designing in any of these components. You will now have reduced the mountain of information available for the millions of devices available, into a manageable size.

By studying the data sheets as you go along, you will be surprised how soon you get the feel for the layout and lingo. Quite soon you will know the important characteristics of your set of selected components and that will be an enormous help in your electronics adventures. You can take a similar approach with integrated circuits. For example, about three opamps will meet most of your needs, and everyone needs to know the 555 timer chip inside out, the same for the LM317/LM337 voltage regulator chips.

 

[kal.a]

spec, what's the chance you're and educator in some capacity? You're amazing.

 

[spec]

spec, what's the chance you're and educator in some capacity? You're amazing.

kal,

you make posting on ETO so gratifying, especially after the flack and waffle you get from certain parties on this site.

I'm not in education, but I have done a fair bit of mentoring at work and have learned about English and logic. The essence is if a person asks how to grow cucumbers you don't tell him how to grow carrots or make out what a wonderful person you are because you have a thousand head of dairy cattle. I also use a set of very simple rules when writing and designing. These rules are applied mechanically and require no thought.

But the main thing is, I remember the awful struggle I had trying to get the hang of electronics. Most of the stuff we were taught was so convoluted and badly explained that it was very difficult to understand, when just a short explanation here and there would have simplified matters greatly.

One example was oscilloscopes. The time base on the scope we used in training were calibrated in cycles a second and there was all sorts of complicated explanations about fly back time etc. I just could not figure it. Then one day I got chatting to a visiting service engineer who had a proper scope calibrated in time per division and then the penny dropped.

We were also taught about signals passing through the ether I just could not figure what the either was and how it worked. That's because the either does not exist. There is only air or a vacuum.

The last such experience was when microprocessors first came out. There was a gang of us who wanted to understand them and build our own home computers. I had samples of the 6502, 8080, 6800, and Z80 chips and all the data sheets, but I couldn't get the hang of the core function. The books on computing at the time were no help either. We had a sub-contract microprocessor applications engineer drafted in to do the company's first micro design and I asked him how you program a microprocessor. He smirked and said, you don't program a microprocessor and that was that. Six months later when I new the score and had designed a couple of home computers I had a look at this guy's work- it was junk.

Finally, I have a great advantage over teachers as I have been a design engineer for ages and front line experience helps you learn most of the tricks in design and also be aware of most of the pitfalls.

If there is anything you need to know and I have the knowledge ask away and I will be only too pleased to help, if I can that is.

spec

 

[kal.a]

Well I'm always grateful that anyone would take the time to read my posts let alone reply in such detail and care. The pleasure is mine indeed spec.

We were also taught about signals passing through the ether I just could not figure what the either was and how it worked. That's because the either does not exist. There is only air or a vacuum.

 

[kal.a]

So I was busy burning stuff and re-reading the thread to absorb as much as possible and was looking at your last schematic and puzzled by your placement of resistors R12 and R23 until I re-read my post and I realized that I wrote the exact opposite of what I wanted to. The Cathode and Emitter are joined not the Anode and Collector. So here how I first **broken link removed** but then I thought that the 2K2 (R2 on my schematic) serves no purpose after moving the 10K resistor. Am I correct?

Then I thought I would be better off using a P_Channel Mosfet instead to connect to the sinking input of the PLC. And redrew the **broken link removed**

But I didn't have a P-Channel so I started fooling around with a PNP transistor in place of the Mosfet and mad some changes it worked well on an LED in place of the PLC.
- I changed the Zener diode to 5.1 V instead and it **broken link removed**( I forgot to use the 22Ohm resistor you had at the gate of the Mosfet. Not sure if it would have the same purpose here too.)

Then I thought that's fantastic maybe I don't need a mosfet after all and rewired it this **broken link removed** and that's when I started burning the BC5647 transistors.
I thought that by limiting the current so much with the 10K resistor it would work. But I guess I still don't understand transistors. I thought they were current driven and for as long as the current was limited the I was good. And in the datasheet Vcbo is 50V and Vceo is 45V . So I was still quite a bit below the absolute maximums. Why is it burning?

Thanks
Kal

 

[spec]

Hi Kal,

Looks like you have been having fun.

I will do a full reply to your post, but in the meantime, here is a circuit using a PMOSFET to turn the motor on/off:

 

[spec]

Here is a version with an NMOSFET:

 

[kal.a]

Hi Kal,

Looks like you have been having fun.

I will do a full reply to your post, but in the meantime, here is a circuit using a PMOSFET to turn the motor on/off:

View attachment 97173​

I don't get the zener diode in this schematic. When the photo detector is dark there should be about 18V (I'm actually getting about 7V on my bread board but 18V in LTSpice) and when "light" or on then there would be 24V which would mean the the mosfet is off all the time as it never gets 0V. I think I'm completely lost. Better get some sleep

 

[spec]

Here is a before and after on the BD140 circuit:

 

[spec]

**broken link removed**
​

As you quite rightly say R2 (2K2) serves no purpose in this circuit configuration. The current through the opto transistor and Q1 is already limited by R3 (10K). But R2 will do no harm and will not stop the circuit from working.

