Low voltage AC power source

Discussion in 'General Electronics Chat' started by dsharp02, Sep 9, 2015.

  1. dsharp02

    Thread Starter New Member

    Aug 8, 2015
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    As a beginner I think I've gotten to the point where I want to explore AC circuits. However I don't have an low-voltage AC power source. I don't feel comfortable trying to make my own transformer, so I'd prefer something pre-made, and cheap. Do they make lab bench power-supplies that provide adjustable AC voltage? I have a limited budget, so cheaper is better.

    I tried to search the forums but the results pretty much all seemed to be people who already had a source of low-voltage AC.

    Thanks for your time.
    Dave
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I use a Variac, connected to the primary of 120V-24V transformer. The Variac allows me to vary the input voltage of the transformer from 0V to ~140V. The step-down transformer provides isolation from the AC line. The transformer serves to limit current in the secondary in the event of a whoops.

    [​IMG]
     
  3. #12

    Expert

    Nov 30, 2010
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    Variac's aren't cheap, but they are the definitive answer to dial-a-volt in the AC realm.
    Two other approaches include a simple transformer to get one voltage at one frequency or a Signal Generator to get a wide range of frequencies and a little bit of voltage adjustment, like maybe 0 volts to 10 volts p-p.
     
  4. dsharp02

    Thread Starter New Member

    Aug 8, 2015
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    Thanks for the responses. Amazon has a variac for $65 USD that looks exactly like the one pictured above. $65 is cheap enough for me. I forgot about signal generators, Elenco seems to have a kit for $33, so I ordered one of those as well.

    Thanks again.
     
  5. #12

    Expert

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    Don't forget you can use the Variac to energize a smaller transformer, like a 1 or 2 amp rated 12-0-12 VAC output. That will give you a voltage range that is useful to power transistor hobbies and it's safer to include a low power, isolated transformer when your fingers get near the circuits. (Edit: MikeML already said that.)

    I'm talking about safety here. A good Variac can melt metal when you make a mistake. Buy spare fuses. If you never blow one, you will be the first person to ever do that.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    And also if a quick DC supply is needed the application of a bridge and a electrolytic is all you need to get any DC supply up to 160vdc but rate the bridge a cap accordingly in Case!
    Max.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    I know it's been mentioned twice but, just to be clear, a Variac does not provide isolation from the mains so you must add a standard (isolated) transformer at either its input or output for safety.
     
  8. dsharp02

    Thread Starter New Member

    Aug 8, 2015
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    So the setup should look something like:

    Mains -> variac -> isolation transformer

    I'm assuming that the isolation transformer doesn't need to step the voltage down, since the variac can do that for me. So a 120v -> 120v isolation transformer would work?

    Thanks again.

    Dave
     
    #12 likes this.
  9. #12

    Expert

    Nov 30, 2010
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    Right. If you have a big isolation transformer, like several hundred watts, you can place it before the Variac without crippling its capabilities much. If you only have a small isolation transformer, it's better to place that after the Variac.

    The Variac has a no-load current requirement. Trying to power a Variac with a 10 watt transformer might use up your whole 10 watts just to get the magnetic field powered.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Yes, any voltage transformer will work.
    The advantage of a lower voltage transformer is that, if it is sufficiently large, it can deliver more current than the Variac current rating (transformer power out VA ≈ power in VA).
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A while ago I required a 240vdc supply that I could bring up from 0v, using a 120vac variac and a 240v/120v transformer, connecting the Txfmr 'backwards' allowed the DC to be variable from 0-240vdc and isolated.
    Max.
     
  12. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    And if you are just getting into electronics,etc.. I'd highly recommend starting with low voltage DC (batteries,etc..) first

    But absolutely make sure you have that isolation transformer with AC..
    In fact this message board doesn't even allow discussions of AC circuits without one.. for safety..
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Wouldn't that be nearly as dangerous as leaving it out altogether? I mean, in either case it would be the circuit breaker that limits the worst-case power.

    I guess the whole isolation thing on this site is a bit of a mystery to me. We can't talk about the "wattless dropper", although it's widely used in commercial devices, but we can talk about big transformers? How about the big one on the pole outside my house?
     
  14. #12

    Expert

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    No. It's still an isolation transformer. Touching any lead after that will not send current through you and your chair to the concrete floor.

    The transformer on the pole is grounded to the planet. The output of the isolation transformer isn't, regardless of which side of the Variac it is connected to.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Oh, duh, yeah. I was thinking about one pole to the other, which could still be dangerous even after isolation, and not one pole to ground, which is much safer with isolation. I guess my head is somewhere else.
     
  16. allenpitts

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    Feb 26, 2011
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  17. hp1729

    Well-Known Member

    Nov 23, 2015
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    Will just a transformer fit your needs? You can get a wall wart that puts out AC .
     
  18. MrChips

    Moderator

    Oct 2, 2009
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    You can pick up a wall wart for a couple of bucks from a second-hand store.
    Be aware that there are three main varieties of wall warts:
    • AC unregulated transformer output
    • DC unregulated transformer rectified output
    • DC regulated switching supply

    This is a one-year old thread. I don't think the TS is still interested.
     
  19. DGElder

    Member

    Apr 3, 2016
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    I have a 10A variac that looks just like the one shown in post #2, a PHC SC-10T, which I got from Amazon for around $180. The secondary winding is isolated from the primary so it will provide isolation if you use a two prong plug in the variac outlets. The third, ground, plug passes ground through from the mains and will defeat the isolation if you use it. I believe the case is grounded as well.

    ...just noticed the OP is a year old :rolleyes:
     
    Last edited: Nov 28, 2016
  20. crutschow

    Expert

    Mar 14, 2008
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    Not true.
    It's the neutral of the standard mains outlet being connected to earth ground that is the electrocution danger.
    The isolation of neutral from earth ground that's performed by an isolation transformer is what provides safety from electrocution in case of the hot wire being touched while being in contact with earth ground.
    Isolation of the safety ground, which is connected to earth, is not a factor in that protection.
     
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