# Circuit design help needed - Variable Current Circuit

J

#### Jamie

Jan 1, 1970
0
Hi all,

I am trying to design a circuit which will alter the current presented to a
solenoid.
The solenoid is a variable pressure solenoid, so the less current it gets,
the higher the regulated pressure.
The solenoid is controleed by a unit which produces a current ramp to the
solenoid, usually starting at around 900ma and tapering off to around 600ma.

I need to increase the pressure,and currently i am doing it by applying an
18 ohm resistor in parallel with the solenoid. This reduces the current by
about 120ma which is pretty much what im after.

Instead of having this resistor fixed, I would like to use a transistor or
FET of some sort to allow me to adjust the current to the solenoid, by
effectively paralleling the solenoid.
This section of the circuit will need to operate from a 1.0 - 4.5 volt input

I have attached a basic circuit of what I am after on
alt.binaries.schematics however it is just a 2 minute design

If anyone can help with a proper design, or anything at all it will be
appreciated.

Please email me jamie_s_22 AT hotmail.com
[AT = @]

Thankyou
Jamie

S

#### Spehro Pefhany

Jan 1, 1970
0
Hi all,

I am trying to design a circuit which will alter the current presented to a
solenoid.
The solenoid is a variable pressure solenoid, so the less current it gets,
the higher the regulated pressure.
The solenoid is controleed by a unit which produces a current ramp to the
solenoid, usually starting at around 900ma and tapering off to around 600ma.

I need to increase the pressure,and currently i am doing it by applying an
18 ohm resistor in parallel with the solenoid. This reduces the current by
about 120ma which is pretty much what im after.

Instead of having this resistor fixed, I would like to use a transistor or
FET of some sort to allow me to adjust the current to the solenoid, by
effectively paralleling the solenoid.
This section of the circuit will need to operate from a 1.0 - 4.5 volt input

I have attached a basic circuit of what I am after on
alt.binaries.schematics however it is just a 2 minute design

If anyone can help with a proper design, or anything at all it will be
appreciated.

+12V o----------+-------------+-----------+
| | |
.-. .-. |
| | 100R | | 10R |
| | | | |
'-' '-' |
| | C|
| | C|
|< |< C|
2SA1015Y |-------------| BD140 |
|\ |\ |
| | |
| | |
| | |
| | |
| +-----------+
0.6 ~ 4.5V |/ |
o--------------| 2SC1815Y |
|> |
| |
| |
.-. o
| | 330R
| |
'-'
|
===
GND
Please email me jamie_s_22 AT hotmail.com
[AT = @]

Nope.

Best regards,
Spehro Pefhany

T

#### Tim Dicus

Jan 1, 1970
0
Jamie said:
Hi all,

I am trying to design a circuit which will alter the current presented to a
solenoid.
The solenoid is a variable pressure solenoid, so the less current it gets,
the higher the regulated pressure.
The solenoid is controleed by a unit which produces a current ramp to the
solenoid, usually starting at around 900ma and tapering off to around 600ma.

I need to increase the pressure,and currently i am doing it by applying an
18 ohm resistor in parallel with the solenoid. This reduces the current by
about 120ma which is pretty much what im after.

Instead of having this resistor fixed, I would like to use a transistor or
FET of some sort to allow me to adjust the current to the solenoid, by
effectively paralleling the solenoid.
This section of the circuit will need to operate from a 1.0 - 4.5 volt input

I have attached a basic circuit of what I am after on
alt.binaries.schematics however it is just a 2 minute design

If anyone can help with a proper design, or anything at all it will be
appreciated.

Please email me jamie_s_22 AT hotmail.com
[AT = @]

Thankyou
Jamie

Hi Jamie,

I would recommend getting away from the parallel circuit. To properly regulate the current, you should put the controlling device in
series with the solenoid.

It might help to know the specs on the solenoid. Do you have a brand and model? Any electrical data on it?

Is the present power supply 12vdc? What voltage drop across the solenoid creates the 900ma current? And what voltage drop creates
the 600ma current? Measure across the solenoid at each current level.

I gotta ask: what is the voltage drop across the 18 ohm resistor when operating? And the rating on the resistor?

