DIY Electronics Controller
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  1. #1
    Why aren't you laughing? ChrisC's Avatar
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    DIY Electronics Controller

    Hi all,
    I've been in a distinctly sparky mood recently. I want to make a few eleccy things and just wondering what everyones thoughts and opinions are on all these ideas:

    1) PC power supply - I'm converting an old PC power supply into a 12V, 6V and 3V power scource. No real modifications except shorting out a wire here and adding a connector there etc etc. Apparently it should be able to supply about 10A @ 12V. It's an old 250W supply (can't remember if it's xt, at or atx).

    To create a stable output on the power supply, a small load is required, do you reckon the 2.2R 25W version of this resitor would be alright or would I need two and stick them in a pair to help with heat dissipation?

    http://www.rapidonline.com/productin...catref=62-8108

    2) Voltage Monitor - if you have access to "everyday practial electronics" magazine, it's one of the projects in the July 06 issue. Basically monitors the 12V, 6V and 3V rails and displays via a simply LED bargraph whether the voltages are too high, high, ok, low or too low, with an alarm as well if any of them strays too far out of line.

    3) Temperature - Again, if you have access to EPE magazine, it's one of the projects in the June 06 issue. It is a digital car display for fuel gages, oil pressure etc and converts an analogue signal into a digital signal, with a low or high alarm. I was thinking that I could put a thermistor in a small silicone sealed box/tube (like one of the glass test kit test tubes, siliconed at the top) and put that in the hang on filter. The surrounding water temp would heat the air inside the little box/test tube whatever and would eventually give an accurate water temp reading. I could then use the alarm setting to control a pair of 12V PC fans or a clip on fan via one of these relays:

    http://search.ebay.co.uk/_W0QQfgtpZ1...3aMEWNQ3aMESOI

    If it works well I could possibly do the same again but use the alarm to control the heater through a min temp function and one of the relays linked above.

    4) Auto top off - assuming PC power supplies can give the 10amps or so I've been told, presumably it'd be fine for the windscreen washer motor (they have a high current draw, anything between 3-5A I've been told)? If so, I'd build a small monostable that, when triggered by the floatswtich, would activate a relay to the pump for about a minute to prevent overflowing.

    Any thoughts on all that? Sorry for ramble.

    Thanks,
    Chris
    "My own opinion is enough for me, and I claim the right to have it defended against any consensus, any majority, anywhere, any place, any time; and anyone who disagrees with this can pick a number, get in line, and... kiss my ass." - Christopher Hitchens

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  3. #2
    Why aren't you laughing? ChrisC's Avatar
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    Could this be a suitable thermistor?

    http://www.rapidonline.com/productin...moduleno=61692
    "My own opinion is enough for me, and I claim the right to have it defended against any consensus, any majority, anywhere, any place, any time; and anyone who disagrees with this can pick a number, get in line, and... kiss my ass." - Christopher Hitchens

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    to destruction & beyond ! davethetester's Avatar
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    1) I can't remember which voltage rail you strap the load across. Remind me & I'll do the calc.

    2) Can't see the point. If it's a PC power supply the voltages will not drift.

    3) Sounds over-complicated. The heater already has a stat so why duplicate ? Why not silicone the thermistor to the side of the HOB and do away with the test tube. Yes if you bias the cct correctly for it to function with that thermistor for the I/P it should work. Can you scan the cct from EPE & post it ?

    4) Again for relyability don't over-complicate. Test to see if there are any heat probs with pump or PSU by running in a bucket or summat. I'm confident there won't be. However washer bottle pumps are not continuously rated (no1 washes their windscreens for very long). It maybe wise to ensure the pump doesn't run too long when topping off. How many float switches do you intend to use in this system ?

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    Why aren't you laughing? ChrisC's Avatar
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    Quote Originally Posted by davethetester
    1) I can't remember which voltage rail you strap the load across. Remind me & I'll do the calc.
    The load needs to go accross the 5V rail. I'm thinking of 2x 4.7R 25W resistors in parralell.

    2) Can't see the point. If it's a PC power supply the voltages will not drift.
    Apart from the auto-top off and fan controller, this project is as much a practice in building things and messing around as it in actual practicality.

    4) Again for relyability don't over-complicate. Test to see if there are any heat probs with pump or PSU by running in a bucket or summat. I'm confident there won't be. However washer bottle pumps are not continuously rated (no1 washes their windscreens for very long). It maybe wise to ensure the pump doesn't run too long when topping off. How many float switches do you intend to use in this system ?
    The mono-stable is to prevent the tank overflowing the case of the switch sticking and to prevent the motor from being bombarded with constant on-off commands as the pump will only top-up till EXACTLY the minimum level required. I plan to run just the one float switch.

    After a little further research I've found the washer motor is impractical as the current will trip the surge protector in the psu. So instead I'm thinking I'll have to use a 240V AC relay to control a powerhead.

    ATM I'm concentrating just on the auto top-off to get it ready before I go off on hol's. Afterwards I'll look into other projects.

    Thanks,
    Chris
    "My own opinion is enough for me, and I claim the right to have it defended against any consensus, any majority, anywhere, any place, any time; and anyone who disagrees with this can pick a number, get in line, and... kiss my ass." - Christopher Hitchens

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    to destruction & beyond ! davethetester's Avatar
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    Gotta question your chosen load resistors:

    Assuming 10% load as instructions .... 1A

    V = IR therefore R = V/I

    5/1 = 5ohms

    Power dissipation at 1A:

    Watts = VI 5*1 = 5Watts approx (err on the side of caution) 1 x 25W 4R7 will be ample.

