24 V power supply from ebay

I just got a 24V 30A psu off of ebay that I was planning to use in my build. I noticed however that although the power supply was advertized as a "switching" power supply rated for "110/220V", there is no switch, and the label says the input voltage is "170-250V". Emailed seller and got a "item should work with 110V, please check it" type of response.

I'm not very knowledgeable about electronics, but common sense is telling me mismatching voltages is not good. I don't feel like having it blow up on me, or otherwise fail to the point where I can't return it (pretty expensive), so I was wondering if anyone has any insight as to whether this item may be safe to even plug in?

Switching is the type of power supply ( [en.wikipedia.org] )it is and this has nothing to do with a physical switch to change input voltage. I can't say for sure if it will work as you did not give a link, model or anything. I know the newer power supplies I buy for LED's that are 24v can be powered from 85v to 220v with no changes. It's possible this is similar to what you have.

Edited 1 time(s). Last edit at 11/28/2014 02:38AM by tmorris9.

All I have is the seller's ebay page. Here's a link:
[www.ebay.com]

It certainly claims to take 110V in the online description, but the label only says: Input Voltage: 170-250V. From what I've seen this could be a European type supply that isn't designed to work with 110V, because normally the label will identify it as rated for both 110V and 220V otherwise. If that's not the case that's great, unfortunately I just don't know enough about these things. I don't want to have it blow up on me when I test it, so I sought help here.

Edited 1 time(s). Last edit at 11/28/2014 03:37AM by anvoice.

I see what you are saying. All I can do is tell you what I would do. I would hook it up to 110v and check the wouldut voltage to see if it were correct. If it is then I would put a load on it and do the same. Then I would let it run a few hours. If it's still OK then I would use it.

If it burns up at this point, it is the sellers fault and you can get your money back (eBay significantly not as described rule).

Thanks, I'll try that. I've got a 24V 350W heater so that should do fine for the load testing.

Took a while, but finally got to testing it. The supply produces ~24.5 volts at the terminals with no load. It also holds up under load as far as I can tell, but the voltage drops to about 19-20V at the terminals. Is this normal?

Its rating is 720W, and the bed should be drawing no more than 450W of power (I can't recall if the spec is 350 or 450V). If the voltage drop is normal, looks like the power supply works fine.

Voltage drop is the normal expected response to a power supply reaching it's power capabilities.

If it is supposed to be able to supply 30A with a 220VAC supply, it may supply somewhat less with a 110VAC supply. Switch mode PSUs basically rectify the incoming AC, then switch it on and off really fast to keep the average voltage at the target value. There is a limit to the duty cycle that it can supply, and when it reaches that limit, the voltage will drop, since the load is draining the buffer capacitors faster than the switched DC can charge them.

So instead of getting 24V x 30A = 720W of power from the supply, you may be getting less current capacity, so it might max out at half that, so about 360W actual. This would be about right if your bed is drawing about 18.75A of current (450W = 24V * 18.75A) so a drop to about 20V at the same current is 20V * 18.75A = 375W. ACTUAL numbers are a bit different, since what remains constant is actually resistance, not current. (Well, is's still more complicated than that, because of thermal deregulation etc, but this is close enough to come up with reasonable results.)

Edited 1 time(s). Last edit at 12/04/2014 03:14PM by SupraGuy.

So basically, the 720W supply I paid >$70 for might basically be a 360W supply?..

Is there a good way to test this to know for sure? The reason I got the 720W is because I felt I needed it to run my electronics.

Most switching supplies have a voltage adjustment. The proper way to set it is to put the unit under the normal load and then adjust the voltage to what it should be (normally a little pot next to the output terminals). If it can't be adjusted properly then it's not properly rated from the factory and you should ask for a refund.

So under 720W (is that "normal"?) of load, I should be able to adjust it to 24V with the pot?

I don't have all the components yet or any resistance wire on hand for all the 720W, but do you think if I hook up the 450W bed and an MKB2 bed (120W) for a totall of about 570W and still can't adjust it to 24V, would that clearly mean the supply is not rated right?

Thanks for you help, it's good to have someone with experience with this.

You should adjust it at the expected working load. But for now you can use the load you have and make sure it can be adjusted higher.

So, you have a 360w load, hook that up and see if you can adjust it to something around 27V if so then it's probably fine, if not then yea it's been improperly rated by the seller.

Tried the 450W load and as much as I turn that pot, it seems to max out at 19.7 or so volts. Using common sense, seems that it can't possibly deliver the full 450W of power given that the resistance of the heated bed is constant, hence the power will be lower...

Of course I'd like to be absolutely certain, but at this point I don't see what I could be doing wrong. Measuring voltage at the terminals is fairly straightforward. I think I'll have to send this one back.

Calculated actual rating of psu at 220V to be about 300W. Trying to return it.

While we're on the topic, does anyone know of a place to buy a 24V 720W psu at 110V? Can't find a single one online that clearly has that rating. I have a suspicion they just don't make them, and the ones with a 720W rating are European.

It'd be nice to avoid setting up 2 power supplies though.

I searched and found one but they are expensive (that is a lot of power)

[www.circuitspecialists.com]