# will it damage a 7.5 volt AC device running it off a DC adaptor? **URL:** \ **Category:** Factual Questions\ **Created:** [January 14, 2010, 6:29am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904 "2010-01-14T06:29:40Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [January 14, 2010, 6:29am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/1 "2010-01-14T06:29:40Z")
I bought a couple of these off ebay: [http://www.monoprice.com/products/product.asp?c\_id=101&cp\_id=10104&cs\_id=1010405&p\_id=4084&seq=1&format=2](http://www.monoprice.com/products/product.asp?c_id=101&cp_id=10104&cs_id=1010405&p_id=4084&seq=1&format=2) shipped to Australia but they came with only 110 volt adaptors. So no problem I bought a 240 volt universal adaptor which is 7.5 volt 800 millamp which it requires… Then I noticed it needs 7.5 volt 800 millamp AC??? never seen any small consumer device that needs AC before. I plugged it for a few seconds with the dc adaptor, the lights come on then I unplugged it. Whats the chances it will damage it permanently running it off 7.5 volts 800 millamp DC instead of AC? ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [January 14, 2010, 6:56am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/2 "2010-01-14T06:56:43Z")
Well if it’s an AC driven unit it won’t even run off a DC adapter so the option does not exist and issues of damage are moot. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [January 14, 2010, 7:12am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/3 "2010-01-14T07:12:20Z")
That’s pretty damn obscure. It won’t work on DC. Jaycar doesn’t seem to have anything at 7.5V AC and neither does Dick Smith. The usual rule is that DC is less likely to damage AC than the other way around so you probably haven’t broken it, you just will need to find something that will do the job. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [January 14, 2010, 7:16am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/4 "2010-01-14T07:16:57Z")
To be more specific when something specifies an AC input using a DC input will usually not drive it properly and may well damage it. A 240V\>120V mains voltage adapter dropping the voltage to 120VAC then plugging in the US adapter should work. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [January 14, 2010, 7:57am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/5 "2010-01-14T07:57:03Z")
Except if the OP is in Australia, **astro**! Then it would be the other way around. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 14, 2010, 9:29am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/6 "2010-01-14T09:29:07Z")
I’d open it up and see if there’s anything that looks like a bridge rectifier in there near the power input (this might be composed of four discrete diodes, or a little square component with four pins). If for some reason the manufacturers did make it this way, I would feel fairly confident about feeding it DC (if the risk were mine, that is - you must decide for yourself). If, on the other hand, there is anything that looks like a small AC transformer in there, the opposite is probably true. ---
**Author:** ![Terry\_Kennedy](https://avatars.discourse-cdn.com/v4/letter/t/c57346/32.png) [@Terry\_Kennedy](https://boards.straightdope.com/u/Terry_Kennedy)\ **Post date:** [January 14, 2010, 9:33am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/7 "2010-01-14T09:33:39Z")
> [@astro](#): > > Well if it’s an AC driven unit it won’t even run off a DC adapter so the option does not exist and issues of damage are moot. Actually, it depends on what type of power supply circuitry is inside the unit. I’ve seen devices which have full-wave bridge rectifiers which are equally happy on AC or DC. Some devices only have a single diode and whether they work on DC or not depends on the polarity of the adapter. It is possible (though rare) that there’s a second transformer inside the unit which will be damaged if used with a DC adapter. One common reason for using an AC adapter is so the device has access to the relatively stable AC line frequency for timing purposes. I have a digital clock which will appear to work just fine on DC, but won’t keep time (doesn’t change at all) because it doesn’t see the line frequency. The best bet is to ask the technical people at the manufacturer, though that isn’t always possible, either because the unit is actually built by some other company, and/or customers get filtered before they get to someone who actually knows the answer. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [January 14, 2010, 11:28am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/8 "2010-01-14T11:28:25Z")
Yes it may work fine with a DC adapter if has been said it is changing the power to DC right after the connector. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 14, 2010, 11:38am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/9 "2010-01-14T11:38:47Z")
One thing that just occurred to me - 7.5v DC is not equivalent to AC with a 7.5v peak… > **[Root mean square | Average electrical power](https://en.wikipedia.org/wiki/Root_mean_square#Average_electrical_power)** > > Electrical engineers often need to know the power, P, dissipated by an electrical resistance, R. It is easy to do the calculation when there is a constant current, I, through the resistance. For a load of R ohms, power is given by: ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [January 14, 2010, 1:23pm UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/10 "2010-01-14T13:23:55Z")
