Variation on the 5v -> 12v newbie question...

Guest
I'm making a simple electronic-transmission controller for my car, but
I seemed to have run into a problem. The basic idea is to control
three solenoids with a BASIC stamp. I've got everything working like
it should, but I'm having trouble on the output circuits to the
solenoids. Here's the problem: I need to send 12v out of the control
box and to the solenoid (they are grounded internally in the
transmission) with transistor switches. I can't for the life of me
however, figure out a way to send out 12v from the controller,
controlled with the 0-5v of the Stamp pins (doesnt matter if the Stamp
needs to switch high or low). I have the 12v available to me in the
controller, so its really a matter of switching the 12v in the box to
the solenoid 3 feet away. I guess what I'm getting stumped on, is that
with everything I try to setup, theres always a voltage drop from what
I want to switch (the 12v) to the base (0 or 5v, its always lower...)

Some specs:
solenoids, 10ohm
transistor im using (NTE11)- NPN, Hfe minimum of 230, max current
capacity 5A

Any ideas?

Thanks in advance...
 
Is it possible to just use relays with n/o contacts rated to handle
the current?
Dave
 
They have to switch pretty fast and I won't really have room. (and its
right next to the steering column--the clicking may be audible)

Larry and Ban-- thanks a lot! Those look a lot better than what I was
coming up with, I'll try em out!
 
On 25 Jun 2005 10:34:26 -0700, cgwiita@ucla.edu wrote:

I'm making a simple electronic-transmission controller for my car, but
I seemed to have run into a problem. The basic idea is to control
three solenoids with a BASIC stamp. I've got everything working like
it should, but I'm having trouble on the output circuits to the
solenoids. Here's the problem: I need to send 12v out of the control
box and to the solenoid (they are grounded internally in the
transmission) with transistor switches. I can't for the life of me
however, figure out a way to send out 12v from the controller,
controlled with the 0-5v of the Stamp pins (doesnt matter if the Stamp
needs to switch high or low). I have the 12v available to me in the
controller, so its really a matter of switching the 12v in the box to
the solenoid 3 feet away. I guess what I'm getting stumped on, is that
with everything I try to setup, theres always a voltage drop from what
I want to switch (the 12v) to the base (0 or 5v, its always lower...)

Some specs:
solenoids, 10ohm
transistor im using (NTE11)- NPN, Hfe minimum of 230, max current
capacity 5A
Click to expand...
I also wondered why, if you are trying to "send out 12v," you are then
trying to drive the solenoid with an NPN? That would be sending out
0V, when ON, right? Larry and Ban are right to point that out. But
I'm still curious why you were thinking NPN, here.

Second, I prefer Larry's arrangement to Ban's (mostly because I don't
think there is distinct benefit for the extra parts.) However, I
wouldn't use Larry's KSB834 because I can't get them from Digikey or
the hobbyist suppliers I checked.

I might use an STmicro STX790A, which is equivalent to the Zetex
ZTX790A. The STmicro part is less than half the cost of the Zetex and
readily available from Digikey (actually, both are.) But one of the
reasons also is that it is also going to have a very low Vce, even
with modest drive current.

Notes: When you apply the 12V to the solenoid, it's probably going to
take some 10s of milliseconds to rise up to the 1A or so you might
expect with the 10 ohms (V/L effect.) Similarly, it's going to also
be even slower to lose its energy, because using the diode will only
allow a small voltage across the inductor during turn-off. Finally,
there may be a bit of voltage "ringing"/oscillation at the PNP's
collector as the solenoid's inductance "fights" with the non-linearity
of the diode near the point where the inductor's energy is nearly
spent. And finally, I'm a hobbyist and not a designer, so take
everything I say in that vein.

Jon
 
On the original issue, be sure to put a clamp of some description ( a
decent TVS rated below the max coil voltage of the relays) across the
12V input line.

Vehicle supplies are not 'clean' in any sense of the word (60V
transients on 'batt+' for instance). In the words of a Linear Tech app
note 'the power supply from hell'. A typical automotive power feed can
be expected to drop to about 4V during cranking and jump to between 50
- 80V during load dump.

I design automotive equipment in my current incarnation, and the main
power feed is one of my biggest issues (not only for the great power
feed, but also because I have to meet pretty tight standards on
conducted and radiated emissions into the power system from my
equipment).

Cheers

PeteS
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top