Tim

S

#### Spehro Pefhany

Jan 1, 1970
0
+12V o----------+-------------+-----------+
| | |
.-. .-. |
| | 100R | | 10R |
| | | | |
'-' '-' |
| | C|
| | C|
|< |< C|
2SA1015Y |-----x-------| BD140 |
|\ | |\ |
| | | |
| | | |
x---x | |
| | |
| +-----------+
0.6 ~ 4.5V |/ |
o--------------| 2SC1815Y |
|> |
| |
| |
.-. o
| | 330R
| |
'-'
|
===
GND

Best regards,
Spehro Pefhany

J

#### Jamie

Jan 1, 1970
0
The only way it is possible is with a parallel resistor.
The unit which controls the solenoid monitors current in the line and
adjusts itself accordingly. Adding a series resistor has no effect as the
device will increase the current up until a certain point where it will go
into fail mode. The solenoid DC resistance is rated 4-6 ohms.
As far as voltage drop goes, I havent measured it, but the 18 ohm resistor
in parallel does what I want it to do, but I would like to have control over
this by using the 1-4.5 volt input to control the current, in an equivalent
way as having a 16 to 24 ohm resistor in parallel.

Currently the resistor is a 10 watt wirewound. It doesnt get hot, maybe just
slightly warm.

Sphero: simulations of your circut seem to be working exactly how I want it
to.. the only think I need is a higher input impendance. I was hoping for
around 5k. What would be the best way to acheive this? use an opamp?

Thanks
Jamie

J

#### Jamie

Jan 1, 1970
0
Actually, simulations were working, until I figured I had the transistors
the wrong way around
When correcting them as per the diagram, I only get 2ma of current change
through the bd140

When I had them around the wrong way, the bd140 CE were reveresed and I also
had the other two transistors swapped... :S

Jamie

C

#### Check It Out

Jan 1, 1970
0
is the controller a current regulated pwm driver? If not potted, it would be
easiest to just find where it combines the feedback with the pwm signal
(look for an integrator) and mod it there.

E

#### Ethan

Jan 1, 1970
0
How are you controlling the logic to turn on and off the solenoid? Is
it a PLC or a mircocontroller or something of your own invention?

I see a couple way to do this, the simple way:

Does your solenoid have any auxiliary contacts? Some contactors have
this, and you could use the extra contacts to switch in your parallel
resistor, so you have a large pull in current, then when the solenoid
moves it automatically switches in the parallel resistor and reduces
the coil current.

And for the complicated way: I have designed a few circuits for PLC
coil drivers for contactors and hydraulic valves, and this type of
problem is common. First of all the coil current might be
programmable by the PLC. A coil driver is usually a MOSFET with a
flyback diode around the coil. Turn on the MOSFET and the coil turns
on. The MOSFET can be driven with a pulse width modulated signal, and
regulate the current by controlling the duty cycle. I used to do this
to be able to use a 12V contactor with a 36V dc rail. You can really
pull in the contactor hard then back off on the current to improve
efficiency, or just to keep from burning up the contactor in the case
of a 12V coil on a 36V system.

The inductance of the coil is usually pretty large so you don't need
an especially high frequency for the PWM signal. I like to use a
frequency of 15-20kHz so you can't hear it.

If you are driving this from a microcontroller you can drive the coil
from a timer/PWM/CapCom/whatever peripheral that almost all
microcontrollers have.

Ethan

J

#### Jamie

Jan 1, 1970
0
The solenoid is actually for a vehicle transmission.
I need to increase the line pressure within the trans, therefore need to
reduce current to the solenoid.
The solenoid is controlled by the transmission ecu.

Currently using the resistors works fine, but i am aiming to design a
circuit which increases the pressure proportional to the throttle position
sensor voltage.

C

#### Check It Out

Jan 1, 1970
0
The problem is the coil changes resistance greatly over temp, hence the
regulation in the first place. That leads to having to measure the coil
current (ON) AND the flyback diode current (OFF). Not doing both screws up
the regulation.

C

#### Check It Out

Jan 1, 1970
0
What does the drive waveform look like? Is it Straight DC or PWM?

