Help with smoothing DC power supply

[MikeMl]

...
The trickle charger works fine on slow days, but when competitions are held they drain fast and the shooting is interupted....

So it is a matter of degree. If a 1A charger wont keep up, try a 2A, 5A, or 10A one. I predict you do not need a 50A one...

 

[bountyhunter]

That's that the machine currently runs on mate. It is a DTL clay pigeon trap, one motor runs constantly to rotate the trap from left to right. And the other motor is switched when the shooter calls pull and runs for about 5 seconds to re arm the trap.

The trickle charger works fine on slow days, but when competitions are held they drain fast and the shooting is interupted. I intend to try and build this power supply to enable us to use the existing 12v trap, rather than paying thousands for a new one. I have been given a 60a transformer it is now up to me to see if it is possible to rectify it to run the two small DC motors

I would recommend using the car battery with a power supply with more current, like a lab supply that can put out at least 20 - 30A. The battery would serve as a giant capacitor and the supply would restore charge pretty quickly.

 

[spec]

Hy Josh,

Just a thought: why not just use a 240V 50Hz motor and call it a day. I bet a half horsepower motor would do the job. There are many motors like that around.

spec

 

[MaxHeadRoom78]

Can your clamp meter measure DC current? Some can, but most can't.

Most now are attachment's to a DMM and will measure both AC & DC.
The maximum peak torque should only occur very briefly at switch on, the normal operating current should occur within a couple of seconds, to an acceptable level of rated continuous torque or less depending on load.
Max.

 

[Tony Stewart]

A car starter normally takes > 100A but in winter with cold oil with all the bearing surfaces and piston rings, it can demand as much as 750Amps in a V6 which is why they rate batteries for CCA.

Hard to find specs like Ah capacity usally start around 50Ah for a new car battery.

All you need is a bigger battery charger. Cost these days is 25 to 50 cents per Watt and a bit more for a smart charger.

 

[Joshua cank]

Would it be possible to strap two 12volt batteries in parallel to give me twice the ah but the same voltage? And then attach a smart charger with a higher output.

Converting the trap to 240 volt would be far to costly due to the price of the spares and having to replace all the control gear, it would come close to the cost of a new trap. Which is why I tried to go down the permanent supply route

 

[spec]

Would it be possible to strap two 12volt batteries in parallel to give me twice the ah but the same voltage? And then attach a smart charger with a higher output.

Converting the trap to 240 volt would be far to costly due to the price of the spares and having to replace all the control gear, it would come close to the cost of a new trap. Which is why I tried to go down the permanent supply route

I take your point about not converting to 240V- just a thought.

Yes, putting two batteries in parallel would be fine, so long as they are the same chemistry, probably lead acid in your case.

I am not sure why you would want to do that though. It would be perfectly acceptable to say put an 80 Amp charger on a 40 A/h battery. That is a typical set up on a car (auto) where an alternator produces say 80A to quickly charge the battery.

You must ensure that the charger voltage does not go above around 15V though, or the battery will be overcharged. A decent battery charger would take care of your battery.

Another avenue worth exploring would be to connect a bridge to the 12V transformer and connect that to your battery:

12 RMS= 12*1.414V= 16.97V peak. Take away 2V forward drop for the bridge gives 14.97V, which should be OK, especially when you take into account other losses. If the battery voltage is too high on charge, just place another diode in series with the bridge output and the battery.

spec

 

[Joshua cank]

I take your point about not converting to 240V- just a thought.

Yes, putting two batteries in parallel would be fine, so long as they are the same chemistry, probably lead acid in your case.

I am not sure why you would want to do that though. It would be perfectly acceptable to say put an 80 Amp charger on a 40 A/h battery. That is a typical set up on a car (auto) where an alternator produces say 80A to quickly charge the battery.

