Transistor Selection for Switching Relay Coil-with a little bit of info on my "ARSC"

[rxforspeed]

It strikes me as overkill for driving garden-variety relays. Do you need that kind of current and voltage capability? The 2N7000 is a venerable old workhorse good to 60 VDC for up to 200 mA continuous duty.

True, I had noticed that on the datasheet after I ordered them. I got a great deal on 'em, though! I believe the total amperage needed for each relay on this prototype is only about 36mA, or somewhere close within that range. Don't quote me on that, it's been a long day-so I could be off a bit. I'm sure it wasn't much over that figure for my intended usage...

I've got 19 T0-92 2N7000's on hand, like you had recommended, so I believe I'll use those. I didn't think I did at first, but I browsed through "My Documents" and found the datasheet there without even having to search datasheetarchives.com for it. I knew that meant that I either had them on-hand or had planned on ordering some at one time, so I checked one of my transistor storage boxes and found them neatly labeled in a little baggie under the "N-Channel FET" compartment...

BTW-the Texas Instruments datasheets I have for the ULN2004 and the ULN2803 don't list the thermal curves for either. I've been meaning to find another manufacturer's datasheet that lists them, but I haven't gotten around to it just yet. I've been back and forth between "The Art of Electronics" and "Programming PIC Microcontrollers with PICBasic" by Chuck Hellebuyck, and have had a rather long day today with my preparation for next semester's courses. I WILL find datasheets with the thermal curves for those Darlington arrays. Thanks again for the help and advice-I really appreciate it!

Sincerely,
-Adam Collins

P.S.-Just wait until I'm asking for help here on some of the problems I encounter with the four semesters worth of Calculus I'll be taking in the coming four years...

LOL-Just kidding about the Calc. questions-I'll figure it out when I get there... I've got a "refresher" course coming up this month to better prepare me. In a little over four years from now, I should be "qualified" to actually assist others! A scary thought, huh?

[mackrackit]

P.S.-Just wait until I'm asking for help here on some of the problems I encounter with the four semesters worth of Calculus I'll be taking in the coming four years...

LOL-Just kidding about the Calc. questions-I'll figure it out when I get there... I've got a "refresher" course coming up this month to better prepare me. In a little over four years from now, I should be "qualified" to actually assist others! A scary thought, huh?

Calculus???
Whats that?

[RussMartin]

BTW-the Texas Instruments datasheets I have for the ULN2004 and the ULN2803 don't list the thermal curves for either. I've been meaning to find another manufacturer's datasheet that lists them, but I haven't gotten around to it just yet.

You might start with Figures 14 and 15 on page 10 of this:

http://focus.ti.com/lit/ds/symlink/uln2003a.pdf

And Figures 11 and 12 on page 5 of this:

http://www.st.com/stonline/products/...re/ds/1536.pdf

[RussMartin]

. . . Just wait until I'm asking for help here on some of the problems I encounter with the four semesters worth of Calculus I'll be taking in the coming four years...
LOL-Just kidding about the Calc. questions-I'll figure it out when I get there... I've got a "refresher" course coming up this month to better prepare me. In a little over four years from now, I should be "qualified" to actually assist others! A scary thought, huh?

Hints for succeeding with your calculus course(s):

1) Memorize as many of the common integrals as possible ASAP.

2) Do as many of the exercises as possible, even ones the instructor doesn't assign.

3) Decide your objective: Do you want to master calculus or (recommended) pass the course(s)? (Bear in mind that many of us who took required calculus haven't used it since school.)

[rxforspeed]

Hints for succeeding with your calculus course(s):

1) Memorize as many of the common integrals as possible ASAP.

2) Do as many of the exercises as possible, even ones the instructor doesn't assign.

3) Decide your objective: Do you want to master calculus or (recommended) pass the course(s)? (Bear in mind that many of us who took required calculus haven't used it since school.)

That's a huge relief to hear-but at least I'll be able to understand more of "The Art of Electronics" once I get familiar with Calculus and Trig. Even the first chapter's lost me with some of the exercises, probably mostly because I just slid through Algebra 2 my senior year in high school. I had enough math credits to graduate, and I stayed busy hanging out with the wrong crowd back in those days; partying and racking up scores like Wilt Chamberlain...

But alas, those glory days are over now; my main focus today is my eight-year-old son, my education, and finding a job to work around my schedule. It feels kind of degrading to search for jobs at gas stations and near minimum wage establishments compared to the jobs I've had in the past ten years, BUT-that's a sacrifice I'm willing to live with to earn my degree. Besides, I've spent a huge chunk of my savings on a hardware, software, and a massive component collection that would make a few smaller distributors jealous. I should be prepared for almost any project I tackle in the next few months or so...

