Hi, I have no name power supply that came with my camper trailer. It is mains input and 12V 10A DC output through a lighter socket. I want to use this to power a Coleman 12V quickpump to inflate airbeds. Yes, I can use the car lighter socket while camping but when visiting relies, i'd rather not run that outside particularly at night. The pump is 12V 64W so should draw around 5.3A so should be fine. However when I turn the pump on it kicks at regular intervals, obviously tripping the power supply overload. So I decided to test the current with my multi-meter. For this, I have the multi-meter between the socket and plug positives. and a separate wire clipped onto the negatives. The motor runs perfectly well with the meter recording close to the expected current.
After investigating all connections, I am satisfied they are good and have concluded that the thinner wiring used to in the meter probes and the negative bridge, not to mention the inbuilt meter shunt, probably provide sufficient resistance to control the quickpumps startup current. While connecting the lighter plug and socket with their thick conductors, is allowing unfettered startup current sufficient to trip the supply overload.
I suppose I could add a very light resistor in series but this would lower voltage across the quickpump.
My next thought is that perhaps a capacitor across the 12V rails (in parallel with the quickpump) might charge safely when the supply is turned on and then assist startup current as it discharges when the quickpump is switched on, thereby limiting the startup draw current demand placed on the power supply.
Would this be a viable solution and what capacitor value to try?
I know I could just buy a dual power airbed pump, but heck this power supply should be able to handle the job and I just want to minimise my pile of gear.
"A lot of these Chinese rubbish landfill pumps have a cheap 12v plug. They warm up a bit & the fuse in the end ends up being an open circuit.
A quality plug will have a proper spring behind the fuse. Maybe it's as simple as that.
It only has a 25% duty cycle, at least they have the courtesy of providing this data."
I don't think it's that simple. The Pump is a Coleman Quickpump and it's connector is reasonable quality, has a spring and a fuse. I checked the pump plug, there is no way for it to short & I re-soldered the connections to ensure no dry joints. The pump's performance is consistently good on everything other than the power supply. It runs off both car sockets and a Jaycar Powertech regulated supply rated at only 1.5A I just don't want to rely on the Powertech as I'm over the current rating by quite a lot.
If anything, I'd suspect the power supply which is Chinese and suspiciously light weight. The socket on the power supply has no moving parts and there is no sign of intermittent problems with it. Whenever I connect the socket to plug via alternate wiring such as the multi-meter probe leads, the pump consistently runs like Trojan. Whenever I plug the pump directly into the socket I consistently get regular power tripping. I mean it trips in a very precise interval cycle. This is why I think it's a motor startup current issue.
The power supply runs my fridge no problems at all but consistently will not run the pump.
I think the supply switching is too regular and fast to be the fuse going open but I guess, it's worth replacing it to be sure.
Ken
-- Edited by Sailfish on Saturday 2nd of January 2021 08:55:57 AM
-- Edited by Sailfish on Saturday 2nd of January 2021 09:02:00 AM
Hi Sailfish; A 10 Amp power supply is way to small for that pump due to the start up current that the pump motor would draw on startup. The current that would be drawn is on the compliance place would show the current draw once the motor is running. The current draw at startup could be around 20+ Amps. This current draw would cause the power supply to go into over current protection. A lot of these switch mode power supplies would be alright for a static load ( resistive ) but when asked for a high current demand would go into some form of protection
The only way around the problem is to use a power supply that has a higher current and regulated 12 Volt output, That or you could use a 12 Volt car battery as a power source.
If the power supply is a Switch Mode type then you will find the compressor motor start up current is way to mutch for it. This is common problem with that type of power supply. Mikes sugestion of a large capacitor may get the problem solved.
My considered opinion That compressor will draw more current when starting and running fast and not plugged into the airbed. I would try crimping the air hose to reduce the air flow until it builds up some resistance which will reduce the current drawn to start and get running. IMHO. Same problem with centrifugal water pumps running with no resistance to flow. Jaahn
Well, I have been a bit preoccupied but took a quick look at the power supply tonight. I am rather surprised by it's complexity considering it's light weight. It is obviously based around a full wave bridge rectifier with 2 x 330 uF electrolytic caps. The main power transformer is rather small which explains the light weight. However, there is a lot of other component in this thing including 3 other smaller transformers and 4 large transistors on heat sinks. I am not going the trace the entire circuit, probably wouldn't understand it fully anyway.
I think as a 1st step, I'll try to check if the supply can actually achieve it's stated 10A. Not sure what I can use for a load able to dissipate that kind of heat.
I may see if I can make up some kind of wire wound variable resistor, maybe. 12 down to 1.2 ohms should suffice.
By the way, the motor is not a compressor, it's a high volume low pressure air bed fan.
There is no airflow at this stage because the motor trips the supply immediately. Probably doesn't even make 1 revolution.
I know that once the motor is running full speed, it draws around 5-6 amps with no pack pressure. So no problem on that score.
The existing plugs and socket pass current just fine so converting to Anderson's at this point would not address the current issue. If anything it would make it worse.
As only a few feet of test prob wire seem to create enough additional resistance to prevent tripping. I think it will not take a lot to get this over the line and work but I do want to get a better idea of whether the supply can manage it's stated 10A without tripping.
Your power supply is a switch-mode type which are virtually universal nowadays, linear supplies are rarely used and generally only in special applications eg. where very high immunity to external RF fields or very low spurious RF generation is required. Wikipedia gives a good explantion:
Inrush current will be the problem, you may be able to put a choke in series to slow the current rise time down or your capacitor might work
if the power supply doesn't trip to it. Else some sort soft start circuit.
That is not the simple light weight solution I am looking for just to inflate airbeds in a domestic setting. I already have all the heavy bulky gear for my camper anyway. I just don't want to lug all that about for an overnight family visit.
Switch mode power supplies have very effective over current protection.
Rather than a resistor I would maybe use a 29A full wave rectifier.
Connect the power supply to the AC inputs and the pump to the DC output.
