# Up to ~1000 A constant current supply 10-40 VDC

So a little background to explain the need. Occasionally we need to test starter motors (for engines) and provide enough power when the load (engine) causes the starter to pull in excess of sometimes 1000 A. Granted, this is for quite large starters. The test would need to last a minute or so, but foregoing the need to alter designs, we'd like to just go ahead with a continuous demand rating.

So I've got a fair understanding of linear power supplies and some common designs of higher currents, such as multiple transformers/taps, bridges, parallel transistors (2N3055, etc.), but in my own experience I have only "engineered", mathematically, relatively simple sub-circuits, RLC, and other simple circuits (555 timers, opamp 747, etc.) using Kirchoff voltage/current, Thevenin, systems of ODE/PDEs, phasors, etc. But really I rely on the community for a suggestion on what to look for. (And ideally a schematic!)

Some of the design requirements are

1. 10-40 VDC (some are 24 VDC starters and a few are 36 VDC)
2. Maximum of about 1000 A at 24 VDC ~ 25000 watts/VAC
3. Be a current supply more than a voltage supply, similar to the idea of having the load in series with a much larger resistor.
4. Lastly (I think), is to be able to use single-phase, two-phase, or 3-phase input.

For larger loads, 110 VAC at the main is not going to work. For some idea of portability (we are a start-up business and have plans to grow and move) it would be nice to be able to run off "single" phase or three phase. This way each leg could supply a good amount of the load. (Can these be joined after the rectifier bridge or smoothing cap?)

By the way, companies such as DynaPower, though awesome, are just way too far out of our budget for right now. Besides, it's much more fun to build it yourself!

For what it's worth, we had thought of purchasing a used portable welder but were worried about the voltage adjusting itself to give the required current, and therefore power, to the load. I wouldn't know how to limit voltages at these currents...

• (2) ... 1000A at 24V = 24000W not 2500. I think your smoothing cap is a bank of car batteries... – Brian Drummond Nov 15 '14 at 20:36
• Ha ha... yeah, for the big loads we will definitely have a couple group-8 batteries nearby! BTW, the use of batteries is what we've been doing but replacing these batteries is expensive. – nate Nov 15 '14 at 20:38
• Why do you need to replace the batteries? Do they have a very short life, even if they aren't being used all the time? – alex.forencich Nov 15 '14 at 21:46
• Well it seems that if they aren't used constantly, at least weekly, they go dead fairly quickly. I prefer bicycling and so if I forget to drive my car, the battery goes bad (sulfates?)... One equipment we have has 4 8D batteries that do not last long since it isn't used much. Just would like something more dependable... – nate Nov 15 '14 at 22:01
• If you leave the batteries on a battery maintainer (basically a float charger), and they don't get exposed to extreme temperatures, they should be good for years. – Connor Wolf Nov 16 '14 at 7:20

I don't think that you will be happy with a regular power supply aproach, e.g. adjustable from 10V to 40V. For 40V and a regulation you need about 42V before the regulation. When you put it to 24V and draw 1000A you will have (42V - 24V) * 1000A = 18KW waste power.

You could mitigate this situation using a transformer with different output voltages for 12V, 24V and 36V. Already taken this step i would also consider if your starter motors realy need a regulated power supply, this would reduce your circuit to a transformer, rectifier and some capacitors. As a last step i would build a separate power supply for each voltage, then you could do some tests in parallel.

If you realy need a regulated power supply i would think about a switching one and considering the impression that i have from your experience in relation to the requirements i would perhaps buy one.

• Aside from design requirement 3, I like everything you said. I'll need to think more about this project for sure... – nate Nov 15 '14 at 23:23

So, use exactly what the big trucks use - big batteries - these can be charged (I think "trickle charging" might be a little of an understatement of course). Don't waste time and effort designing something that might take a year to properly engineer - use what the trucks use (or the buses, lorries, trains, planes or whatever).

You say that replacing the batteries is expensive - I bet it's cheaper than the effort to design a 40kW DC power supply.

• I should have added this to the question - a DIY that humbly suggested up to 500A and in the VDC range of a 12 Volt automotive systm: sound.westhost.com/project77.htm From here I thought it might be cheaper than almost $600/year for batteries. (More than just for this but if this worked maybe another could be built to alleviate some/most of the need for batteries. Short of draining them and refilling them before use they don't seem to last long...) I also figured the design could be simple since it doesn't need great filtering, can stand drift, etc. etc. Do you see this link favorably? – nate Nov 15 '14 at 21:06 • @nate - the author kind of describes it as a basic 10A regulator and it uses 4 power transistors that could possible be dissipating maybe 5 watts each. Now muliply this topology up by 400 times and you are having to get rid of 2kW in heat. I'm not going to analyse this any more - gut feeling tells me it's the wrong approach. The problem with the specification is of course the current and not the voltage and trying to find ways of getting several smaller supplies working in parallel is not easy. – Andy aka Nov 15 '14 at 22:19 • Okay, well thank you for your time and looking at the link! Experience has told me batteries are also the wrong approach so maybe I need to bite the bullet and look into a DynaPower type of approach ;) – nate Nov 15 '14 at 22:26 Batteries sound like a simple relatively inexpensive approach as well. I would suggest using the the battery pack from a forklift as the basis for your project. they should last for quite a few cycles. • Thanks for the advice - I have no experience with them at all :) I hope I can find one cheap - I've just priced only a group 78 (31s are often found in truck tractors and are better) at mscdirect and they want$360+freight. I guess my budget was off!! If we stay in business for 5 years and only buy two batteries a year, that's around \$3500... All the more reason to try and find an alternative - and these weren't even primo batteries like Odyssey or Crown! Maybe these forklift batteries are better than "automotive". Thanks again for the idea! – nate Nov 16 '14 at 5:44