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Posts posted by Gazza

  1. The rating of the iduction motors that I am going to control is above 5 HP, 415V, 3-phase wound rotor or squirrel cage .
    The available materials for me are , 8085 and 8086 microprocessors with their interfecing devices , interfacing cards like DAC and ADC , 8051 microcontroller.
    Thank you Gazza.

    Sounds good to me, so you just need to program the PWM sequence and build the power circuit.

    It is going to be a lot of work, I will be interested to see how it turns out :)

  2. Hi Folks, i hope you can help me out with this real irritating problem;

    I have a radio scanner installed in my vehicle with an extension speaker plugged into the rear 'extension speaker' socket. It works OK, if a little quiet. So I built a 7w amplifier and hooked it up to the same output of the radio using a loading resistor across the amp input. The amp works wonderfully except that it is picking up bad noise from the vehicles electronic systems. I've tried regulating the supply line, adding extra caps etc, altering the load.... To check the noise wasn't coming down the signal line i coupled the amp to a 12v gel battery and it runs without any noise at all. What do you think guys???.......

    I think this is a better place for you to get an answer :)

  3. Hi there i have problem with my transformer power supply. The input is 415v and the output is 130v. Now i need 110v from the 130v, so how to get 110v from 130v? Only need to reduce 20v to get 110v. What electric part or electronic part to reduce this voltage???? Plz help to solve my problem. Thanks.

    How many KVA is your existing transformer?

  4. Hi there,
    I have an electric bike. As per its manufacturer's instructions, the SLA battery needs a charging time of about 8 hours to reach 90% charged from entirely discharged battery. In my point of view, it is too long and not convenient, especially when I need to charge it in the day time.
    So, anyone can let me know if I can increase charging voltage/current to shorten the charging time, for example down to 3 or 4 hours???
    Thanks a lot for your suggestions.


    Can you be more specific on the type of battery. Make. Model, Ah rating and Wattage rating

  5. I would like to build a circuit to automatically measure the dryness of the soil in a hanging basket and then water it when required. Some type of soil probe and a water solenoid set up.
    Any ideas?

    If I was doing this, I would use a Microcontroller. I have no experience in soil sampling, so I don't know what the proper device is, I did find this. http://www.berryhilldrip.com/Irrometr.htm#Model%20'R

    You could set up the controller to monitor the output level of the sensor and set up some limits, at a low level for moisture, trigger a solenoid to a water reservoir, at the high level of moisture turn off the solenoid.

  6. acdc,
    Is there a chance that this unit has a schematic attached inside that you could scan and post? It would be much more helpful to see how it is already setup.
    As ante has mentioned, there are too many parameters to be able to give a "one size fits all" type of answer.


    I know this is an older post, but I have a lot of UPS experience.

    It sounds like you found an old Emerson UPS with bad batteries. Most likely there is only a few bad batteries in the string so you wont need to replace all of the batteries.

    What you need to do is get a constant current supply and inject each battery block with 10A of current. With a DMM measure the voltage across each cell. You should get a voltage in the 80-150 mv range. If the drop is above 1V there is an open cell and you need to replace that block.

    Since all of the batteries are in series they all need to be functional for the string to operate.

    As far as the generator is concerned, are you trying to use a DC generator to power the DC bridge? Or using a transfer switch have a standby AC generator.

    I guess it doesn't matter because neither will work.

    DC Generator: If you use a DC generator to feed the DC Link during a power failure, what is going to turn the generator? The Power is off, that's why you bought a UPS.  I can think of at least 3 more major problems this would cause.

    AC Generator: This wont work without batteries either. The function of the batteries is to bridge the gap between the beginning of the power failure and the time it takes the generator to ramp up to full voltage.

    If you want your UPS to work, you are going to have to invest in the new batteries.

  7. Hey all,

    I'm not familiar with PLCs. I only have experience working with the BASIC Stamp microcontroller. Are there any good reads for a newbie like myself?

    --thank you.

    For the money, if you want to teach yourself, try www.automationdirect.com.

    It is a fantastic site.

    Also if you just want to try using a simulator, google "The Learning Pit" They offer an excellent AB PLC 5 simulator. I am pretty sure it only costs $25 and it comes with a bunch of examples.

  8. LOL! Not entirely convinced! Interesting!

    For a simple low-pass RC network, the cut-off frequency is 1/(2.Pi.R.C). The voltage gain for a simple sinusoidal input at that frequency is

    Vout/Vin = 1 / Sqrt(2) = 0.707 = 71%

    The power gain is the square of this (power is proportional to square of voltage):

    Pout/Pin = 1 / 2 = one half. Very convenient.

    Expressed as decibels:

    10 log (1/2) = -3.01dB

    Not exactly 3, as I said. But conventionally engineers have decided that it's close enough to 3 to be able to call the cut-off frequency the '3dB point'. It could be argued that '10 log(Pout / Pin)' stinks of arbitraryness, but Mr. Bell found that log-base-ten simplified the maths quite a lot where power (not amplitude) is concerned, and engineers subsequently noticed that multiplying the logarithm of gain by 10 made 10dB equal to a power gain of 10. All for convenience. If Mr. Bell had settled upon the natural logarithm as the way to go, then the numbers would be a nightmare.

    I can imagine the meeting: 'Hey, 3 is nice round number, and half is a fantastically convenient gain, AND it happens to be roughly the frequency which is the inverse of the time constant of the system. Let's use it to define where a system stops or starts passing signals, and we can call it the 3dB point. Waddya reckon?" Surely it must have gone down to a vote.

    It didn't escape these guys that if you created a second order filter by cascading two identical first order RC filters, at frequency 1/(2.Pi.R.C) the response would be -6dB (1/4), which wasn't so convenient. They still wanted to define the 'half power gain' point of their systems, and so Butterworth and Chebyshev and all the other second order filter folk, for convenience, worked out the frequency where their own designs had a power gain of a half (the 3dB point), and used those instead of the 6dB points to define the passbands of their designs. Just for consistency, you understand.

    For 3rd order filters, which increased attentuation to 9dB at that f=1/(2.Pi.T) frequency, they did the same. "Let's not use the 9dB point to sell our designs", they said. "Stick with the conventional 3dBs" was their choice.

    So, to eat my words, 3dB was chosen to describe higher-than-one order active designs, but only because nature had a convenient "nearly 3" response in it's own passive systems.

    That is an excellent answer. I am convinced.

  9. Gazza,
    Definition of the cut-off frequency has nothing to do with the sampling theory. In your diagram to allow a signal to have a bandwidth of f2-f1 we need to at least sample it at 2f2.

    Also as quoted from 'All About Electronics', the amplitude at the 3dB frequency is around 70%(sqrt(2)/2) of the centre frequency; and the power is half.

    Although not fully convinced, the most interesting is Cabwood's post.
    Mathematical Convenience

  10. *1 The cut-off frequency is not strictly defined as the frequency at which attenuation begins. In Butterworth type low-pass filter, the cut-off frequency is defined as the "–3 dB point of total characteristics". In a Chebyshev type low-pass filter, the cut-off frequency is defined as the "first point beyond the maximum ripple amplitude of a passband". Roughly, however, the cut-off frequency can be defined as the frequency at which attenuation begins.

    That quote from this page says it all. http://www2.okisemi.com/site/faq/speechfaqs/speechgeneral/general6

  11. Dear kebloks, Gazza
    I think Sukhbinder already knows what you said.
    His question (if I got it right) is: why do we use the -3dB figure for calculating our cut-off freq.s why not 2dB or any other value.
    This has been on my mind for a long time.
    My guess is there are some practical reasons behind it
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