Shop OBEX P1 Docs P2 Docs Learn Events
Simple low power isolated power supply — Parallax Forums

Simple low power isolated power supply

JonathanJonathan Posts: 1,023
edited 2011-12-16 20:34 in General Discussion
Hi All,

I have a panel meter that I would like to use to monitor the voltage of a battery bank. The meter isn't common ground, so it can't read its own power supply. So, I need a simple isolated power supply that can provide 9V or so at a couple of mA. I don't want to use 9V batteries. The only AC available is from an inverter running from the battery bank, but I don't want the losses involved in a transformer set up. Any ideas how I can take the 12V from the battery bank and isolate 9V from it?

Thanks!

Jonathan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-06-03 16:12
    You have to use a transformer. There's no other practical way to get the isolation and to transfer power without it. If you have a source of AC, you can use that, otherwise you have to provide another inverter just for the meter's supply. There are all sorts of off-the-shelf small isolated supplies available and they all use a high frequency inverter and transformer.

    TI has some modules for producing up to 1W: focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=562&uiTemplateId=NODE_STRY_PGE_T

    It looks like these cost maybe $5 to $10 in 1K quantities. I don't know what they'd cost in singles. You might be able to get a sample.

    Post Edited (Mike Green) : 6/3/2009 4:22:29 PM GMT
  • JonathanJonathan Posts: 1,023
    edited 2009-06-03 16:40
    Mike,

    That's not the answer I wanted!! :-0

    But thanks. I had a feeling it might be that way.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
  • Mike GreenMike Green Posts: 23,101
    edited 2009-06-03 16:52
    I did find one capacitor isolated 9V supply here that will avoid the transformer:

    www.techlib.com/craig/craigs_isolated_9_volt_supply.htm

    It looks like the 4011 used only has an output current of about 2mA, probably not enough for your needs. You might be able to use a different IC with a higher output current. The 74AC00 might work. It can handle up to 24mA.

    Post Edited (Mike Green) : 6/3/2009 5:16:34 PM GMT
  • JonathanJonathan Posts: 1,023
    edited 2009-06-03 22:46
    Mike,

    Thanks for digging up that schematic! I happen to have all the parts on hand so I'll give that a shot.

    As a backup plan, I can use a super cap and a DPDT momentary switch. Press the button, charge the cap and the meter will run for a little while, long enough for me to read the voltage. It's been running for 15 mins so far on a .5F cap charged to 9V. Has the advantage of not using juice when I'm NOT looking at the meter. Although the meter draws only a mA or so.

    Still, I'm going to build the circuit you linked and play with that. Good stuff to learn if nothing else.

    Thanks again!

    Jonathan

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
  • Mike GreenMike Green Posts: 23,101
    edited 2009-06-03 23:13
    Remember that the 4011 will work with a supply voltage up to 15V, so it will work directly off your 12V battery. If you substitute a 74AC00, that'll be destroyed with a supply voltage much above 6V. Make sure you put some filtering on your supply input. There's probably a bit of noise on the 12V supply.
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2009-06-04 00:38
    Mike Green

    Thank For sharing that link to the one capacitor isolated 9V supply

    I might have a use for that some time soon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    ··Thanks for any·idea.gif·that you may have and all of your time finding them smile.gif

    ·
    ·
    ·
    ·
    Sam
  • JonathanJonathan Posts: 1,023
    edited 2009-06-04 15:16
    Mike,

    Actually, I'll have to use a 12V vreg, as the batteries are occasionally equalized at 15.5 - 16VDC. I plan on trying out some switch mode regs for better effeciency. Its funny how one starts counting EVERY watt when off the grid. Keeps you honest! :-0

    Thanks for the help.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
  • Mike GreenMike Green Posts: 23,101
    edited 2009-06-04 15:26
    I think the CD4011 has an absolute maximum supply voltage of 18V. You might check a couple of datasheets since these are made by several manufacturers. This would be a good place to use 2 or 3 1N914 diodes in series to drop the 16V to under 15V. If you have a largish (220uF 16V) capacitor across the output of the diodes, that would keep the meter running for a little while if the batteries were disconnected.
  • vkssinghvkssingh Posts: 3
    edited 2011-12-14 22:46
    Mike Green wrote: »
    I think the CD4011 has an absolute maximum supply voltage of 18V. You might check a couple of datasheets since these are made by several manufacturers. This would be a good place to use 2 or 3 1N914 diodes in series to drop the 16V to under 15V. If you have a largish (220uF 16V) capacitor across the output of the diodes, that would keep the meter running for a little while if the batteries were disconnected.

    I want to build the circuit to power a panel meter of the car which require isolated 9V supply, but the circuit does not show any connection to pin 7 & 14 of the IC? How are these IC pins connected? Are the left open?

    If I do not use part "OK1" in the circuit but in oreder to regulate the voltage put a 9V zener in paralle to C5 will it work? I would use a fixed voltage regulator 7809 to reduce the input voltage fron 12v to 9V.
  • kwinnkwinn Posts: 8,697
    edited 2011-12-15 21:06
    They are the power pins. Pin 7 is Vss (- or ground) pin 14 is Vcc (+)
  • vkssinghvkssingh Posts: 3
    edited 2011-12-16 00:43
    That's exactly what my question is, if your see the schematic no connection has been shown to pin 7 & 14. The scematic is at link http://www.techlib.com/craig/craigs_isolated_9_volt_supply.htm

    can you explain what leads are to be connected to these pins for the circuit to work?
  • ajwardajward Posts: 1,130
    edited 2011-12-16 09:54
    Mike Green wrote: »
    I did find one capacitor isolated 9V supply here that will avoid the transformer:

    www.techlib.com/craig/craigs_isolated_9_volt_supply.htm

    Thanks for that link Mike! So many cool things to experiment with!!!

    Amanda
  • kwinnkwinn Posts: 8,697
    edited 2011-12-16 19:08
    vkssingh wrote: »
    That's exactly what my question is, if your see the schematic no connection has been shown to pin 7 & 14. The scematic is at link http://www.techlib.com/craig/craigs_isolated_9_volt_supply.htm

    can you explain what leads are to be connected to these pins for the circuit to work?

    Pin 7 goes to the ground or negative side, and pin 14 goes to the positive side of the power supply that is providing power to the circuit.

    BTW the C1 and C2 voltage rating needs to be high enough to withstand any difference between the supplying and supplied circuits. Don't try to power a circuit floating at 500V with capacitors rated for 100V. Results in loud noises, smoke, and flames.
  • vkssinghvkssingh Posts: 3
    edited 2011-12-16 20:34
    Considring your reply I have to connect the positive side of 9 v supply both to pin 14 and also to pin 2 of IC. Please confirm.

    The C1 & C2 cpacitance value in not shown in the schematic , I guess it is 1 nF, same as C3 & C4. I would use 25V or higher rating for these capacitors, is it OK.

    My second question is, If I do not use part "OK1" in the circuit but in order to regulate the voltage, put a 9V zener in parallel to C5 will it work?
Sign In or Register to comment.