Jamie said:
The solenoid is actually for a vehicle transmission.
I need to increase the line pressure within the trans, therefore need to
reduce current to the solenoid.
The solenoid is controlled by the transmission ecu.

Currently using the resistors works fine, but i am aiming to design a
circuit which increases the pressure proportional to the throttle position
sensor voltage.

Jamie said:
Hi all,

I am trying to design a circuit which will alter the current presented
to
a
solenoid.
The solenoid is a variable pressure solenoid, so the less current it gets,
the higher the regulated pressure.
The solenoid is controleed by a unit which produces a current ramp to the
solenoid, usually starting at around 900ma and tapering off to around 600ma.

I need to increase the pressure,and currently i am doing it by applying an
18 ohm resistor in parallel with the solenoid. This reduces the current by
about 120ma which is pretty much what im after.

Instead of having this resistor fixed, I would like to use a transistor or
FET of some sort to allow me to adjust the current to the solenoid, by
effectively paralleling the solenoid.
This section of the circuit will need to operate from a 1.0 - 4.5 volt input

I have attached a basic circuit of what I am after on
alt.binaries.schematics however it is just a 2 minute design

If anyone can help with a proper design, or anything at all it will be
appreciated.

Please email me jamie_s_22 AT hotmail.com
[AT = @]

Thankyou
Jamie

J

#### Jamie

Jan 1, 1970
0
dunno, havent ever checked it
Id say it would be straight DC, if it were PWM, there would be some sort of
inductors and flyback diodes within the ECU, but I dont think there is (I
did open it up around 6 months ago)

Check It Out said:
What does the drive waveform look like? Is it Straight DC or PWM?

Jamie said:
The solenoid is actually for a vehicle transmission.
I need to increase the line pressure within the trans, therefore need to
reduce current to the solenoid.
The solenoid is controlled by the transmission ecu.

Currently using the resistors works fine, but i am aiming to design a
circuit which increases the pressure proportional to the throttle position
sensor voltage.

to
applying
current
transistor
or
FET of some sort to allow me to adjust the current to the solenoid, by
effectively paralleling the solenoid.
This section of the circuit will need to operate from a 1.0 - 4.5 volt input

I have attached a basic circuit of what I am after on
alt.binaries.schematics however it is just a 2 minute design

If anyone can help with a proper design, or anything at all it will be
appreciated.

Please email me jamie_s_22 AT hotmail.com
[AT = @]

Thankyou
Jamie

C

#### Check It Out

Jan 1, 1970
0
OK, then it should have a feedback resistor inline with the coil. Double the
resistor, you cut the current in half.

Jamie said:
dunno, havent ever checked it
Id say it would be straight DC, if it were PWM, there would be some sort of
inductors and flyback diodes within the ECU, but I dont think there is (I
did open it up around 6 months ago)

Check It Out said:
What does the drive waveform look like? Is it Straight DC or PWM?

Jamie said:
The solenoid is actually for a vehicle transmission.
I need to increase the line pressure within the trans, therefore need to
reduce current to the solenoid.
The solenoid is controlled by the transmission ecu.

Currently using the resistors works fine, but i am aiming to design a
circuit which increases the pressure proportional to the throttle position
sensor voltage.

Hi all,

I am trying to design a circuit which will alter the current
presented
to
a
solenoid.
The solenoid is a variable pressure solenoid, so the less current it gets,
the higher the regulated pressure.
The solenoid is controleed by a unit which produces a current ramp
to
the
solenoid, usually starting at around 900ma and tapering off to around
600ma.

I need to increase the pressure,and currently i am doing it by
applying
an
18 ohm resistor in parallel with the solenoid. This reduces the
current
by
about 120ma which is pretty much what im after.

Instead of having this resistor fixed, I would like to use a
transistor
or
FET of some sort to allow me to adjust the current to the solenoid, by
effectively paralleling the solenoid.
This section of the circuit will need to operate from a 1.0 - 4.5 volt
input

I have attached a basic circuit of what I am after on
alt.binaries.schematics however it is just a 2 minute design

If anyone can help with a proper design, or anything at all it will be
appreciated.

Please email me jamie_s_22 AT hotmail.com
[AT = @]

Thankyou
Jamie

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