# Newbie needs help

Discussion in 'The Projects Forum' started by jonrcast, Oct 13, 2009.

1. ### jonrcast Thread Starter New Member

Oct 13, 2009
3
0
I have not done a whole lot with circuits before, but consider myself a fast learner. I have a small circuit that I am trying to figure out. I have one already, but it is so messed up with glue and jbweld I can not figure out what is going on with it. It is a very simply circuit I believe. I would rip apart the one I have, but hate to ruin a good unit. I just would like to create a spare.

So, to describe what I have. It consits of 5 wires (3 inputs and 2 outputs) and a perforated circuit board.

Wire 1 entering is a constant 12V positive.
Wire 2 is a ground.
Wire 3 taps into an existing wire that is connected to a solenoid. It gets its voltage from this solenoid signal wire.

The 2 output wires are:
Wire 4 is a ground
Wire 5 is a positive wire to connect to an identical solenoid described in the other set of wires.

The 2 solenoids have to have exactly the same voltage for the system to work correctly. The ouput positive can not feed back into the input positive so a diode is used which I can see. Also, I can see a adjustable voltage regulator I believe. This is about all I can see on the board.

Does anyone have any ideas as to where I can start? Any help would be much appreciated.

I know this drawing looks terrible but I tried. R1 and R2 must have exactly the same voltage. The voltage of R1 changes all the time. So, R2 needs to change along with R1.

Last edited: Oct 13, 2009
2. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
What kind of current do the solenoids require?

What is the voltage range across the solenoids?

Something like this would be fairly easy to do with a power opamp, as long as the + side voltage doesn't get too close to the positive rail (+12v). The voltage wouldn't be exact, but it would be within a few millivolts.

3. ### jonrcast Thread Starter New Member

Oct 13, 2009
3
0
I am not sure exactly what they are. Would it make a huge difference? Can the signal wire from R1 control say a lm317 to regulate the appropriate voltage to the r2 wire?

4. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
Yes, the solenoid current is a necessary piece of information. If you don't know, then post what the resistance of the solenoid is.

If the solenoid requires 0.6A or less, a relatively inexpensive power opamp could be used. If more current is required, a more complex and/or expensive solution will be required.

As far as an LM317 being used; that might work if it were used as a voltage follower for a low-power opamp, although I'm not certain what the response time would be.

There is also a minimum voltage drop of 1.7v across an LM317 from input to output; at high current the dropout is more.

5. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
See the attached.

The opamp for U1 A should really be something like an L2722 dual power opamp.

The opamp for U2 should really be something like an LM2904.

U1 A is trying to drive a solenoid coil that has a resistance of 150 Ohms. As you can see from the simulated O-scope trace below, the yellow trace doesn't follow the input signal very well; the opamp just can't supply enough current.

U2 driving the LM317 does a much better job - and the coil on the right is only 15 Ohms. Where it starts falling flat is when it gets near the voltage rails; it can't go lower than 1.25v or higher than about Vcc-1.8.

The diodes are required to absorb reverse-EMF in case the current to the solenoid coil is suddenly cut off. Otherwise, the LM317 or the opamp could be damaged.

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