Calculating Size of Resistor needed?

[SystemTheory]

So far you're on track. I would insert three R1's and three R2's, one for each transistor, and run two of the R1 inputs to a single push button switch input. This might even make the board layout simpler. Your design might work as is, but I haven't thought about it.

Direct current power dissipation is P = V*I. Each resistor must be rated to dissipate enough power in the design, and the transistor must be rated to dissipate power when it is turned on. For example, the spec sheet says saturation voltage V = Vce(SAT) = 1.5V and you plan to pass collector current of iC = 0.5 amps. So the transitor without a heat sink must safely dissipate 0.75 watts. Check the spec sheet for the power dissipation rating at various temperatures. Then run a similar calculation to verify power ratings for each resistor.

The best practice is to test the circuit performance before soldering it up to a board. In this case some meter measurements of voltage and current, and verify the transistor saturates with the actual loads. You can defer the board design until after the circuit is tested, unless you wish to keep revising the board layout as you go along.

 

[Wetmelon]

300mA Solenoid:
P = IV = .3 * 1.5 = .45W

From the TIP41C Datasheet:
Collector Dissipation (TC=25°C) 65W
Collector Dissipation (Ta=25°C) 2W

So it can handle it bare with no problem.


Resistors:

For R1 (300mA solenoid)
V_BE(sat) = 2.0V
P = 2.0V * .065A = .13W

For R2
10V = I * 1850ohm
P = 10V * .0055A = .055W

For R3 (500mA solenoid)
V_BE(sat) = 2.0V
P = 2.0V * .039A = .078W

For R4
10V = I * 3100ohm
P = 10V * .00325A = .0325W

But I need someone to double check these to make sure I have any clue what I'm doing. I'm pretty sure I have R1 and R3 right, but I am really unsure of R2 and R4.

 

[Adjuster]

Well done, you're getting the hang of it now... Except for some resistor ratings!

Noting that a bit less than the full supply voltage is across R1 and R3, you might like to round them down to the next preferred value. Use the preferred values when making the resistor wattage checks.

As for the resistor dissipations, please note that for the circuit in your diagram, most of the supply falls across R1 and R3. It's the other resistors that get V_BE! Let's do a sanity check: The 500mA I_C transistor has about 65mA base current, so if we have about 10V across R3 it would dissipate roughly 650mW.

Actually, you should assume the maximum possible battery voltage. If you are using a lead-acid battery, this could reach about 14.5V on charge. For R1 and R3 dissipation, don't assume the absolute maximum V_BE. Something more like 1V V_BE may be more appropriate. Thus you could have about 13.5V on a 180 ohm R3, giving a shade over 1W. Remember P=V²/R

Don't forget the anti back-emf diodes, and you might think to add series resistors if space permits.

Now I have to go and try to fix the lights on my Christmas tree, so that's all for now.

 

[whome9]

You need to take into account the Safe Operating Area of the transistor. National Semiconductor has application notes on this subject.

 

[SystemTheory]

Sorry I forgot you're not EE so my instructions did not explain how to find the voltage across each component. But you have the right idea.

Instead of building and testing the circuit (or before doing so) you could build it in a free student version of SPICE. Depends on whether you want to handle the learning curve.

I've never used 5spice, but it appears to be free and easy to use:

http://www.5spice.com/download.htm

The TIP41 is a special part so you'd need to import a device model.

Google search: tip41 spice model

Result: http://www.onsemi.com/PowerSolutions/supportDoc.do?type=models&part=TIP41

I'm not sure how to model the solenoid coils however, you may need an inductor value and a resistor value which are unknown ...

The point of simulation would be to study the circuit performance in terms of voltage, current, power ratings, and transistor saturation before building anything in hardware.

Attachment shows the rough equivalent circuit in the base-emitter loop when the button is pushed. The base-emitter (internal) diode turns on and has a small voltage of 0.7 volts in small transistors, it shouldn't be much more in the TIP41, but I don't know how much. Anyway each R1 may see significantly more voltage than in your calculation.

 

[Adjuster]

Someone just mentioned the dreaded Safe Operating Area. It's a valid enough point, but I think a TIP41 is man enough to handle a 500mA relay on a 12V supply, provided you don't forget the coil transient protection.

TIP41 looks good for up to 40V continuously for currents up to 1A. See Fig.3 on Page2 of the attached data-sheet.

 

[SystemTheory]

I decided to try 5Spice and ran this saturation model of the TIP41C with success. A load resistor RLoad = 24 ohms will draw 0.500 amps at 12V, so I put this in as the coil resistance.

R1 = 330 ohm 0.5 watt rating 5% tolerance.
R2 = 3.3k ohm 0.25 watt rating 5% tolerance.

Base current is just 0.034 amps with the switch closed (shown as 12V battery). Power dissipation is about 0.38 watts in R1, so use 0.5 watt rated component. Power dissipation in the saturated transistor is a few tenths of a watt. When I set the input battery to zero volts the transistor turns off and a nano-amp current flows in the load resistor.

You can use the same values of R1, R2, and D1 (diode) for both the 0.300 ampere (40 ohm load) solenoid and the 0.500 ampere (24 ohm load) solenoid and the transistor should saturate properly in both cases.

The only cautionary issue is that simulated saturation voltage was quite low, less than a tenth of a volt, and I am not sure why the datasheet has such a high saturation voltage listed. It is probably because the collector current is 6A under the spec sheet saturation test. Yes, I think that's why, and with collector current at less than 1/10 that value in the solenoid design the simulator is probably fairly accurate.

 

[Wetmelon]

SystemTheory:
Hurray! My calculations were rather close, then! I suppose this clears me for breadboard stage testing :P

Adjuster:
From what I know of the car's electrical system, we get (surprisingly) little transient voltage. Our main problem is electrical/magnetic fields created by the alternator...

OTOH, if you would like to explain to me how to include protection in the circuit without throwing off the calculations too much, feel free :)

 

[Adjuster]

Surge Protection.

Unless the solenoids all have with internal anti-back emf measures (check, don't guess), you need to include a flywheel path for each coil. As a minimum, connect a diode like 1N4002 etc. in parallel with each coil with the cathode (bar end of symbol) to positive supply and the anode (triangle end) to the collector.
The coils will turn off more briskly if you can add a resistor in series with each diode, of value roughly equal to the coil's resistances (say, 24 and 39 ohms) and rated to carry the coil current briefly.

For protection against external transients, consider adding a Transient Voltage Suppressor between Collector and Emitter of each transistor, something like 1.5KE33A (aka 1N6283A), might fit the bill - see attached datasheet.

The above measures will have no effect on your calculations so far.

Finally, there is something that has to be said. FOR YOUR OWN SAFETY AND THAT OF OTHERS, PLEASE ENSURE THAT YOUR DESIGN IS CHECKED BY A COMPETENT PERSON BEFORE IT IS TESTED IN PRACTICE. However sensible any advice you obtain from this forum may appear to be, you don't really know where it comes from. Neither do we know exactly your situation. So do yourself a favour, get it checked by someone you can be sure to be competent.