You must ensure that the charger voltage does not go above around 15V though, or the battery will be overcharged. A decent battery charger would take care of your battery.

spec

Okay mate thanks for the advice, the arm motor will throw anywhere up to 2000 clays in a day so with the oscillation motor running constantly as well it tends to drain the batteries quite quickly. I will look into the spec of the existing trickle charger as I suspect it to be a little under rated for the job in hand. I was just curious to see if I could build a permanent 12v DC supply to run off the existing 240 sockets in the trap house

 

[spec]

Okay mate thanks for the advice, the arm motor will throw anywhere up to 2000 clays in a day so with the oscillation motor running constantly as well it tends to drain the batteries quite quickly. I will look into the spec of the existing trickle charger as I suspect it to be a little under rated for the job in hand. I was just curious to see if I could build a permanent 12v DC supply to run off the existing 240 sockets in the trap house

No sweat Josh

As the other ETO members have said, by far your best bet is to buy a standard 240V/12V power supply at whatever current you want. They are not that expensive. If you let us know what current you really need, I'm pretty sure that one of us will be able to find you a suitable PSU at a reasonable price.

spec

 

[ChrisP58]

Does the motor run continuously, or is it cyclic? If cyclic, what is the on/off ratio?

For instance, if it's only running 10% of the time, then your 50 Amp motor will only draw an average of 5 Amps out of the battery, and a 10 Amp charger would more than keep up with the demand.

 

[tcmtech]

12 RMS = 12*1.414V= 16.97V. Take away 2V forward drop for the bridge gives 14.97V which should be OK especially when you take into account other losses. If the battery voltage is to0 high just pop another diode in series with the bridge output and the battery.

That's the way I would go. Buy an off the shelf commercial 1 KVA buck/boost transformer with a 12/24 VAC secondary set and put a set of 150 amp diodes on it and call it good. Let the batteries do the rest.

Add in line and transformer losses and whatnot at load and the odds are it will sit right in the 13.8 - 14.4 volt range all day long thus not harming your battery.

 

[bountyhunter]

Does the motor run continuously, or is it cyclic? If cyclic, what is the on/off ratio?

For instance, if it's only running 10% of the time, then your 50 Amp motor will only draw an average of 5 Amps out of the battery, and a 10 Amp charger would more than keep up with the demand.

Depends. It might drain the battery in two hours which could be less than a 10% duty cycle for a full day..... but would still kill the battery with a low current charger.

As already noted: this system can't be specified or designed without knowing the load current required as well as the loading profile (time it runs and rests). With that it will be easy.

Hypothetically: if it is a motor that runs for most of the day, he might need a pretty high rated power supply because the motor might be able to knock the battery down in an hour or two. Most car batteries are in the 60 A-hr ballpark.

 

[bountyhunter]

That's the way I would go. Buy an off the shelf commercial 1 KVA buck/boost transformer with a 12/24 VAC secondary set and put a set of 150 amp diodes on it and call it good. Let the batteries do the rest.

Add in line and transformer losses and whatnot at load and the odds are it will sit right in the 13.8 - 14.4 volt range all day long thus not harming your battery.

It's feasible but you would need some stud mount rectifiers bolted to a LARGE heatsink to handle the power dissipation those rectifiers will be seeing. Maybe even a fan.

 

[tcmtech]

It's feasible but you would need some stud mount rectifiers bolted to a LARGE heatsink to handle the power dissipation those rectifiers will be seeing. Maybe even a fan.

Large is relative! I have worked with large industrial power supplies all my life and have a way oversized collection of parts so for me a 500 amp diode and the 1#+ heatsink it is attached to is normal.

Now my 2200 amp 1200 volt Hockey puck diodes and SCR's and their heatsinks, Well now you're talking fairly big but not as big as the ones that have three 600 amp 1200 volt half bridge IGBT bricks on that can dissipate 2 - 3 KW's passively . Those are huge!

For a DIY 50 amp independent four diode full bridge or a two diode half bridge running near-continuous duty I would recommend using a good sized CPU heatsink on each mounted in a way to get reasonable passive airflow around them.

By touch of hand they may feel hot but by diode heat ratings they would be well under their critical limits.

 

[spec]

Large is relative! I have worked with large industrial power supplies all my life and have a way oversized collection of parts so for me a 500 amp diode and the 1#+ heatsink it is attached to is normal.

