circuit diagrams

kmacken2

Hey everyone,

I'm in a group project that is using several valves and sensors, which we hope to program all with propeller. I've attached a word document with draft circuit diagrams and specs on the equipment we are using. Before we power anything and begin programming, it would be great to get feedback to see if our diagrams look right.

Nothing is too complicated, but we received some on a few parts and wanted to check how we can validate the components (i.e. size of resistors).

Thank you!
TeamWHOI_Circuitry.doc

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kmacken2 wrote: »

Hey everyone,

I'm in a group project that is using several valves and sensors, which we hope to program all with propeller. I've attached a word document with draft circuit diagrams and specs on the equipment we are using. Before we power anything and begin programming, it would be great to get feedback to see if our diagrams look right.

Nothing is too complicated, but we received some on a few parts and wanted to check how we can validate the components (i.e. size of resistors).

Thank you!
TeamWHOI_Circuitry.doc

'
I couldn't open your attachment. Can you convert it to another file type? (txt,jpeg,etc)
'
I hate to leave a newbie hanging.

Duane Degn

I used OpenOffice to open it.

Here it is as a pdf.

TeamWHOI_Circuitry.pdf

I'm not a transistor expert. I'll take a pass on this one.

Mike Green

The attachment opens with Microsoft Word under Windows. It describes a typical interface to a humidity and temperature sensor using some kind of 4 channel ADC. It also describes how to drive a 5A solenoid from a 24VDC power supply using a standard Darlington transistor and a diode across the solenoid coil.

For your information, the base-emitter junctions of the transistor are in series with the input from the microcontroller I/O pin and that will reduce the voltage available to drive the transistor by about 1.4V. There still should be plenty, but the calculation of the base current through the 1K resistor is wrong as a result. With the Propeller, you'd have about 3.0V - 1.4V = 1.6V. With a 1K series resistor, that would provide about 1.6A output from the transistor with about 1.6mA base input. Make sure the transistor's gain is at least 3000, so you'd have about 4.8A available for the solenoid.

Regarding the ADC. Look in the Object Exchange for ADC drivers and choose the ADC you're using based on that. It's always easier to use a pre-tested piece of code than writing or modifying something to fit what parts you have.

Tapperman

Consider using a DS1620 or some other device close to this, for the Temperature Sensor ... nothing to do but hook it up, download the driver from OBEX and then use it! That is, if you sense range is in the neighborhood of -30 to 250 F?

Duane C. Johnson

I would not use a Darlington in this circuit.

The problem is the lowest voltage across the main transistor will be about 0.7V, probably a bit more though.
The power dissipated would be about 0.7V * 5A = 3.5W. This will probably need a heat sink.

A better solution would be to use an IRF3708 MOSFET. See:
http://www.redrok.com/electron.htm#nch
http://www.redrok.com/electron.htm#IRF3708
http://www.redrok.com/MOSFET_IRF3708_30V_62A_12mO_Vth2.0_TO-220.pdf
This MOSFET has an Rds(on) of 29mO. So the power dissipated would be:
5A^2 * 29mO = 0.725W and would not need a heat sink.
This MOSFET costs about $1.95 and can deliver in excess of 7A disipating no more than 2W
so no heatsink would be requited..
The TIP142 is similar in cost at $1.82 but the heat sink adds considerably to this.

Duane

StefanL38

The voltagedivider for the humidity sensor is aprox 14 Ohm. The resulting current will ve 5V / 14 ohm = 0.357 A. The resistor and the humidity-sensor have to dissipate a power of

7 Ohm * 0.357^2 A = 1.8W. This is pretty much. Of course you can buy 2W, 5W even 100W resistors but I guess the humidity-sensor does not stand such a high current.
is it a typo that you meant 6.95 kilo-ohm??

I would take a close look into the datasheet of the humidity-sensor how much current this sensor can stand.
As you wrote:

before power up we want to check everything. hm - you have to check a lot of things earlier in the state of choosing the components to use.

Sometimes this is an iterative process of having chosen a component, choosing other components on the demands of component A
and maybe recognising on analysing component C that it does not work. So you have to go back to component A and even have to choose a new type for component A
and do all the checking of the specs for component A, B and C again.

I took a look into the datasheet http://www.omega.com/Temperature/pdf/TH-44000.pdf

It says resistance 2250 up to 30000 ohms. So which one do you want to use?
How much EMV do you have in your project? High resistances mean more trouble with EMV

best regards

Stefan

kmacken2

Mike Green wrote: »

The attachment opens with Microsoft Word under Windows. It describes a typical interface to a humidity and temperature sensor using some kind of 4 channel ADC. It also describes how to drive a 5A solenoid from a 24VDC power supply using a standard Darlington transistor and a diode across the solenoid coil.

