Instant Hot Water Heater Schematic

Discussion in 'The Projects Forum' started by CmdrDick, Jan 12, 2013.

  1. CmdrDick

    Thread Starter New Member

    Jan 12, 2013
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    I have a Titan N120 instant hot water heater that has been driving me nuts.
    It continues to fry the SCR's and I believe that the controller board is at fault. This is a very small surface mount board and I'm definitely not digging into that. What I'd like to do is to find a schematic for a design where I can build it out of discreet electronic components. I'd like to stay with the original hardware design, but I don't quite understand the dual S6065J SCR's where one is wired hot lead to Anode and the other is hot lead to cathode. They are soldered in sets, so there must be a reason, possibly each works off a half wave?? The hardware is pretty basic except for the board.
    The unit has a simple magnetic reed switch sensing the water flow and a thermal overheat breaker and a temperature probe. One side of the 240AC power goes thru the SCR's and into two 5500 watt coils and back thru the overheat breaker to the other leg. The gate signals come from the board. If anyone knows of a schematic that will make use of the present hardware, it would be sincerely appreciated. Titan is a terrible outfit to try to get any help from. They promise to ship and never do. After a month of cold water I'm going to build a controller board myself. I've got a ton of circuit parts in the shop and plenty of experience with AC, I just need a simple control circuit that can be hand built with TTL/CMOS or simpler components..
    Right now I've got one side of the coils wired directly from the 60A knife switch panel and start the water running before I close it so we can take showers. It's a pain, but this time of year the water is cold, brrrrr!
    Thanks, Dick
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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  3. JDT

    Well-Known Member

    Feb 12, 2009
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    You are right: each SCR handles a half wave. This is common in higher-power circuits where a triac is not man enough. You have two 5500W heaters in parallel running from 240VAC? If so, this is 45.8A. Large SCRs will be required, on heatsinks.

    There is a safety issue here: It is vital that the heaters switch off immediately the water flow stops. I take it the over-heat switch is in addition to the SCRs. So that even if the SCR's fail the unit still does not boil?

    You could use a Crydom Solid-State-Relay. At least 50A, more is better and connected so that the flow switch controls this. The SSR must have a heat-sink. Then you could probably ditch the existing electronics.

    Thinking that the SSR could be mounted on a copper tab attached to the incoming cold water pipe instead of the heatsink. Safety, as above, and proper earthing is essential!
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
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    Hi CmdrDick,
    1. Can you post some pictures of the unit showing where the the major components are?
    2. Do the SCRs fail right away or take longer? Overheat then fail?
    3. Perhapes the SCRs fail because of a design error.
    Regards,
    Ifixit
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,011
    3,233
    The SCRs could be failing if only one is being turned on instead of both.

    You could use a large SSR such as JDT suggested but you would want to control it from both the flow sensor and the temperature probe (AND function). Otherwise at slower flow rates the water may get very hot until it trips the overtemperature sensor, which would give a fluctuating water temperature.

    Do you have an information on the temperture probe characteristics? You would connect the probe signal to a comparator with some hysteresis so it would turn the heat on and off to maintain the desired water temperature. Can you measure its cold resistance and warm resistance?
     
