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Let's say an op-amp or similar device requires, say, +- 19V DC to operate; that is, there are separate pins marked +19V and -19V respectively and let's say, I just happen to have a couple of 19V laptop power supplies sitting about collecting dust. Obviously, one of the adaptors can be connected with due regard to polarity, to the +19V pin but my question is this; can the other adaptor be connected to the -19V pin with the Earth rail connected to the pin and the active rail connected to the circuit ground? I've pondered this at length and decided, given that a DC voltage is just a potential between two points, it should be OK but I'm sufficiently skeptical to get a second opinion first. Thoughts???

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I don't understand how you want to wire the component. Try to add a schematic, you can use one of the online ones like circuitlab.com – trygvis Nov 22 '12 at 5:51

4 Answers

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There seems to be some confusion here.

Most laptop power supplies have three input connections:

  • AC Hot
  • AC Neutral
  • Ground

Every laptop power supply will be floating relative to the AC inputs. This is very much a safety necessity, as if someone wired a plug backwards, and the DC output ground was connected to the AC neutral pin, you would get mains voltage on the DC output.

However, many (most?) laptop power supplies connect the output ground to the input power ground pin. As such, if you break the ground connection (i.e. the third pin on the power connection), it will be floating.

Basically, if you disconnect the ground pin of each adapter, the output is pretty much guaranteed to be floating, at least with respect to DC (AC common-mode garbage is another matter).

@DavidKessner's point about asymmetrical start-up time is a valid one, so it would be a good idea to put a diode in series with the + and - rails, to ensure that if one power supply starts faster then the other, you don't wind up reverse biasing the other.

If you do do this, it is a important safety necessity that you make sure you connect the DC ground to the wall-ground.
In other words, make sure that the center-point where you connect the two adapters is properly grounded to the third pin of the AC outlet.
This way, if one of the adapters fails, it will properly trip the AC circuit breaker. If you do not have this ground, and one of the adapters fails, you could wind up with mains voltage everywhere in your circuit.

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If you don't mind the cost of a DC to DC convertor with +15V, 0V and -15V output, then this is probably the securest way to go especially as the noise issue on two series-connected laptops supplies would be avoided. Traco make a decent range and this range: -

http://www.tracopower.com/fileadmin/medien/dokumente/pdf/datasheets/tel3.pdf

Should do the trick. In particular I think the TEL 3-2023 should be ideal.

Several other manufacturers provide similar products.

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Yes since the charger is floating you can connect them in series with center grounded.

However just because they are floating does not mean there is good CMMR, in fact there is significant CM hum so if you have any high impedance inputs the CMMR of the Op Amp will be challenged by their capabaility to reject 50/60Hz hum & high frequency pulses inside the chargers and so you need to either suppress the CM hum by connectting the center to a real ground or use a large CM choke on both DC lines. By the way, this is a common fault on Laptop external mic that get hum and the solution is the same.

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Think there are any potential problems (no pun intended) with the two adapters sharing a common ground through the wall socket? – Samuel Nov 22 '12 at 4:29
Since they are floating , there is no potential risk in operation, unless you use it in the shower and have a ground fault. – Tony Stewart Nov 22 '12 at 4:32
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The OP didn't say they were floating. I have several non-floating laptop supplies. Also, depending on the load, if the two supplies do not turn on exactly at the correct time then one supply might be reverse-biased by the other supply. Since the supplies are not designed for that they might be damaged. The worst case load for this is a resistor between +19v and -19v. An opamp (what the OP used as an example) is very close to that worst case. – David Kessner Nov 22 '12 at 4:55
I beg to differ. grounded plug laptop power supplies are always floating on the DC side. the AC ground if used is only used for sheilding. In the 2nd comment, if you had a huge low ESR cap across the total 38V load, it would back drive the other supply. Consider the charger already has a low ESR large cap with over current and reverse protection built in, it won't happen. THis protection is needed when connecting an unpowered charger into a battery powered load. – Tony Stewart Nov 22 '12 at 5:09
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@Richman I just measured two supplies from two different manufacturers. One was isolated and one was not. Obviously your mileage may vary. I also measured the voltage at the DC jack of my laptop and it was 0v. Obviously my laptop charger does not need to protect itself from the "unpowered charger into a battery powered load" scenario. Again, your mileage may vary. I have no doubt that there are some laptop supplies that would work with this. But I also have two common ones here (ASUS and DELL) that obviously won't. – David Kessner Nov 22 '12 at 5:38
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up vote -4 down vote

Interesting question. However... why not just try it, connect the wires and see what happens? The worst you can expect is a smoked op-amp. They're cheap. There's nothing wrong with your assumption that it should work.

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He could also end up with a smoked laptop power supply. Depending on how they are grounded. Not nearly as cheap to replace as a single op amp. – The Photon Nov 24 '12 at 16:42

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