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The control curcuit for the watermaker could have been done with a microcontoller but I wanted to keep it as simple as possible and avoid programming so I went with relays and analog circuits. There are 3 sections, the relay board, the RPM switch and the flush timer.
Electrical Power Requirements
While the engine driven pump eliminates most of the power required by a motor driven system, the controls do consume some amps. Even though the alternator is generating power while the pump is running, the amps required by the watermaker are not available for charging the batteries so it is a good idea to examine exactly how much we are using. The electric clutch requires 3.5 amps, the boost pump needs 5 amps and the diverter solenoid uses 1 amp so the total while producing water is about 9 to 10 amps. In flush mode, the boost pump and diverter solenoid are off so the load drops to about 4 amps plus about 40 watts from the inverter to operate the bypass and fresh water solenoids.
The Schematic.
The Flush timer is in the lower left, RPM Switch is at the right and the control relays are in the center. There are 5 connector busses. Clodkwise from upper left are the control panel circuits, next are the 120V solenoid and TDS meter circuits, the high pressure and boost pumos and the interlocks, the RPM sensor connectiona and at the bottom the power connections. All of the parts came from Digikey. The relays are Omron G2R series 12V coil relays that can handle 5A except the bost pump relay which is a Potter and Brumfield 30A.
The ICs and relays are all regularly used in automotive applications so they can stand up to the 14-15V charging voltages.
Circuit Explanation
The start relay is activated by grounding one side of the coil with the start switch. This side is also connected through the interlock circuit to the NO position of the first pole of the relay which then goes to ground. The interlock circuit consist of the over and under speed relays, the oil pressure switch, the system over pressure switch and the Off button. Once activated the relay remains latched until the interlock circuit is broken. The Normally Closed position of the first pole enables the flush curcuit. This is done to prevent activating the flush mode while the system is operating. The second pole of this relay activates the boost pump.
The RPM switches are a pair of National LM2907 frequency to voltage converters. One of the primary uses for this IC is for sensing wheel speeds in anti-lock break systems. The frequency that the switches operate is controlled by the resistor capacitor combination. The trimmer pots provide adjustment. Pulses are recieved from an Omron E2EC-C2R5B1 PNP motion sensor on the pump/alternator mount. The sensor is activated by the 6 bolts on the Aquadrive flange. The underspeed relay closes when the engine speed exceeds 1400 RPM or 140 pulses/second. The overspeed relay opens when the engine exceeds 2100 RPM or 220 pulses/second.
The flush curcuit is a 555 timer. Flush time is controled by FL-trim from about 10 to 90 seconds. The backwash time is a delayed of circuit made up of capacitor C10, transistor 2N2222 and BW-Trim. Te time can be varried from about 10 to 45 seconds.
Here is the component layout of the printed circuit board. The board is 3.75" x 4". I placed the 120VAC connections at the bottom to avoid any confusion with the low voltage circuits. Trimmer pots for the RPM switch are at the top left and flush/backwash are at tne bottom left.
This is the top copper and pad layout. It is not exactly to scale but if you can print it 4"x4.25" in black you could get a board from it. If there is enough interest I will have some more boards made up.
Here is the back side. I always make the back side of my boards a ground plane. It helps reduce RFI problems and makes layout a lot easier. There are only two vias. One at the 120VAC power connection and the other to the left of the control panel connection.
Bill of Materials:
All of the electronic parts are available from Digikey. Here is a list of the parts I ordered with the Digikey catalog numbers.and prices as of 1/10/04. I had some resistors and capacitors so you will have to look them up. Total invoice was about $40.00
Qty DIGIKEY # Value Device Unit Price
2 .01uF CERAMIC CAP
4 10K OHM 1/4 WATT RESISTOR
1 100K OHM 1/4 WATT RESISTOR 2 500K OHM 1/4 WATT RESISTOR 3 Z143-NDRELAY PWR PC MNT SPDT 12A 12VDC 03.40 3 Z147-NDRELAY PWR PC MNT DPDT 8A 12VDC 04.03 1 PB302-ND RELAY PWR SPST 30A PC MNT 12VDC 02.47 2 LM2907N-8-ND IC CONVERTER FREQ TO VOLT 8-DIP 01.80 1 296-3518-5-ND IC HEX INVERTER 14-DIP 00.36 1 LM555CNFS-ND IC TIMER SINGLE 0-70DEG C 8-DIP 00.50 1 P11222-ND CAP 820UF 25V ELECT FC RADIAL 01.86 1 P10274-ND CAP 470UF 25V ELECT FC RADIAL 00.78 2 P813-ND 10UF 25V MINI ALUM ELECT (KA) 00.30 1 PCC2279CT-ND CAP .068UF 25V CERAMIC X7R 0603 00.07 1 478-1235-1-ND CAP CER .047UF 10% 25V X7R 0603 00.29 1 277-1023-ND CONN TERM BLOCK 3POS 5MM PCB 02.05 6 277-1024-ND CONN TERM BLOCK 4POS 5MM PCB 02.72 3 1N4004GICT-ND DIODE 1 AMP 400V DO-41 00.04 1 P2N2222AOS-ND TRANS SS GP NPN 40V TO-92 00.39 1 490-2279-ND POT 10K OHM TRIM 25TRN TOP ADJ 00.73 3 490-2280-ND POT 100K OHM TRIM 25TRN TOP ADJ 00.73
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