  6. CmdrDick

    Thread Starter New Member

    Jan 12, 2013
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    Hi All: The SCR's in the original circuit are S6065J rated at 60 amps, and mounted on a bronze block which is part of the incoming water pipe.
    The 5500 watt coils in parallel are fed one from each SCR, that is what confused me about the half wave theory. In any event I found a source for S8065 SCR's from Malaysia and bought several of them. These are rated at 80 amps and should more than do what is needed here.
    The original flow sensor is a tiny magnetic reed switch mounted on the incoming pipe. It closes each time that the magnetic device in the copper water line rotates one turn. The leads went directly to the board. There must be a circuit on the board that senses this switch contact that also eliminates the possibility of the magnet stopping at the switch closed position. Otherwise the heater could stay on when no water flows. The original board also has four selector switches that power the coils at four heat levels. I don't care about trying to duplicate this part of the circuit as we always kept it at the highest position. The house is a two story and it takes a lot of heat to make it to the upstairs bath in winter.
    The temperature sensor is a tiny loop that is connected to the board. The sensor end is coated with silver paste and slips into a tiny hole drilled into the water coil housing. The sensor measures 70k Ohms at room temperature and 9k Ohms at 200 degrees F. This MUST be used as the heat control input.
    The overheat safety breaker is a separate 2 pole component on the #2 legs that is somewhat slow to react and the system has to get really hot before it pops. Then it takes a LONG time before it resets. This is a last line safety and can not be considered a heat control at all, just an overheat disconnect.
    I Don't think there is a design error, more likely a motherboard component failure as the unit worked fine for about a year. It's also possible that this was fried by lightning as we have hellacious storms in the summer when this unit got fried. The circuit is protected by a 250 volt 60 Amp breaker in the main panel. I plan to make hi-res scans of the bottom of the older board which may allow me to draw a schematic. The newer board is much more complicated.
    I'll get some photos of the components and learn how to upload them this weekend. Thanks
     
  7. CmdrDick

    Thread Starter New Member

    Jan 12, 2013
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    Hi:
    I justed returned from traveling and hope to finish the Tankless Water Heater Circuit soon. As stated earlier, the problem is a fried big bucks control board that seems a recurring problem with our lightning surge AC power. I'd like to replace that board with a hand built circuit that will control the two SCR's (S6065J or S8065J) and accept inputs from the Flow Sensing Reed Switch and the Temperature Probe (70-200 Ohm). The first photo is the unit as removed from the wall.
    [​IMG]
    The second is the wiring diagram.
    [​IMG]
    The third shows the desired schematic wiring points.
    [​IMG]
    The last is a blank sheet with connection points for convenience.
    [​IMG]
    I know that I could probably use a PID for the temp control, but that would be bulky and I'd still need a flow control circuit.
    The problem there is that the circuit needs something to eliminate the power staying live if the flow sensor stops in a position where the magnet holds the reed switch closed. I saw a no-bounce circuit for a computer keyboard that might work if modified a bit. It eliminated all but one pulse to the circuit and then reset. You guys are the Gurus on this stuff and I'm not afraid of TTL or CMOS, but I'd like to keep this at least that simple. Space is no detriment as I can move the circuit into another box, but if I can build it on a 3-4" square board, all the better.
    Thanks again for your time. It is hugely appreciated.
    Dick
     
  8. ifixit

    Distinguished Member

    Nov 20, 2008
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    What are the part numbers on R27, R28 & U5? I can't read them clearly. It might be easier to fix the design flaw (if there is one) than to re-design.

    Thanks,
    Ifixit
     
  9. CmdrDick

    Thread Starter New Member

    Jan 12, 2013
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    R27 & R28 are both 221 and U5 is a moc3063
     
  10. CmdrDick

    Thread Starter New Member

    Jan 12, 2013
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  11. ifixit

    Distinguished Member

    Nov 20, 2008
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    Hi CmdrDick,

    I drew a sch of the power supply section of your board and simulated it. Seems to work okay and suppresses transient surges well. I guessed at the value of VD1 VZ1 C1 VR1 R31. Can you see what they are or measure? This part of the PCB is easy to test and ensure that it is working so you can re-use it for your new "simple" design.

    The SCR interface is straight forward as well. The MOC3063 is a TRIAC opto-isolator. I sketched a schematic of that as well. I also have ideas for the thermistor and flow sensor interfaces. I can present them if you are interested.

    By the way, you have done very nice pictures and graphics to show what you want to do.

    Regards,
    Ifixit
     
  12. CmdrDick

    Thread Starter New Member

    Jan 12, 2013
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    Yes, thank you. I printed the Power Supply schematic and I'm trying to absorb it. I'd be very interested in seeing the additional schematics, especially where the mag reed switch is implemented. That has been a real hang-up for me.

    If / when I get this completed, I'll document it well so the next guy has a jump off place.

