Hi all, After a half dozen re-writes, I realize I may be askingthe impossible, but here goes... What I need to do is supply 100 ADC to a .001 ohm loadand reverse the polarity to this load every four seconds.I'm seeking a (least expensive) solution that can be used inlieu of multiple relays because I believe most contactorrelays within my budget would be short-lived. The most desirable characteristics in order ofimportance are: 1) Robust design (failure intolerant situation) 2) tolerate 50% duty cycle @ 100 Amps (100% if devicesshare the same case/heat sink?) 3) operate at as low a voltage as possible 4) Low thermal loss (lowest switching device saturationvoltage) 5) easily mounted device(s) to facilitate heat sinking(i.e. TO-220 case style or chassis mountable) 6) easiest and/or most convenient drive requirements Thus far, considerations have included; 4 3PST contactorrelays, 2 contactor relays + 2 power sources, 4 SCR's, 2SCR's + 2 power sources, MOSFET(s), and powertransistor(s). IGBT devices were abandoned because I couldnot find one with low enough saturation voltage, and DCSSR's due to cost. Optional control scenarios included programming the powersource to zero current to turn off SCR's, and in the case ofrelay's, additionally firing an SCR across the p/s output tocircumvent contact arcing (might not be possible if o/pvoltage is under 1 VDC?). Obviously, an additional goal isa favorable MTBF of p/s('s) and component choice(s)... . I have some junk box parts, consisting of usedcontactors that will support 100 ADC with paralleledcontacts (coils are 24VAC, high wattage...), 4 SCR's are onorder to at least try (MITSUBISHI TM200DZ-H's X2, but Idon't expect these to meet characteristic #4 above), a fewSSR's suitable for p/s input or contactor drivers, andseveral large heat sinks. Can anyone recommend alternative device(s)(manufacturers & part numbers?!) or even a fullswitching/control scenario utilizing as much of my...

Why? An explanation would help to focus the answer.FWIW, have you looked at magnetic blowouts?

Try a google search using keywords: multiphase buck converter (no quotes)or else try searching the sites at or The goggle search even turned up a wikipedia entry for generalinformation on the concept.

How can I switch 100 Amps at VERY low voltage?

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Peter A Forbes 19 years ago

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"double

...

Double contact break is where there is a contact bar across the input and output terminals, so you actually have two contacts that break together, increasing the gap very quickly and preventing an arc forming. Magnetic Blowouts pull the arc out of the gap and extinguish it.

by

They do a specific motor-reversing relay that would be up your street.

It's protecting the switching device against back-emf that is the biggest problem.

I have their current data sheets at the office (which we are currently moving premises with) but I can look at the folder in the morning and pass your email details to them if you want?

We use the SPDT SW201A and the SPST SW200 as well, both/all are double contact break types. We are using one of their new range as a PCB mount contactor on a small 110V DC 10A charger, we use the contactor to disconnect the charger in the event of a fault or over-volt condition.

We've had good responses in the past from the guys here, happy to return the favour when we can :-))

Albright only build to order, so if you get a few 'funnies' with the US distributor, give me a shout and we can probably get something moving this side. There are also people in the fork lift truck market who carry piece parts and who can make these up quicker than the Albright people, costs you more though.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

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Gary Reichlinger 19 years ago

Try a google search using keywords:

multiphase buck converter (no quotes)

or else try searching the sites at

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or

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The goggle search even turned up a wikipedia entry for general information on the concept.

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jdworley 19 years ago

This looks like a perfect match for my needs and a very affordable price - thank you! The only difficulty I can see is the package itself is not something I am used to where it comes to making connections and heat sinking. I don't suppose I could impose on you to ask if there is a similarly handy device in a more easily mounted package (something more "hobbyist class")?

I didn't look at (scrutinize) the spec sheet yet, but I do wonder how many amps these will support without any heat sink ... perhaps with 2 to 4 in each leg I could get by with a cooling fan?

Yes, the next hurdle after choosing a device ... the driving circuit(s). As much work as getting to that point has been, I think I'm looking forward to that .

THANK YOU RL!!!!!!! I SUSPECT MY SEARCH FOR THE RIGHT DEVICE IS COMING TO A CLOSE.

Now to concentrate on the other aspects ... making a circuit board, finding the high amp capable hardware, and, perhaps, canceling on the two SCR's (? --- simpler solution, but I bet the on resistance is much higher than can be achieved this way).

Thanks again, Steve K.

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Mook Johnson 19 years ago

Try the IRF2204PB from digikey for ~$3.50 each.

16 will cost you about $50.

this is an H bridge circuit. Since your supply voltage is so small you don't need T5, T6 R1, or R2 to drive the high side gates. You can drive them directly.

A good quad mosfet driver for this would be the TC4467COE also from digikey.

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To operate you turn on T4 then T1 for positive voltage across the motor. Then turn them both off. Then turn on T3 then T4.

If you turn on the top and bottom fet at the same time you effectively short your supply t ground through the mosfets. Usually the would blow the FETs or the supply but in your case it might not be so critical.

Each of the 4 mosfets in the H bridge should actually be a parelleled est of

4 mosfets to reduct teh RDS to below 1mohm.

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when dealing with 100 amps, you had better have very heavy wires and PC board traces.

You also need a decent heat sink to mount them too. 10-20 watts will be dissipated in the H-bridge mosfets.

good luck

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Ross Herbert 19 years ago

R Legg has suggested the most practical method, ie. the use of the H-Bridge using parallelled MOSFETS.

What you seem to be trying to achieve is akin to the H-bridge inverter used in sinewave (or modified sine) DC - AC inverters for 50/60Hz power generation, except that you are switching at a much lower frequency.

University of Texas Prof H. Mack Grady has some interesting course material on-line here

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Look under Course Materials/Materials for EE362L... for stuff on H-bridge inverter, MOSFET firing etc.

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Homer J Simpson 19 years ago

I'd be inclined to look at a rotary switch for the sort of rates you are talking about. But it will be a very special rotary switch!

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Homer J Simpson 19 years ago

Yikes!

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Homer J Simpson 19 years ago

I'd be trying to think in a 'plumbing' mode - where will the heat be and how can I get rid of it?

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Jamie 19 years ago

DC drive of around 150 HP will work just fine. one for each direction operating in Armature feed back..

Or just get a DC amplifier.

"I'm never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5

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jdworley 19 years ago

Switching control comes from the bridge...

I've got a good start with all the info this thread has provided - thanks to all!

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Gary Reichlinger 19 years ago

You never mentioned just what you plan to use for your original power supply to get the 100 ADC. If you can allow some dead time in between transitions, turning the power supply on and off might be the easiest way to go. This is essentially what I was talking about with the buck converter multiphase approach. The converter would actually be converting the power from a higher voltage/lower amperage source.

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Mook Johnson 19 years ago

I would recommend having the driver IC very close (within 1 - 2 inches) of hte modfet gates. I would also recommend putting a small 10 ohm resistor directly to the gate and a 16V zener between the gate and source of each mosfet. This will kill and oscillation between the wiring undctance ang the gte capacitance and clamp and spikes that might make it get on the gate.

You don't want long wires between the driver and the mosfets.

I would strongly recommend twisting the source and return wires for the 100A to reduce the amount of magnetic field generated.

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