The PMOSFET has a few problems:
(1) It is a 20V max VSD device and it is being used with 24V VSD
(2) The source and drain connections are swapped
(3) The IRF7404 PMOSFET is not suitable for driving a motor- it is not man enough

Once the above points are fixed the circuit will work OK but it would be operating the opto receiver transistor with a VCE lower than is specified in the characteristics defined by the data sheet.

 

[spec]

**broken link removed**
​

This is a light on function. ie when light is shining on the opto receiver transistor, the motor will run.

The reason why Q1 (BC546B) is overheating is that there is nothing to limit its collector current and hence the base current of Q2 (BD140).
The opto reciever transistor can conduct 18ma for a really good sample. Q1 (BC546B) can have an hFE of 300 so potentially Q1 collector current could be 18mA * 300= 5.4 Amps. The voltage across Q1 collector emitter is 24V, so Q1 could possibly be dissipating, 5.4A * 24V = 129.6W. If you had a power supply with a current capability of 5.4 A and the Opto reciever transistor were particularly good and Q1 had an hFE within its data sheet specification there would be an explosion and all that would be left of Q1 would be the collector, emitter, and base leads.

To fix the problem all you need to do is put a 2K2 resistor between Q1 collector and Q2 base. This will limit Q1 collector current to around 24V/2K2 Ohms= 10.9mA, slightly different to 5.4A

This circuit will work with the addition of the 2K2 resistor, but once again it is relying on parameters not given in the dara sheet.

 

[spec]

I don't get the zener diode in this schematic. When the photo detector is dark there should be about 18V (I'm actually getting about 7V on my bread board but 18V in LTSpice) and when "light" or on then there would be 24V which would mean the the mosfet is off all the time as it never gets 0V. I think I'm completely lost. Better get some sleep

​

Hy kal,

Firstly let me assure you that the above circuit is correct and will work as intended. I suspect that you have the PMOSFET incorrectly wired. I also suspect that that your PMOSFET is blown. The IRF7404 type is not suitable for this application anyway.

P Type MOSFET Operation

A PMOSFET is the compliment of an NMOSFET. This means that all the operating voltages are reversed. Under normal operating conditions the drain of a PMOSFET is more negative than the source. If the gate and drain are at the same potential no drain current will flow. If the gate is more negative than the source drain current will flow. So in the opto sensor the PMOSFET source would be connected to 24V and the drain would be connected to the top of the motor.

Circuit Function light on

Light impinges on the Opto Receiving Transistor (ORT) this causes an ORT collector current to flow from the 24V supply line through the emitter base junction of Q17 (consider R7 to be a short circuit at the currents concerned).

The current flowing through Q17 base emitter will generate a Q17 collector current of hFE Q17 * Ib to flow in the collector of Q17, but this current is limited by R5 (2K2) to 24V/2K2 = 10.9 mA.

As the whole supply line voltage is dropped across R5, the collector of Q17 will be at 24V. This voltage will also be on the gate of PMOSFET Q16 (R8 has no effect). As the drain of Q16 is also at 24V there is OV between the gate and source, so Q16 is turned off.

Circuit Function light off

No light impinges on the ORT so only a small dark current flows in the ORT collector. This small current flows through R24 (10K) and does not generate enough voltage (600mV) to turn Q17 on.

As Q17 is off R5 tries to drag Q17 collector down to 0V but Zener D8 prevents this and starts conducting at 24V-18V = 6V. The net result is that gate of PMOSFET Q16 is at 6V and its source is at 24V so the gate is 18V more negative than the drain. This means that Q16 is fully turned on.

Zener Diode

You ask what the Zener diode is for: simply to protect Q16. It serves no other purpose. The data sheet for the PMOSFET says that the gate should never be more than +-20V with respect to the source. Without the Zener the gate could be 24V more negative than the source which could potentially destroy the PMOSFET.

What does R7 (2K2) do?

R7 in the base of Q7 probably looks a bit odd but it is there to protect the collector of the ORT and Q17 base from destructive current flow from the 24V supply rail to 0V. You might say but the data sheet says only a maximum of 18 mA will flow in the ORT and you would be right, except:

(1) Radio frequency interference could cause the ORT to conduct heavily

(2) Electrostatic discharge could cause the ORT to conduct heavily.

(3) High energy radiation (Gamma, X, etc) could cause the ORT to conduct heavily.

(4) When you turn a circuit on it is very difficult to know which voltages are applied to the circuit during the finite switch on period. During this time a combination of voltages might arise that would cause excess current to flow thru the ORT if no over current protection is provided.

Also an arrangement like that is like a loaded gun waiting to go off, and even if an excess current could not possibly flow in theory, you would still protect against excessive current as a matter of good design- the addition of a single resistor does not add significantly to the cost or complexity of the circuit.