    Thanks, and by the way, I have over 10,000 mostly used electronic parts here. I inherited my uncles electronics bench and garage full of stuff and I don't know what to do with it all. He was a big time surplus electronics nut back in the sixties and seventies. If you need something that is here, mention that in your reply. I've been selling it off a piece at a time on eBay (A1_Surplus_Sales), as I have not been able to find an interested party for the whole lot. There is stuff here that I have no clue to what it does, mostly ancient mil test instrumentation. Thanks again, CmdrDick
     
  13. CmdrDick

    Thread Starter New Member

    Jan 12, 2013
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    The following are the components on the Tankless heater board
    One of them does not match it's ID number (see R27,28). Having the same ID number and different measurements indicates perhaps a blown resistor.
    On the small caps, C2,C3, C8, I could not identify them.

    C1 SRMA 105k 400v
    C2 220 25v
    C3 ???
    C8 ???
    C9 88nF in circuit
    J1 6 pin conx
    P1 Pot 5k

    R1 104 100k Ohms
    R2 5620 .56k
    R3 4702 47k
    R4 4702 47k
    R5 5620 .566k Ohms
    R6 5620 .566k Ohms
    R7 5620 .566k Ohms
    R16 4702 42 Ohms
    R19 6042 38k Ohms
    R21 1003 100k Ohms
    R23 2201 2k
    R24 1016 9k
    R25 3300 .33k
    R27 221 8.6 MegOhms
    R28 221 .36 MegOhms
    R29 1023 38k Ohms
    R30 1243 124k Ohms
    R31 ------- 179 Ohms

    U1 TS78L05
    U2 PIC 16F689
    U3 TS358CS
    U4 SN 1020 41050I
    U5 MOC 3063

    VD2 Diode S1G E9
    VR1 Sincera 14E471k
    VZ1 PJ0169 925A
     
  14. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi
    Your uncle and I have something in common... we both collected old electronic junk from days gone by. My wife would kill me if I took in anymore. :) However, I think we have members that live in, or near Phoenix and they might be interested in "unloading" your garage.

    Your board uses a PIC micro controller to do the job and that is by far the best way to go for a modern design approach. However, if the problem can't be resolved I have attached a schematic for a possible simple solution. The circuit needs to be reviewed carefully before any building takes place, but at least we'll have something to chat about.

    You will have to run some tests to determine the pulse dutycycle and frequency and how this relates to water flow rate. I heard an installer say on TV that his unit would not fire up unless 2.5 gallons per minute, or more, was flowing. The flow detector circuit is very simple, but I would like to have time to work on a digital design. I would like to improve reliablility. I have biased the reed switch so that it doesn't matter if the reed is stopped on or off.

    The fact that R27 & R28 measure high impedance tells me that they are likely blown open. They should be 220 ohms. It is not suprising that this happens when the SCR fail open. Hopefully, the Opto-isolator MOC3063 was saved by the resistors and is not damaged. If you were to replace R27 & R28 & the SCRs, the system might work again, but we still wouldn't know why it failed in the first place.

    If you think power surges or lightning is a problem then you should consider installing a whole-house surge protector at your service panel.

    U1, TS78L05 is a linear voltage regulator that supplies +5 volts for all the circuits. I need to verify that it and the input circuits can supply the milliamps needed. I'll re-simulate now that I know the values for R31 and C1. R31s colour bands, from bottom to top, should be brown-grey-brown and gold. Correct? 180 ohms and you measured 179.

    Where is P1, the 5K pot? I don't see it in your pics.

    Regards,
    Ifixit
     
  15. John P

    AAC Fanatic!

    Oct 14, 2008
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    What I'd expect would be a kind of charge pump operated by the flow sensor. Every cycle would deliver charge through a capacitor/diode circuit to charge another capacitor which would discharge fairly quickly in the absence of pulses. Then a comparator would drive the optoisolator based on that voltage and the temperature sense.
     
  16. CmdrDick

    Thread Starter New Member

    Jan 12, 2013
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    Thanks guys, I've got a couple of days of classes and then I'll get back to this.
    The P1 is a micro sized trimmer pot that may be covered with hot wax in the online photos.
    P1 is just to the right of the blue contact terminal strip. It looks like it's tied to the top of R24. and the farthest right terminal (reed switch). I'll look closer when I have time later this week.
     
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