Power supply question

[challenger]

I have a tennis ball machine that I purchased a power supply for. The machine is supposed to be battery powered by a 12v 7ah battery. The battery went bad and it's convenient for me to use a cord. I ran the battery wires outside of the unit and powered it with a 4amp,old style, battery charger and it worked fine. I ordered a 7amp power supply and the unit runs but the low battery light glows and the feed tray won't rotate. Any ideas why this isn't sufficient power?
Thanks

[SLK001]

Any ideas why this isn't sufficient power?
Thanks

Probably a power supply that is specified at levels greater than it is capable of making. Is the 7A supply a switcher (is it really heavy, or light)?

[polaraligned]

Measure the voltage when machine is running. I would bet the machine is pulling more current than the supply can reasonably deliver, so the voltage is being pulled down.

[ed_h]

If it's a switcher (lightweight), they sometimes don't like certain kinds of dynamic loads, like motors being switched on. Overcurrent protection circuits can get triggered. Dunb linear supplies are typically much more forgiving.

Ed

[J Tiers]

The problem with SMPS type power supplies is peak current. If the current EVER exceeds the maximum the supply is designed to provide, it will either shut down, or cut back the current, which causes output voltage to drop.

Most likely the machine AVERAGE current is low, but there may be times when it has a high current draw. Those switching supplies cannot handle it unless they are rated for the maximum current (serious overkill most of the time), so that they don;t shut down.

Batteries don't care, and supply whatever until discharged. Battery chargers, particularly old-style ones, also supply plenty of current.

[Mike Amick]

I doubt this is the case but technically it could have been designed to run for 30 min at 14amps.

The fact that the battery level light is dim is pretty much all the proof you need that the supply you bought isn't making it.

You can pick up a PC power supply for practically nothing, and have a 12v supply that will put out 20 or 30 amps.
There is a tiny hack that needs to be made that is silly simple.

If you don't like doing the small hack needed to the pc supply you can always buy this cool adapter that just plugs in and takes
care of everything.

https://www.wish.com/search/pc%20power%20supply%20adapter/product/5c949de35e18c31d56bc59ce?source=search&position=0&share=web

Oh, and I find it coincidental that I have a plug in tennis ball machine that I wanted to make battery powered, but isn't practical so am building my own.

[psomero]

Not knowing anything about the supply you bought, I would guess that it can't handle the instantaneous load of your device running at max power (namely, inrush of motors turning on) and it's dipping out.

Most any battery has better dynamic response to a heavy load (it pushes current and doesn't think about it unless there's a management circuit, and even then it will push up to the limits imposed by the BMS up to the point where it gets disconnected for its own safety). A small supply might not be able to handle any overload for any period of time beyond the current that it nominally states on the label, however.

[psomero]

how "bad" is the battery? what is the chemistry? you might be able to leave the battery in parallel with the psu and have it handle those brief spikes when your motors are kicking in. if you are keeping the bulk of the power coming from the PSU and the internal impedance of the battery isn't shot to hell, it can be a rather cushy life for a battery to be in this kind of scenario, as long as you don't violate its fundamental over/under limits for voltage and currents...

[nickel-city-fab]

I wonder if the issue could be overcome by sticking a large electrolytic capacitor across the lines to the motor? Give it all the inrush current it wants for a split second.

[psomero]

also, what voltage is the supply rated for, and if it's adjustable, are you on the low end of the range? one of those simple gotchas that can be sneaky

[PStechPaul]

A 12V lead-acid battery will normally read more than 12.5 volts and as much as 13.5 volts when fully charged. 12 volts is a typical reading when about 50% or even 25%, so that might trigger the low battery warning. But that doesn't solve the issue with not being able to rotate the feed tray. The large electrolytic across the supply, as mentioned above, might do the trick, but sometimes this will cause high inrush current that will trigger the PSU's overcurrent circuit, so it might need a resistor to limit the charging current until it is mostly charged. Then you can close a switch or relay across the resistor for normal operation.

Can you provide details on the power supply you bought?

[polaraligned]

I wonder if the issue could be overcome by sticking a large electrolytic capacitor across the lines to the motor? Give it all the inrush current it wants for a split second.

You are going to have to deal with the inrush to the capacitor. On DC servo mills we use an inrush limiter. I don’t know how well this work with a switching supply.

[J Tiers]

You are going to have to deal with the inrush to the capacitor. On DC servo mills we use an inrush limiter. I don’t know how well this work with a switching supply.

That depends on the supply's response to overload. As I mentioned, some just shut down, which is no help. Those are normally the cheaper ones, but not always.

Others will regulate current at or below their limit, which would eventually refill the capacitor. The question is whether they automatically return to normal operation, and how long they can stand to operate in current-limiting mode.

Problem is that you need to provide some "impedance", probably a small value resistor, between the supply and the capacitor. That allows the capacitor voltage to fall some amount without forcing the supply to refill it instantly, which would likely cause another overload.

The value of the capacitor depends on the magnitude of the current surge the load takes, and it's duration. The resistor value needs to keep the supply from being overloaded by increased current due to whatever voltage drop the surge causes.

An inrush limiter may not help much, as they are often NTC devices. They normally have a higher resistance, which falls as they heat up due to current. Being thermal, they do not "reset" quickly, and may have little to no effect after the initial "filling" of the capacitor, since the average current needs to keep them hot and low resistance.

The NTC (or a plain resistor) has some resistance no matter what, and that takes away from the voltage regulation of the supply. So does using NO inrush protector or resistor, if the supply cuts back current when overloaded. That would have a similar effect to a resistor. And the supply would have to supply enough current to operate the load, AND refill the capacitor in a reasonable time.

It can work fine, but can be "complicated".

[challenger]

Probably a power supply that is specified at levels greater than it is capable of making. Is the 7A supply a switcher (is it really heavy, or light)?

I'll try to address the questions. I thought I was multi quoting them but it didn't work.

There is no battery in the unit.
The low battery light isn't dim.
When the low battery light is on the feed tray won't turn.
The two main motors both run at the proper speed, it's the feed tray motor that won't turn.

I am leaning towards making a bare bones power supply.
Just a transformer and a full wave bridge rectifier.

[J Tiers]

So, CAN the feed tray work? If it runs when the low battery light is off, then it's OK.

It sounds a bit as if the device is set up to NOT feed if the "battery" is found to be low. So what you see may be "intended behavior", and the real issue is that the device is "detecting" a low battery condition in the first place.

That puts us back to the question of the power supply and it's ability to provide current enough to operate everything.

The power supply in the picture is for a constant load type, it says LEDs and security system. That is generally unchanging, and the power supply may have no overload capacity to speak of.

Does the thing work if it has a real 12V battery connected to it?

If so, can you measure the DC current it draws from the battery?

Knowing not just the average current draw (is that 7A?), but some idea of how much it varies, would be very helpful in deciding what to do. The bare bones supply MAY be able to handle the load better than what it in the picture. It would still be helpful to know what the maximum draw is, to be sure to allow for that when setting up the supply.