Ok....I built a circuit to control solenoids in a transmission. Normal ops: Solenoids have 12vdc applied all the time and the computer applies a ground to each solenoid in the appropriate combinations for each gear...no problem. I build a circuit to manually control each solenoid for each gear using TIP41's to apply the grounds rather than the computer. This part appears to work.
In manual the car shifts manually like it is supposed to except for a small delay between 1st and 2nd. Im sure its hydraulic and not electric but this led me to build a 7 segment display to indicate what gear was selected.


So, the 7 seg display works flawlessly in automatic (computer controlled)...however it never works in manual even though the gears are shifting normally. I don't understand why it wont work. Im curious to hear what everyone might think it is...The ground for each solenoid comes from the TIP41's Emitter through the collector to the solenoid...Its like the TIP41's are not really "grounding" the solenoids enough to both actuate and indicate the correctly. With the computer grounding the solenoid I read a perfect ground at the 7 seg logic...from Tip41's It drops from 12vdc to about 10vdc at the logic. I will have to post pictures later of the circuit...just wanted to know if the TIP41's are up to the task. 12vdc to solenoid (22 ohm) to ground through TIP41.

THANKS!!!

 

The TIP41's appear adequate to operate the solenoids (if they have sufficient base current).

You'll have to post a schematic for an answer about your display problem.

 

did you isolate the computer from the attached circuitry by say an opto-coupler? How about the base drive for saturation of these transistors?
One thing is sure that the solinoid would not release immediately , due to back EMF sustaning the current thro the coil for little more time, coutesy the diode we provide. Instead, try to use an MOV or an R-C snubber, it would perform better, I feel.

__________________
Regards,
Sarma.

 

ok...the 3 attached schematic pics may help...sorry it took a while but I have been getting eaten alive at work recently.

Page 1: is the core of the schematic. At the top grounds coming from the computer go directly to the solenoids for normal operation. (The switch in the diagram is actually a capacitive switch controlled relay) When switched the grounds are supposed to come from the TIP's

Page 2: This is the core base logic for decoding the hydraulic switch signals into the appropriate combinations in manual. In reality there are no nand gates...just a single 4028 to do this.

Page 3: This again is the logic for decoding the solenoid combinations to display either 1, 2, 3 or 4. Again there are no nand gates...a single 4028 decodes and through a few diodes supplies the appropriate logic to the 4511 7 seg driver.

So, as it sits right now in automatic it works flawlessly displays 1 through 4 with no problem. The issue only occurs when its switched to manual and the TIP's provide the grounds rather than the computer. The digit "3" is always displayed meaning both solenoids are off (no ground provided) even though it shifts into the appropriate gear. When digit logic voltage is checked in manual it only drops about 2.5 volts or so...the 4028 still see's "high" and the digit never changes. If I add a hard ground to the solenoid wiring the digit goes without a problem...WHAT GIVES!!!! Any help appreciated!!!!!

***Edit: the resistor values are all 4.7K and transistors to the left of TIP's are 2n2222's***

Attached Images

	SCHEMATIC PAGE 1.jpg (60.1 KB, 24 views)
	SCHEMATIC PAGE 2.jpg (69.2 KB, 15 views)
	SCHEMATIC PAGE 3.jpg (71.8 KB, 14 views)

 

swap the tip41's for fat N channel mosfets. I suspect that the saturation voltage of the transistors may be seen as a logic 1. A fat FET will have an on resistance of a few milliohms, virtually no volts on the drains.

 

The TIP41's need more base drive. They will have Vce(sat)~100mV if you provide Ib=Ic/10. Change the base pullup resistors to 220 ohms, 1 Watt. Or use MOSFETs, as spuffock suggested.

__________________
Ron

 

Thanks for the help! Can anyone suggest a MOSFET that would work for my application? Ive never used one before...in the mean time I will try the base resistor swap suggested by Roff. THANKS AGAIN!!!