> [@Mangetout](#): > > One thing that just occurred to me - 7.5v DC is not equivalent to AC with a 7.5v peak… It’s equal to 7.5Vrms though. It may or may not work for the OP’s device for the reasons already given. I agree that he should contact the manufacturer of the item he wants to use. ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [January 15, 2010, 12:28am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/11 "2010-01-15T00:28:49Z")
If it doesn’t have a bridge rectifier how much time running DC through it would it take to permanently damage it? Eg if I actually connect it to some VGA signals for 10 minutes how likely is that to stuff it up? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 15, 2010, 12:30am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/12 "2010-01-15T00:30:02Z")
> [@coremelt](#): > > If it doesn’t have a bridge rectifier how much time running DC through it would it take to permanently damage it? > > Eg if I actually connect it to some VGA signals for 10 minutes how likely is that to stuff it up? It’s exceedingly unlikely to hurt the device - it just won’t work. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [January 15, 2010, 2:44pm UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/13 "2010-01-15T14:44:27Z")
According to this spec sheet, it will accept AC or DC: [http://www.sunumsistemleri.com/images/cable/dc\_matrix.pdf](http://www.sunumsistemleri.com/images/cable/dc_matrix.pdf) Problem is your DC adaptors are a little light on maximum current - 1.5 A specified. But you might get away with it - they won’t the devices. On edit - oops, looks like only one model takes DC, or at least comes with a DC adapter. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [January 15, 2010, 3:01pm UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/14 "2010-01-15T15:01:16Z")
I’m finding it really tough to believe this thing _needs_ an AC input - it’s got to use DC internally. The AC-output adapters are listed as “package contents” rather than a specification for the unit. I’ll bet they’re using them because they’ve built nice well-filtered DC converters into the units themselves, and they’ve got a cheap source of the AC supplies. But the AC supplies can’t supply enough power for the 16-output model so they’ve substituted a higher-current DC supply for that model. I’d put money on them all being able to use a DC supply. ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [January 15, 2010, 4:26pm UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/15 "2010-01-15T16:26:39Z")
> [@matt](#): > > I’m finding it really tough to believe this thing _needs_ an AC input - it’s got to use DC internally. > > The AC-output adapters are listed as “package contents” rather than a specification for the unit. I’ll bet they’re using them because they’ve built nice well-filtered DC converters into the units themselves, and they’ve got a cheap source of the AC supplies. But the AC supplies can’t supply enough power for the 16-output model so they’ve substituted a higher-current DC supply for that model. I’d put money on them all being able to use a DC supply. thanks matt, thats the conclusion I’ve come to, the ac adaptor is probably 20 cents a unit cheaper or something like that… I’ve just never seen an ac ac- adaptor before. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 15, 2010, 4:50pm UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/16 "2010-01-15T16:50:56Z")
> [@coremelt](#): > > thanks matt, thats the conclusion I’ve come to, the ac adaptor is probably 20 cents a unit cheaper or something like that… I’ve just never seen an ac ac- adaptor before. It’s not as common az it once was, but AC adapters can be used to provide a negative rail for OpAmps and RS232 drivers. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [January 16, 2010, 2:09am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/17 "2010-01-16T02:09:41Z")
> [@beowulff](#): > > It’s not as common az it once was, but AC adapters can be used to provide a negative rail for OpAmps and RS232 drivers. Sure, but unless the provided AC adapter has a three-terminal connection, that would require a second transformer in the unit itself. Theoretically possible, but not very logical. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 16, 2010, 3:19am UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/18 "2010-01-16T03:19:26Z")
> [@matt](#): > > Sure, but unless the provided AC adapter has a three-terminal connection, that would require a second transformer in the unit itself. Theoretically possible, but not very logical. Not true. You can get plus and minus voltages from a two-wire AC supply. Just use one wire as a common and use two diodes connected in anti-parallel to the other wire. I’ve done this many times. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [January 16, 2010, 3:59pm UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/19 "2010-01-16T15:59:56Z")
Never encountered that one - have to remember it! ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 16, 2010, 5:04pm UTC](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904/20 "2010-01-16T17:04:28Z")
> [@matt](#): > > Never encountered that one - have to remember it! It’s a simple half-wave rectifier, one for positive and one for negative. [Next page](https://boards.straightdope.com/t/will-it-damage-a-7-5-volt-ac-device-running-it-off-a-dc-adaptor/524904.md?page=2)