Here I go with another "long-winded" ramble... I'll get to the point:

For some reason, I couldn't get that second link from your other post to open, Mr. Martin. It's probably my computer-I've got so much RAM occupied with all the windows, browsers, PDF datasheets, MCS, etc. open and minimized for quick reference that it really slows down this notebook. I've got a collection of parts to build a nice desktop, but that hasn't really been a priority as of yet...

I did save the first link to the ULN200X-ULQ200x datasheet and I see exactly what you mean about the "thermal curves". I don't believe I'll have an issue with the Darlington arrays on the main prototype I'm building. Please correct me if I'm wrong, but I believe I remember that P=VI, with P being power (in watts), V being voltage (in volts), and I being current (in amps). I know from Omron's G5LA datasheet that the coil power consumption for both the 5V and the 12V relays is 360mW, so to find the total amperage I'd rearrange the equation to: I=P/V (I=0.36W/12V=0.03A). Sound right, 30mA? That's one relay; I'm using four on this prototype, so 4*30mA=120mA through the array.

I may be wrong in my assumptions about the duty cycles for these components, but my experience with duty cycles comes from my use of ARC and MIG welding machines where the duty cycle is measured in half-hour increments. I.E., 20% duty cycle on a 90A MIG setting means that the machine can be run for only six minutes at a time (30min*0.2=6) and then must rest without use for the remainder of the half-hour, or 24 minutes...

This datasheet for these components isn't identical to the arrays I'm using, but I'll even assume the thermal characteristics are worse for the ULN2004AIN's I have. The charts show between an 80-100% duty cycle for all four relays operating at 30mA each (120mA total), but the program I've written only samples one corner at a time, opening the "fill" valve for 100msec (plus the time it takes to execute the instructions minus the 10msec "operation time" then plus the 5msec "release time" for the relay's electromagnetic coil-so I'd still say roughly 1/10th of a second at a time per corner). Just to be safe, however, I have added an additional "PAUSE 50" statement after each sample (that's about 40msec more than I need for the A/D conversion with the ADC_CLOCK defined at "3", or internal RC oscillator, right?)...

The program loops via a variable "ANINRTNE" (abbrieviated "ANalog IN RouTiNE") to decide which corner to check next and is regulated by the pressure reading from the tank's psi sensor (if the tank pressure is below 90 psi, or "207" from my calculations with an 8-bit resolution and the Vref established at 1.525VDC, then the program waits an additional second before checking the tank psi level again and continues this routine until there is 90 psi or more in the tank)...

In order to reach 50 psi and 40 psi in both the front and rear corners, respectively, I know this won't be the quickest route for the program but the owner of the "Phantom" will not be in a rush to get anywhere with it; it's his show car. I'll concentrate on speeding it up when I get more acquainted with PBP and interrupts, but I believe this should be adequate for the time being. I really just need to get a prototype in the car so the owner can flip the switch to power it up, get up to "ride height" in about a minute or so (less time than the poorly engineered GM TPI V-8 takes to properly warm-up), then be ready to go. I'll practically have all winter (minus class and study time) to work on learning interrupts and exactly what I need to add to (or change/delete from) the program to make it a bit more efficient.

I hope the calculations and assumptions I've made for the project are at least "in the ballpark", and I'm sure there are quite a few variables I'm overlooking, but I think I've pretty much gotten this down (or at least I'm VERY close to it). One more quick read through the PBP manual and the '690 datasheet and I should have it in order. If I have problems with the code, I'll try to search the forum and all the other references I have handy to resolve the problem before posting a cry for help on here...

Sorry for the "long-winded" post-I just wanted to *attempt* to clear things up, although I may have done the complete opposite and just totally confused some of you, but I really doubt it. Most (if not all) of the members here that have helped me out have proved in some way or another that they really "know their stuff", so even if I haven't confused you then I'm sorry to make you read this short novel. I sincerely appreciate all the time, effort, knowledge, and help you guys have provided and shared here-it really means a lot to me!

Thanks!

[RussMartin]

For some reason, I couldn't get that second link from your other post to open . . .

Try http://www.st.com/stonline/products/ and then use the second box to do a part number search for ULN2803 or ULN2803A.

[rxforspeed]

Try http://www.st.com/stonline/products/ and then use the second box to do a part number search for ULN2803 or ULN2803A.

That worked, thanks again sir! I see exactly what you mean, too-the more ouputs you're using and/or the higher the current draw through the array equals less overall time the array IC can be used. I don't believe that my program and the current needs of the relays I'm using will be too much for these.

I will, however, have a back-up-just in case, with the 5V relays (power supply isolated from PIC's) and the 2N7000's. Thanks again for all the help!

Sincerely,