I promise this isn't my homework.
I have a 6 volt controller that runs a 2 wire DC motor forward and
swaps the leads to run reverse. I want to run a 12v motor from this. I
assume that is a couple transistors and diodes but I can't figure it
out.
Any links to a place that can help me?
The basics:
The existing controller could have one terminal of the motor
to ground, or to 6 volts or to somewhere else. That is
important to know. The controller output could just be a
voltage that can be adjusted between 0<>6 volts, or could be
a switching type that produces a train of pulses whose width
determines motor speed, or something else. That is important
to know also. It may easily be that the controller itself
could work off 12 volts with little or no modification. It
could be a very simple design or a very complex one, with
internal feedback loops that rely on the motor being present
to work properly. Or something in between.
There is such a range of possibilities - posting you just a
link or two is probably not going to be doing you much of a
favour. If you post more information about the controller
and the motor, you may get more focused advice.
So anything you can find out about the controller and post?
These include:
1) Identifying the type and value of the components used -
especially for things like capacitors, which should have a
voltage marked on them as well as a capacitance value.
2) Tracing the circuit board may be viable if there are few
components.
3) Close up photographs of the circuit board may help an expert.
4) Looking at what the output does at various settings, with
an 'scope.
5) What the motor is and what you are using the motor for.
So, the more information you can post, putting things like
photos and diagrams on a web page and posting links to them,
the more likely you are to get focused advice. Which could
range from, "it should be fine on 12v - you don't need to do
a thing" to, "forget it - the modifications/ new circuitry
would simply be too complicated".
+-------|<------+
| |
ControllerA ---+--->|---+---[FwRelay]---+ (Forward Relay)
| |
+---|<---+---[BaRelay]---+ (Backward Relay)
| |
+-------|>------+
|
ControllerB ----------------------------+
+12 ---+------------------+
| |
o o All Relay points
\ Fw1 | Ba1 shown are N/O;
o o shown with Forward
| | relay energized
+---[A]Motor[B]---+
| |
o o
| Ba2 / Fw2
o o
| |
Gnd ---+-----------------+
Conceptually for transistors, see below.
+12 ---+------+-----------------+-----+
| | | |
e/ ----- ----- \e
1---| Q1 / \ / \ Q2 |---2
\ --- --- /
| | | |
+------+---[A]Motor[B]---+-----+
| | | |
/ ----- ----- \
3---| Q3 / \ / \ Q4 |---4
e\ --- --- /e
| | | |
Gnd ---+------+-----------------+-----+
ControllerA ---+---[Diode>]---[R]--- To 1
|
+---[<Diode]---[R]--- To 2
ControllerB ---+---[Diode>]---[R]--- To 4
|
+---[<Diode]---[R]--- To 3
Q1 and Q2 are PNP; Q3 & Q4 are NPN. R limits base current, yet
allows the transistor to be driven into saturation.
Mosfets make better H-bridge transistors and could be used with
your circuit with a level converter from 6 to 12 volts:
+12 ---[10K]---c e---Gnd
\ /
---
|
Put 6 volts on the base, and c goes to 0; put
0 volts on the base and c goes +12
That's just the basic idea - Google on H-bridge for a lot
of information.
Ed
wrote:
Thanks Ed I have a circuit that uses all NPNs. The bottom 2 are in
emitter follower and the top 2 common collector.
http://www.mitedu.freeserve.co.uk/Circuits/Switching/dcmotor.html
If the rails are 12v and ground on the bridge and the input is 6v will
it still work? (for s1 and s2)
I don't suspect a problem with the emitter followers but if the
"ground" on the 6v ends up being .7v it might trigger the wrong common
collector. I suppose I can bias that one away.
What I am trying to do is use a 6v RC module to drive a 12 v motor.
Nope. The transistors on the top will have no B-E
current when their emiiters are at ~5.4 volts, so they
won't put 12 volts on the motor. In addition, the 2.2K
base resistors limit the base current to ~2.7 mA on the
lower transistors, way too low. Even at 12 volts with
about 5.4 mA, current to the motor is anemic. (The best
hfe shown on the datasheet is 50; 50*5.4 = 272 mA) That
may be fine for a tiny motor, but that circuit would be
better designed with power mosfets. Turn them on with
12 volts at their gates and the current to the motor
is limited by their rds, which is low. Taking .2 ohms
as representative (and you can get lower), the curent
would be limited to 30 amps.
You need to design based on what your motor draws.
Relays will work with any motor. Bipolars can be
used with low current motors. You want to drive
the bipolar hard into saturation. Mosfets can go
with either low current or high current motors.
Ed
I was afraid of that. I guess relay it is.
I can replace S1 &S2 with reed relays I guess.
I am starting to wonder if the original unit would run at 12v. I can't
read enough off the chips to figure out what they are.
There's a voltage drop/space penalty for bipolar transistors
and a current limitation penalty for reed relays.
Those penalties don't apply to mosfets or regular relays.
regular relays suffer a space penalty, but some come in
a DIP footprint and may be sufficient for you. Much
depends on the current your motor needs.
Reeds aren't usually good for high current (which
you may need) or double pole (which you do need).
In context, high current would be > 500 mA
For low current, you could use RLY-625 from Allelctronics
with a current limiting resistor in the coil circuit.
$1.50, fits in a DIP foot print, operates on 5V and
contacts are rated at 2 amps. RLY-642 is larger than a DIP
(still fairly small) but will handle 5 amps is a 6V relay.
I'd keep the current to 1/2 the contact rating.
If drawing a lot of current from the 6V source is an issue,
use mosfets. Here's a sample circuit plus decsription:
http://www.armory.com/~rstevew/Public/Motors/H-Bridges/Blanchard/h-bridge.htm
(watch the line wrap)
I would modify that circuit to put a fuse between each N-channel
mosfet and ground. That will protect them if there is an error
where both mosfets on one side turn on. Make the fuse a slow-blow
at a few amps over motor current and a few amps under the mosfet
max current rating. For example, an 8 amp fuse would be fine for
say 5 amp motor current and the mosfets below. If the motor current
is under say 6 amps, an STP12PF06 12 amp To-220 should be fine for
the P-channels, and a 16 amp IRF640 for the N-channels, but you can
use just about any power mosfet of the proper polarity and current
rating.
Your input, using mosfets, would be as shown at the web
page, but you would need to add 2 diodes:
A----|<----Gnd
B----|<----Gnd
where gnd is the 12V ground.
Ed
Hi,
Just to add somethings may be its can help a little bit.
A transistor is used not only to gain currents but it also can act
as ON/OFF switchs based on currents base (if not make mistaken).
you may get supply from collector leg direct to motor as assuming
forward and anothers
collector legs (pair) as assuming reverse. Before you do so, please
check
and measure the voltage form each legs (pairs) after you varied the
supply to pair transistor.
Tks
magic
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