Flight controller -Electrical checking
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yes. The R1:R2 ratio must be large to keep current low through the igniter, but this will also lower the differential voltage into the op amp. You need to find a good ratio where you're amplifier can accurately measure the voltage but there is not a lot of current through the igniter.
Yes, it allows the continuity tests to work while it's disarmed.
it won't with those resistor values. this gets sort of complicated... for the input to go high, the voltage over that resistor (call it R1) will have to go above the Vh of the MCU inputs (obviously). This will be:3) Last questionHow would the safe/arm input going to the MCU ever go high?
If the arm switch is off, then the MCU input would be grounded by the 1k resistor, as the resistance to +9v is a fair bit higher. If the arm switch is on, then it would be a direct ground and hence no change.
Vin = 9(R1)/(R1+Req)
where Req is the parallel resistance between the three resistors circled in your picture (call them R2). Therefore, if you want Vin to be 3V, then R1 should be half of Req. however, to make it more complicated, Req will change depending on how many igniters are plugged in (more igniters means smaller Req).
Also, you should add a high resistance in front of the safe/arm input. Something in the megaohms. This will protect the mcu from overvoltage if the mosfets fire while unarmed.
so here's a "quick" way to figure it out:
define R1 as the 1K resistor at the bottom
R2 is the value of the three circled resistors
RL is the resistance of the loads
Req is the equivilant resistance of all R2 and RL
So, here are your requirements. The values for max load current and minimum load voltage are assumed. you'll have to find them or decide them, along with your minimum input high voltage, yourself.
Vin < 3.0 (input voltage can't go above 3 volts)
Vmin < Vin (input voltage, when unarmed, must be greater than minimum input high voltage)
IL < 3.0ma (load current must always be less than 3ma)
0.5mv < VL (load voltage must be greater than 0.5 millivolt to read continuity)
IL = 9 / (R2 + R1 +RL) when unarmed
so 9 / (R2 + R1 + RL) > 0.5 ma
IL = 9 / (R2 + RL) when armed
so 9 / (R2 + RL) < 3.0 ma
Vin = 9 (R1) / (R1 + Req)
with one igniter attached, Req = R2 +RL
with three igniters attached, Req = (R2 + RL)/3
so 9 (R1) / (R1 + R2 + RL) > Vmin
and 9 (R1) / (R1 + (R2 + RL)/3) < 3.0
So you need to find values that satisfy these requirements. I don't know if there is a solution. I don't have time right now to work it out.
Also, read the end of my last post if you didn't already.
Hello
I think I have it!
R1=1K
R2 resistors =3K3 each
This assumes:
RL = 2ohms. (resistance of ematches)
IL > 0.0005A when unarmed (See my question about this one below)
IL < 0.01A when armed (max test current for pyro's, I think this is what you were refering to?)
Vin > 1.9V (Vhmin on MCU pins)
Vin < 3.0V (Vmax on MCU pins)
This gives me results of
IL unarmed = 0.00209... OK
IL armed = 0.002725....OK
Vin(1 pyro) = 2.09205....OK
Vin(3 pyro) = 4.28299...Not ok, but im sure I can fix this with a series resistor
Also, when you say 9(R1) in your equations, I assumed you meant 9R1? (9*R1)
Different way of writing things where I was taught I think
Allllso...
Where these statments refering to the same thing? I got a bit confused...
I think I have it!
R1=1K
R2 resistors =3K3 each
This assumes:
RL = 2ohms. (resistance of ematches)
IL > 0.0005A when unarmed (See my question about this one below)
IL < 0.01A when armed (max test current for pyro's, I think this is what you were refering to?)
Vin > 1.9V (Vhmin on MCU pins)
Vin < 3.0V (Vmax on MCU pins)
This gives me results of
IL unarmed = 0.00209... OK
IL armed = 0.002725....OK
Vin(1 pyro) = 2.09205....OK
Vin(3 pyro) = 4.28299...Not ok, but im sure I can fix this with a series resistor
Also, when you say 9(R1) in your equations, I assumed you meant 9R1? (9*R1)
Different way of writing things where I was taught I think
Allllso...
Where these statments refering to the same thing? I got a bit confused...