Now my 2200 amp 1200 volt Hockey puck diodes and SCR's and their heatsinks, Well now you're talking fairly big but not as big as the ones that have three 600 amp 1200 volt half bridge IGBT bricks on that can dissipate 2 - 3 KW's passively . Those are huge!

For a DIY 50 amp independent four diode full bridge or a two diode half bridge running near-continuous duty I would recommend using a good sized CPU heatsink on each mounted in a way to get reasonable passive airflow around them.

By touch of hand they may feel hot but by diode heat ratings they would be well under their critical limits.

and I think a 30A rectifier is big.

any pics?

 

[spec]

Hy again Josh,

Just a general point about your battery: an ordinary car (auto) type will soon lose its capacity in your application. Instead, you need a deep discharge type as used in boats, campers etc.

Another point about lead/acid batteries is that you can't practically use all the charge in them. I ended up working on a figure of 65%. I can't remember why this is, but I learned the hard way and about deep discharge batteries too.

We needed to trial some equipment that consumed around 5A at 12V. I didn't give the battery power supply much thought and simply put a car battery, bought from a garage down the road, in a box with a switch, 10A fuse, and a couple of terminals. The equipment worked fine, but the damn battery was nothing but trouble. It only gave half the expected duration even when new and, in six months, it was down to 10% capacity. Then I found out why.

spec

 

[Tony Stewart]

This thread could have been answered in one answer, if you provided the much needed current measurements or expected ratings for battery, motor and charger for average and max.

My guess is the charger barely just keeps up to idle motor motor/gear load current with moderate use. I suspect the peak/average load current may be 20x bigger, but then my best guess of duty cycle of a clay pigeon motor is 2% so the overall demand might be 20*0.02 or 40% more .. thus doubling your battery capacity or charger current may be two workable solutions.

The choice is limited by your lack of given constraints for cost or availability of free solutions.

Commercial solutions would consider battery chemistry, electrolytics and pulse current rates for optimal stability and lifetime. The charger ought to be capable of supplying most of the peak current to reduce the aging of battery, while the battery smoothens the voltage. But this is a tradeoff between short term economy and long term reliability.

 

[tcmtech]

and I think a 30A rectifier is big.

any pics?

Just look up Hockey puck SCR and there you go. The biggest ones I have are the 2.9 inch dia ones.

I don't recall the exact specs for the ones I have but I think this would be close enough to give you an idea of what it takes to burn one out.

https://www.nteinc.com/specs/5500to5599/pdf/nte5563.pdf

 

[spec]

Just look up Hockey puck SCR and there you go. The biggest ones I have are the 2.9 inch dia ones.

I don't recall the exact specs for the ones I have but I think this would be close enough to give you an idea of what it takes to burn one out.

https://www.nteinc.com/specs/5500to5599/pdf/nte5563.pdf

I meant pics of your equipment and I misinterpreted the term hokey puck. I imagined you were a hockey buff, especially because of your avatar, and had built some sort of machine for propelling hockey pucks, at a goal for example, for practice. I was thinking that the puck would be going at some speed with all that power.

spec

 

[bountyhunter]

Large is relative! I have worked with large industrial power supplies all my life and have a way oversized collection of parts so for me a 500 amp diode and the 1#+ heatsink it is attached to is normal.////
By touch of hand they may feel hot but by diode heat ratings they would be well under their critical limits.

I have a large collection of heatsinks as well and I designed power converters for many years. Doing "ball park" estimates in this case would put power dissipation in the 100 - 150W range on the rectifiers... depending on how much money he wants to spend to get the forward drop down. Very high current schottky diodes might get down to 0.6 - 0.8V range, cheaper silicon would probably be twice that with corresponding doubling in power diss. Bottom line is you have to have a heatsink with Rth in the 0.6 C/W range to keep the heatsink's rise above ambient down to about 60 - 80C, allowing for rise of junction to case for the diodes. Like I said, the design is possible but it's a pretty big heatsink.....and those kind of extruded sinks are pricey for those not fortunate enough to acquire them as a fringe benefit of their job like I did.