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Transistor selection confusion
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bson:

--- Quote from: phaseform on May 20, 2018, 08:16:44 am ---
--- Quote from: bson on May 19, 2018, 03:38:55 am ---RE = (2.5-0.7)/6m = 300
--- End quote ---
Not familiar with this equation? so VE = voltage across R2/IC?
...
how is R1 not mentioned in transistor base resistor calculations generally?

--- End quote ---
Vbe is a well-controlled property in that while it varies it doesn't vary much.  0.65 or 0.7V is a pretty reasonable value for BJTs (it goes up a little with current, maybe 0.05V or 0.10V).  It's a hallmark of BJTs.  If the voltage on the base is Vb, then the emitter voltage is Ve=Vb-Vbe; loop analysis (Kirchoff's voltage law) tells us this.  (One of the school exercises I still recall from the early 80s is loop analysis around BJTs.  Lots of them.)  Ohm's law tells us Ve = R1 * Ie, or R1 = Ve/Ie.  With a large base resistance (R1*Hfe) we know the base current Ib doesn't contribute much and Ie = Ic, to within 1/Hfe.  For an Hfe of 200 that means within 0.5%.  If Hfe varies from say 100-200 between individual units of a particular BJT, then this discrepancy between collector and emitter currents will vary between 0.5% and 1%.  Ic and Ie are pretty close, certainly no worse than the tolerances of a jellybean resistor.  So, R1 = (Vb - Vbe)/Ic to within ~1% or less.  It's also worth noting that a higher Hfe results in a small Vbe change, since they have the same root cause.

R1 is very much discussed in BJT loop analysis.  It's an essential part of it...  A lot of online material however tends to be rather incomplete, and you can't really provide a formula to plug numbers into for every situation.


--- Quote from: phaseform on May 20, 2018, 08:16:44 am ---Looking at the datasheet supplied by jaycar Vcesat should be below 1 V.

--- End quote ---

For small currents Vcesat doesn't matter, as long as you get the desired current.  In fact, an SSR, being a semiconductor device (typically a LED), switches on at a threshold voltage and is current driven.  Hence you need to drive it with a current, and Vce will be whatever it needs to be (pretty high) to set the input side (LED) current.  Too much current and input LED is gonski (as Dave would put it).
phaseform:
I've created a schematic for this circuit, not sure if a BC639 will be suitable, don't fully understand driving for constant current, so I'm still unsure of what I'm trying to solve here. I don't really have much room for VCE for the SSR to operate fully.


Attaching a 10 Ohm resistor in series with the SSR connected to 5V source (wired as: 5v - SSR - resistor - 0V), I get approx .097 V, hence IC = .97/10 =~ 10 mA

The outcome I'm looking for is VCE < 1 V @ 10mA, in order to have max voltage across the SSR.

Open D0 is between 1.5-2.7 V depending on sensor current.. so Ideally, I'd like IC = 10 mA for D0 between 1.5-2.7 V...

But I'm confused as for crunching the numbers / transistor suitability. - I'm not sure the constant current circuit would leave enough voltage across the SSR
bson:
R1 = (1.5-0.7V)/10mA = 80ohm

Why a BC639?  That's branded as a "High Current NPN Bipolar Transistor".  10mA is not "high current".  It's tiny small-signal stuff.   The jellybean of jellybeans BC548 (or 546, or 547 - they just differ in voltage tolerance, and are all perfectly fine) would be a better choice.  The BC548 has a Vcesat of 90mV typ and 250mV max @ 10mA, with a max Ic of 100mA.
Zero999:

--- Quote from: phaseform on May 16, 2018, 10:45:52 am ---I'm using a current sensing board from ebay to trigger an SSR, although the 'digital' output from the sensor board isn't ideal for the SSR.
I'm getting 0.2v low, and 2.2v high from the sensor board (with settings best I can figure to achieve), I want to use a transistor as a switch to send the full 5v through the SSR (3v min). The SSR draws ~5ma.
I have a BC639 (datasheet) I've found, which seems like overkill (1A max). Just from looking at transistor equations to figure out a base resistor, and whether this transistor is suitable.. I've gotten a bit confused. I'll have another look at it tomorrow, and probably get a lower power transistor from Jaycar.. Anyone with actual electronics knowledge that can shed some light on this..? - I was confused with min Hfe values - wont the base current just be lower in this case with a very low collector current?

--- End quote ---
Something isn't right.

Is that 0.2V low and 2.2V high from the sensor board, with the SSR connected to its output?

The sensor board has an MCP6041 op-amp on it, which is being used as a comparator to generate the digital output. It has a rail-to-rail output, this means with no load, the output voltage swing equals the supply voltage. In this case it has an LED and 1k resistor (these are on the board) connected to the output, which will drop the voltage a little but not much. With a 5V supply, it should give 0V low and a little below 5V high, say 4.8V, when measured  with a meter.
http://www.dexsilicium.com/Microchip_MCP6041.pdf

All you need is this circuit.

http://electronics-course.com/image/bjt-switch-inductor-load.png

In this case a solid state relay is not inductive, so the diode is not needed.

Suppose the output voltage from the sensor is 4.8V and the solid state relay draws 10mA.

Set the base current to 1/10 of the collector current, which is 1mA.

Calculate R1.

R = (V - VBE)/I

I = 1mA = 0.001A
V = 4.8V
VBE = 0.6V

R = (4.8-0.6)/0.001 = 4200Ω

In reality, the resistor is not critical: 3k3, 3k9 or 4k7 will do.

To answer the original question: the BC639 is overkill but will work. The BC548 is more than adequate, but if you only have the BC639, then it's fine.
phaseform:
so yes, I am using a BC546, BC639 is one I had already and decided it was not ideal, so I got some BC546 parts.. D0 is giving 0.3 V low, and at least 1.5 V high (up to 2.7 V no load), D0 varies with sensor current, and isn't really a digital signal in my config at least. I played around with the adjustment pot and that's the best I could get, don't fully understand how the tuning worked. I went and got one of these arduino compatible relay boards because I wanted the SSR switching to be discrete (on/off), it draws 150mV through the sense pin over a 10 Ohm resistor = 15mA (wired as: 5v -  resistor - relay board sense pin).
So next is to figure BC546 (derp) resistor values to trigger this relay board for >= 1.5 V D0 output... it will have to wait for now. So much for a simple project... as with every case.
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