Someone else threw me the idea of instead of using an op amp for each signal, one could use a single op amp with an analog multiplexer selecting its inputs. This would be efficient if you wanted to measure a lot of voltages with the same gain. So instead of maybe 6 six precision op amps each with their own resistors and each needing an input pin, you would just need one op amp with resistors plus the multiplexer and one input and three outputs (to select the input). But if it's only 3, it's probably not worth it. I'm not sure how an analog switch would handle such low voltages either, though many of them claim to operate rail to rail. They also get expensive. Something like the MAX4582ESE might be a good compromise.
Also, if the space of discrete components is getting big, consider resistor arrays. They are cheap as dirt (like 4 cents for a 4 X 0603 package) and tiny. They will not be precise, though, so you shouldn't use them to replace some resistors (like gain resistors).
Also, if the space of discrete components is getting big, consider resistor arrays. They are cheap as dirt (like 4 cents for a 4 X 0603 package) and tiny. They will not be precise, though, so you shouldn't use them to replace some resistors (like gain resistors).
Hmmm interesting idea.
The way I have laid out my op-amps, they take up about the same space length wise as the ADC, and about twice the width, so as you rightly guessed, its probably not worth it in this case
I am using 3x OP777 (1 op-amp package) with a bunch of resistors. I have been trying to use the OP747 (4 op-amp package) but am having difficulty with track layout as I am trying not to run many tracks under the component. Might give it another go though
Dont worry about the tracks under the component. As long as you dont mix up things like high speed databusses or switched power lanes with sensitive analog circuits, you shouldnt experience any problems. If I understand you correctly, you dont intend to measure the resistance of the igniter very precisely, so its even less critical. If youre interested in single opamps, with a smaller footprint, look out for devices in the SOT 23-5 (or -6) package. The SC-70 package is even smaller, but I guess it may be nasty to solder. Unfortunatly, you are looking for a very special OPA (low Vos, R2R Input, Vsupply >5V), so there is only a limited selection available. The best one, that I found is the LMP7701, but its input offset is higher (200µV). Therefore, the sensing current should be at least in the region of about 1mA.Originally posted by basil4j
I have been trying to use the OP747 (4 op-amp package) but am having difficulty with track layout as I am trying not to run many tracks under the component. Might give it another go though
A transistor is a simple semiconductor amplifier. It transforms a small input current (or voltage in case of a FET) into a bigger output current. If the input signal is increased, it acts more like a switch, because the output current is then limited by other parts of the circuit (if not, the transistor can be considered a mini smoke bomb
).More information can be found in the Wikipedia:
https://en.wikipedia.org/wiki/Transistor
Reinhard
Good points. I think the inputs to the FETs would qualify as sensitive analog traces though, since any noise will effect the outputs significantly. However, you could get around this by just having the ADC sample and average a bunch several times to attenuate its effect. I wouldn't be too worried about interference with the continuity checkers. The sensor outputs, MOSFET gates, and power supplies should be your biggest concerns.
As for the op amps, the OP7xx series is really the only reasonably part I could find. A quad TSSOP package for <$6 with very low quiescent current and offset is about as good as it gets.
Ugh, I remember when I first dived into electronics alone... transistors completely eluded me. There are so many ways to model them that I was constantly confused. I think I burned about two packs of radioshack transistors before realizing that it was a good idea to put limiting resistors on themA transistor is a simple semiconductor amplifier. It transforms a small input current (or voltage in case of a FET) into a bigger output current. If the input signal is increased, it acts more like a switch, because the output current is then limited by other parts of the circuit (if not, the transistor can be considered a mini smoke bomb
More information can be found in the Wikipedia:
https://en.wikipedia.org/wiki/Transistor
Reinhard
Originally posted by mtwieg
As for the op amps, the OP7xx series is really the only reasonably part I could find. A quad TSSOP package for <$6 with very low quiescent current and offset is about as good as it gets.
Yeah, Ive been trying to find a smaller alternative but had no luck. I think Ill stick with the OP777, its not too much of a problem now as I have decided to mount some components on the bottom layer to relieve some board space pressures.
Originally posted by Reinhard
(if not, the transistor can be considered a mini smoke bomb
Just a smoke bomb? I got some shrapnel in my eye once from an exploding transistor
That'll teach me for trying to see how quickly I could blow one up With regards to laying the tracks to the FET via the little transistors, should I keep the track width fatter than normal signal? The run will be about 1, maybe 2" long from the MCU to the transistors, and about 0.5" to the FET. My normal track width for signal is 12mil.
It does not pass under any components on the same layer.
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