    I think if you have any worries you should include a 2nd level switch as a belt braces protection against flooding ..... ie if it reaches this level stop everything.

    Where ya going on hols ?

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    Why aren't you laughing? ChrisC's Avatar
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    Gotta question your chosen load resistors:

    Assuming 10% load as instructions .... 1A

    V = IR therefore R = V/I

    5/1 = 5ohms

    Power dissipation at 1A:

    Watts = VI 5*1 = 5Watts approx (err on the side of caution) 1 x 25W 4R7 will be ample.
    Too many number for me. I was recommended two resistors in parrallell to aid temp dissipation, as apparently most failures are cause by over-temp, and they're only £1.07 each anyway.

    I think if you have any worries you should include a 2nd level switch as a belt braces protection against flooding ..... ie if it reaches this level stop everything.
    Yeah, I'll probably fit one of those some time.

    Where ya going on hols ?
    Turkey (the country that is ). Not for a few weeks yet, but I'm skint and I've been told to stand in a puddle in Wales for a week by school before then anyway.

    Thanks,
    Chris
    "My own opinion is enough for me, and I claim the right to have it defended against any consensus, any majority, anywhere, any place, any time; and anyone who disagrees with this can pick a number, get in line, and... kiss my ass." - Christopher Hitchens

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    to destruction & beyond ! davethetester's Avatar
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    If you want to err on the side of caution temp wise the resistor value is wrong. Remember 1A taken to stabalise the supply is 1A less from the usable output. As I remember the instructions said 10% was OK. So why draw more than 1A stabilising current ? If you use 2 x 4R7 resistors in parallel across the 5V rail you will be drawing more than 2A stabilising current and therefore having to get rid of more heat. How's that safer ?

    Wales sounds great ...... & Turkey

  10. #8
    Why aren't you laughing? ChrisC's Avatar
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    If you want to err on the side of caution temp wise the resistor value is wrong. Remember 1A taken to stabalise the supply is 1A less from the usable output. As I remember the instructions said 10% was OK. So why draw more than 1A stabilising current ? If you use 2 x 4R7 resistors in parallel across the 5V rail you will be drawing more than 2A stabilising current and therefore having to get rid of more heat. How's that safer ?
    Cool, that's interesting and not something that I hadn't thought about. Thanks a bunch for the help.

    Wales sounds great ......
    Wales will not be great. It's one of schools optional or die trips, which is basically the teachers releasing all that built up anger over the year on us.

    I'll be ordering some parts on Rapid Electronics this weekend, so I'll get:

    a) 4mm connectors from power supply to various other things
    b) 25W 4R7 Resistors
    c) 2 110mm acrylic boxes, one to house 12V cuircritry, the other to house the mains voltage relay

    Thanks for the help.

    Thanks,
    Chris
    "My own opinion is enough for me, and I claim the right to have it defended against any consensus, any majority, anywhere, any place, any time; and anyone who disagrees with this can pick a number, get in line, and... kiss my ass." - Christopher Hitchens

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    to destruction & beyond ! davethetester's Avatar
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    Is it loads of hiking type stuff then ?

  12. #10
    Why aren't you laughing? ChrisC's Avatar
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    DAY 1 - Set up tent. Argue with each other for who's going in the middle.
    DAY 2 - Rock climbing - should be a highlight.
    DAY 3 - Hike.
    Day 4 - Hike. (no doubt lose a shoe along the way
    Day 5 - "Water-activities" (sounds like buckets and water guns to me )
    Day 6 - Pack and leave... finally.

    Thanks,
    Chris
    "My own opinion is enough for me, and I claim the right to have it defended against any consensus, any majority, anywhere, any place, any time; and anyone who disagrees with this can pick a number, get in line, and... kiss my ass." - Christopher Hitchens

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    to destruction & beyond ! davethetester's Avatar
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    Can I come ?

  14. #12
    Why aren't you laughing? ChrisC's Avatar
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    Quote Originally Posted by davethetester
    Can I come ?
    Go for it mate. Turn up at Birkdale School (I really think they should change their name as all the pupils get to call it BS school. ) next Monday and look for the gaggle of distressed looking teenagers being forced into vans.

    Thanks,
    Chris
    "My own opinion is enough for me, and I claim the right to have it defended against any consensus, any majority, anywhere, any place, any time; and anyone who disagrees with this can pick a number, get in line, and... kiss my ass." - Christopher Hitchens

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    Registered User 2manytanks's Avatar
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    Chris,
    son your either going to be an Electronics wizz or a mad scientist (im rooting for the second one )
    2manytanks, doesn`t post enough to be a mod

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    Why aren't you laughing? ChrisC's Avatar
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    "My own opinion is enough for me, and I claim the right to have it defended against any consensus, any majority, anywhere, any place, any time; and anyone who disagrees with this can pick a number, get in line, and... kiss my ass." - Christopher Hitchens

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    to destruction & beyond ! davethetester's Avatar
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    More thoughts.

    If you want 50W power dissipation (which is what you would have with 2 x 4R7 in parallel) then why not go for a 4R7 50 watter from rapid (same page) at £1.20 ea. This makes far more sense and would limit the stabilizing current to roughly 1A. As I said before 1 x 25 watter is overkill anyway.

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