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Shopping for power supply

Discussion in 'Power Electronics' started by Micro Farad, Jun 22, 2014.

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  1. Micro Farad

    Micro Farad

    7
    0
    Aug 8, 2011
    I am looking for advice on a power supply to buy for my electronics hobbies. I have been using 9V batteries and unreliable power transformers for a while. I recently started taking some of my electrical engineering coursework with lab work and have found the rack mount power supplies in the lab far more useful even than the oscilloscopes. Just having a reliable power supply makes all the difference. Here are the features I want:
    Adjustable dual supply to at least -12V/+12V
    Quality product but affordable price, a tool I can be happy using far into the future without spending myself broke
    Switched mode? (more expensive but lighter and more efficient, do you recommend linear for me?)
    Short circuit and overload protection
    Adequate current and power ratings for hobby electronics (I'm sorry this is so vague, I need advice)

    I am wondering partly what power supply you recommend I buy, but also what current delivering capabilities my supply might need. Most of my projects are quite low current, well under an amp, but I don't want to hinder myself from projects with higher current demands in the future. I want a tool that will be useful for me for a long time to come. I am investing in my hobby now because if I'm going to buy the right tool, I might as well buy it as soon as possible so you can use it for as long as possible. That is my thinking. I need some info on a few power supplies you recommend and what you believe their advantages and disadvantages might be.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    I've found that dual power supplies can be oddly expensive.

    I bought a pair of single power supplies and use them. Mine were under $100 each. 0 to 30V 0 to 5A digitally controlled. Not too bad.

    If you go this route, make sure there are three output terminals, positive, negative, and a separate ground. This will ensure that the supply is floating which allows you to connect both supplies together to create a double-ended power supply.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    Unless it really matters to you (like if you're designing low distortion preamps, or something really noise sensitive) either linear or switchmode should be fine. The ones I have (and I can't find a link at present) are linear, but switch voltage taps on the transformer using a series of relays to keep dissipation relatively low.

    I find digital to be better because I can dial in a current limit of 15mA, or a voltage of 9.75 volts simply rather than reading it from a meter. Mine do up to 5A, but the presets are typically set for 100mA or less.

    There are 5 main types of controls

    1. 2 rotary controls, one for current, one for voltage. If these are single turn, they can be hard to get accurately set, if they are multi-turn it can be annoying to make a large change.
    2. 4 rotary controls, coarse and fine for voltage and current. Whilst you can make both coarse and fine adjustments, it can be a bit annoying if you want to go just that bit higher or lower and you've hit the end of the fine adjust. You have to use the coarse adjust, overshoot, then trim back.
    3. digital single knob. Uses a rotary encoder to set both voltage and current depending on some mode setting. Almost always allows you to adjust the size of each step. Takes a while to get used to. If used poorly it has all the disadvantages of (1) and (2). If used well, it has all the advantages. Mine work like this. The major drawback is that you need to be certain you're adjusting voltage or current. My PSUs flash the one you're adjusting.
    4. keypad input. Allows direct entry of values. seems wrong to someone who likes controls that turn. Needs to have some form of up/down control to adjust voltage/current or you'll tear your hair out. I have one of these for my 100V 1A power supply. If you want to go from 1.000 volts to 1.001 volts you have to press <set><1><.><0><0><1><set>. To change the current from 23 mA to 24mA, you go <set><set><0><.><0><2><4><set>. It's really easy to end up setting the wrong value Aaagh!
    5. And the fifth is via an interface to your PC. It is rarely available on its own, but as an option. Often also allows monitoring of voltage/current. Most likely only available on higher end and digital units. Mine have this option, but I didn't get it.
    What voltage range do you need? I don't know what you need, but I wanted at least +/- 18V. 30 came stock standard, so that's what I have.

    What current range? For me, I wanted at least a couple of amps, but I rarely use anything more than a couple of hundred milliamps. Whatever you need, get something capable of supplying at least 50% more. There's no point in running a PSU at close to full load all the time.

    Dual power supplies that allow you to set both voltages simultaneously and to connect them (via a load switch) simultaneously are an advantage, but not one I bothered with. Having 2 power supplies gives you some added flexibility if, for example, you want asymetric rails, or you want two positive rails, etc.

    Oh, load switches. I recommend you get a supply that has a load switch. you want to be able to set the voltage and current without the load being powered. It's also useful to quickly remove power if smoke stars leaking out.

    My power supplies have an additional mode where rather than current limiting they disconnect the load if the limit is exceeded. Sounds great, but I rarely use it. Any input capacitance at all can cause it to trigger when power is applied, and it may be too sensitive to transients. If you get it, great, but I wouldn't pay extra for it.

    Beware of the output capacitance of bench power supplies. Mine has 100uF. This can provide a pretty hefty spike of current regardless of the current limit settings. Lower is better here -- but higher helps the manufacturer get lower noise and ripple.
     
  4. Micro Farad

    Micro Farad

    7
    0
    Aug 8, 2011
    I hear what you're saying about a load switch. I wondered why there was not such a thing on the rack supplies in my school's EE lab. Every time I connect everything up, then remember the voltage isn't set, disconnect the cables, turn on the power, dial in to the correct voltage, reconnect the cables... it's a major pain.
    That's a clever trick they do with the relays. The rack supplies I've used make a clacking when you go past certain voltages, so I suspected something like that was going on when I first used it. I've not been satisfied with the 2 rotary controls type supply, I think digital sounds nice, particularly keypad input. I've used an oscilloscope with the exact sort of wheel control you're talking about and I always end up finding the keypad it also has much more convenient.
    I also like your suggestion of possibly getting two single floating supplies. That makes a lot of sense to me. Can you point me to some good brands?
     
  5. Ehsan

    Ehsan

    100
    1
    Jun 12, 2014
  6. Micro Farad

    Micro Farad

    7
    0
    Aug 8, 2011
    Yes, I have considered building a linear power supply. I have all of the parts and know-how already but I would need to make a nice enclosure and buy some good connectors. My only problem with that plan is a commercially available power supply will almost certainly have better characteristics and have a built in voltage reading, which is a feature I greatly desire.
     
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