For your information, the base-emitter junctions of the transistor are in series with the input from the microcontroller I/O pin and that will reduce the voltage available to drive the transistor by about 1.4V. There still should be plenty, but the calculation of the base current through the 1K resistor is wrong as a result. With the Propeller, you'd have about 3.0V - 1.4V = 1.6V. With a 1K series resistor, that would provide about 1.6A output from the transistor with about 1.6mA base input. Make sure the transistor's gain is at least 3000, so you'd have about 4.8A available for the solenoid.

Regarding the ADC. Look in the Object Exchange for ADC drivers and choose the ADC you're using based on that. It's always easier to use a pre-tested piece of code than writing or modifying something to fit what parts you have.

Thanks Mike -

You mentioned the base current, but do we need to know the load current Ic as well?

Would it be better to use a different size resistor in order to get larger base input and so we wouldn't need a transistor was a larger gain?

kmacken2

updated schematics for the sensors

TeamSmith_Circuitry-1.doc

kwinn

Very nice job on the documentation. That circuit should work very well. FYI, the 0.29A rating for the relay coil is a "typical" rating based on the rated voltage and coil resistance (I=E/R or 23/78 = 0.2948A). The coil resistance will limit the current since the 3.3V output from the micro and the transistor gain could provide much more current ( (3.3-1.4)/1000 = 1.9mA and 1.9mA x 1000 = 1.9A).

Paul M

Just a very minor error: your schematic shows wire junctions with black "dots" - there should be 2 more at the R1/TR1b junction and at the TR1e/TR2b junction of the TIP142 schematic.

kmacken2

Hey!

So we ended up buying a MOSFET, but our coach recommended a FQP30N06L. He told us the following when comparing the MOSFET to the TIP142


The data sheet I've included says this part has an on resistance of .045 Ohms with 5 Volts of gate voltage. It's available at Digi-key and costs about
a dollar. Let's see how it stacks up against the TIP142. Power across the FET can be expressed as Current squared times resistance so, .307*.307*.045 = .004
Watts. That's less than you put into the base of the TIP! The TIP also dropped a Volt, let's see how the FET does .307*.045 = .014 Volts. I don't
think we'll miss 14 mVolts.

We just tried running a test to confirm our circuity and understand voltage and current relationships. Instead of wiring the solenoid valve, we used a resistor (78 Ohms). Also, instead of wiring directly to the microcontroller we used a power source. The solenoid valve is powered by a wall-wart we've connected to the breadboard. When the 'microcontroller pin' was set to 'on' (supplying 3.3 V), we wanted to measure voltage and current changes across the 'solenoid valve'. But we didn't measure any change, the only thing that happened was that the power from the microcontroller only reached -0.189 V.

Shouldn't we be expecting a current drop or change when the pin is turned on? Please correct me if I'm misunderstanding how the circuit should work!

Duane C. Johnson

Hi kmacken2;

If your driving the FQP30N06L MOSFET from a Propeller pin you will not get 45mO. The speck says you must use 5V to get that resistance.

I suggested you use the IRF3708. See #6. Yes, it costs $1.81 but it has a specified on resistance of 29mO all the way down to 2.8V on the gate, easily driven by the Propeller. The FQP30N06L does not. See the spec:
http://www.redrok.com/MOSFET_IRF3708_30V_62A_12mO_Vth2.0_TO-220.pdf

Duane

kmacken2

Hey Duane,

Sorry we didn't take your advice before! I don't know if I remember correctly but I think at the time we looked it up it might have been out of stock on digi-key and we weren't having luck finding it anywhere else. Where did you find it?

Also after you clarified that our voltage from the pin was too low, we did another test and increased the voltage to 5 V with the power supply. We're still not seeing a current or voltage change across the 'solenoid' (the resistor of the same size).

Duane C. Johnson

I get them at:
DigiKey:
http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=US&WT.z_homepage_link=hp_go_button&KeyWords=IRF3708&x=0&y=0

Don't forget the snubber diode across the relay.
It wouldn't hurt to put a 4.7K, or so, series protection resister from the pin to the MOSFET gate.

Duane

kmacken2

Hey,

I realized I didn't explain the test very well. Gave the 4.7 K a try, and just like the 1 K we're not getting any change in current across the solenoid resistor but we are getting a voltage fluctuation. But the voltage change doesn't correspond to when the pin is turned on or off. Also, the power supply never reaches the limit we set it to (5 V), it only gets up to about 0.5 V

I'm afraid to plug in the solenoid valve until I really can see how turning on the pin affects the current drop across the valve.