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Power Supply Rise Times

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Dave Marshall

Mon Aug 16 2010, 02:43AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I'm in the final stages of designing a multistage, low current, constant voltage coilgun, and I've reached the point where I can't put off designing the power supply any longer.

I've avoided this mostly due to a lack of information and the belief that this was going to be unbelievably expensive, but new developments have convinced me otherwise. My criteria are: Voltage at least 60V but no more than 300V, maximum power output of 4kW, minimum of 2kW (As voltage increases, I can afford to lose some current because I can overcome more resistance in the coil, providing greater flux density through a larger coil). This basically shakes out to 60V @ 70A, or 300V (rectified 220V mains) at about 15A on the high side.

I've narrowed it down to 3 practical methods:

Rectified mains (110v or 220v)
A bank of 5-10 12V ~15AH SLA batteries
A bank of 5-10 12V Lead Acid car batteries

At this point, all are equally reachable. The mains solution has the benefit of being the cheapest, with the down side being the very real potential to burn my house down if something fails hard. The SLAs I have 5 of already. They are 15AH batteries pulled from UPSes and known to be good. The car batteries I would have to acquire, but used car batteries are fairly easy to come by. The big question is, which is going to deliver the best rise times?

My mains connection is a standard 110v residential connection, and 220v is achieved by going rail to rail with a second 110v line. The pole pig is about 100 yards from the meter, on a utility pole down the street. I would be able to connect to the breaker box less than 3' from the meter, removing most of the house wiring from the equation. I would most certainly have the lines connected through circuit breakers in case something died short.

The SLAs are easy and cheap to get. I don't know how good their rise time will be though. My understanding is they are capable of delivering fantastic current pulses for very brief periods, but I don't know how fast it will ramp up. This is where I need the most insight.

I suspect the car batteries will be the best solution over all, certainly for maximum peak current, but they'll be the most time consuming and expensive solution. They also present the very real issue of weighing something between 500lbs and 1,000lbs, depending on how big my bank is.

What direction do I need to take this thing?

-Dave

radiotech

Mon Aug 16 2010, 03:46AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

How is the circuit between the batteries or rectifier closed and what inductance is being switched? Maximum pulse power from batteries happens when the internal resistance of the batteries equal the load resistance, this happens at .5 open circuit terminal voltage. Same with the rectified mains. So stiffness of power supply and the inductance are going to determine rise time. The stiffness of power supply will be in series with the coil resistance for the for the R of the L/R The number of turns and form will dictate inductance and the amps/area of the wire you decide on, what coil resistance will be.

Or you can convince a university student to work it out on matlab ~!

Dave Marshall

Mon Aug 16 2010, 04:50AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I know the load will have a significant effect on rise time. I'm primarily looking for best case scenarios, or dead short conditions.

I'm considering a hybrid Mains/Capacitor system that would use the capacitor to offset the rise time on the mains. Just started looking at it today, so I'll have to poke around a bit more before I make a decision on that end.

-Dave

doctor electrons

Mon Aug 16 2010, 09:47PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

My guess is that you will get your best rise time out of the car batteries. I am sure you have heard the term
"cold cranking amps". In theory, you should get a faster rise time from the car battery vs. the sla, which from
my understanding is meant to deliver a steady amperage over time. A large "pulse" or surge may not make them
too happy. If i was doing this project for myself i would use the car batteries and some pulse rated caps. I would also
make a safety circuit using a contactor and a ct to measure amperage, and drop out the contactors coil before a dangerous
output amperage is reached. One other benefit to using the batteries would be a good zero point, sometimes strange things
can take place when using your mains. Once i had worked on some wiring at a friends house, with his main breaker shut off
there was about 80 volts at the load side of the breaker! After wepco came in and had a look it turns out the soil under his house
was acidic, and his neighbors was basic. Made a giant battery!!! If you end up going with car batteries, there is one out there
called optima, or ultima, cant remember for sure but it looks like a plastic six pack. I have seen them used in demo derby cars,
also have seen them still work when cut in half!

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