# How can I switch 100 Amps at VERY low voltage?

J

#### [email protected]

Jan 1, 1970
0
Hi all,

After a half dozen re-writes, I realize I may be asking
the impossible, but here goes...

What I need to do is supply 100 ADC to a .001 ohm load
and reverse the polarity to this load every four seconds.
I'm seeking a (least expensive) solution that can be used in
lieu of multiple relays because I believe most contactor
relays within my budget would be short-lived.

The most desirable characteristics in order of
importance are:

1) Robust design (failure intolerant situation)
2) tolerate 50% duty cycle @ 100 Amps (100% if devices
share the same case/heat sink?)
3) operate at as low a voltage as possible
4) Low thermal loss (lowest switching device saturation
voltage)
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 contactor
relays, 2 contactor relays + 2 power sources, 4 SCR's, 2
SCR's + 2 power sources, MOSFET(s), and power
transistor(s). IGBT devices were abandoned because I could
not find one with low enough saturation voltage, and DC
SSR's due to cost.

Optional control scenarios included programming the power
source to zero current to turn off SCR's, and in the case of
relay's, additionally firing an SCR across the p/s output to
circumvent contact arcing (might not be possible if o/p
voltage is under 1 VDC?). Obviously, an additional goal is
a favorable MTBF of p/s('s) and component choice(s)... .

I have some junk box parts, consisting of used
contactors that will support 100 ADC with paralleled
contacts (coils are 24VAC, high wattage...), 4 SCR's are on
order to at least try (MITSUBISHI TM200DZ-H's X2, but I
don't expect these to meet characteristic #4 above), a few
SSR's suitable for p/s input or contactor drivers, and
several large heat sinks.

Can anyone recommend alternative device(s)
(manufacturers & part numbers?!) or even a full
switching/control scenario utilizing as much of my on-hand
items as practical - that will generate the least amount of
excess heat without eating up my two used power supplies
that would cost me $4000+ each to replace if I had to do it during a test cycle... . Thanks in advance!!!! Stephen Kurzban H #### Homer J Simpson Jan 1, 1970 0 After a half dozen re-writes, I realize I may be asking the impossible, but here goes... What I need to do is supply 100 ADC to a .001 ohm load and reverse the polarity to this load every four seconds. Why? An explanation would help to focus the answer. FWIW, have you looked at magnetic blowouts? P #### Peter A Forbes Jan 1, 1970 0 Why? An explanation would help to focus the answer. FWIW, have you looked at magnetic blowouts? Albright International have motor reversing contactors with magnetic blowouts and double-contact break on most of them. We use their products a lot and have found them to be very reliable. Carriage/Post might be a cost problem if you are outside the UK, but worth looking at. The SW180 is probably the one to look at for single pole switching, but look through their site and see what suits you: http://www.albright.co.uk/ Peter E #### Eeyore Jan 1, 1970 0 What I need to do is supply 100 ADC to a .001 ohm load and reverse the polarity to this load every four seconds. Is it an inductive load ? That makes a *big* difference. Graham P #### PeteS Jan 1, 1970 0 Eeyore said: Is it an inductive load ? That makes a *big* difference. Graham What rise / fall times can you tolerate? Even assuming a non-inductive load, it's going to take some time to switch that amount of current. Note that even a non-inductive load will still have some inductance simply due to connectivity; very small admittedly on a good set of planes. Cheers PeteS N #### Nermal Jan 1, 1970 0 How about mercury relays. Four spst NO relays will work. These relays have very good overload capabilities. They will emit a lot of RFI when they open on an inductive load. J #### [email protected] Jan 1, 1970 0 Homer said: Why? An explanation would help to focus the answer. Resistance measurement similar to those taken with the galvanometer in a current comparitor bridge - the system is currently missing the accessories, one of which is a DPDT 100A high current switch. I'm designing a replacement for it. FWIW, have you looked at magnetic blowouts? Please tell me more What is a magnetic blowout? The biggest problem I'm seeing (besides cost) in a mechanical solution, is lack of DPDT contact arrangement, heat generation in driving the relay coil(s), and duty cycle. I'll do an internet/Google search later today for "magnetic blowout", but I am primarily interested in a solid state solution because of the potential for mechanical failure due to the high duty cycle - thank you Mr. Simpson! J #### [email protected] Jan 1, 1970 0 Peter said: Albright International have motor reversing contactors with magnetic blowouts and double-contact break on most of them. I found reference to them last night but no prices (yet) - and have the telephone number for their U.S. distributor so I can call them tomorrow. I do not know if I can afford their product line until then, but also, I have no idea what a "double contact break" is either (was planning on asking them that question). Same with the magnetic blowout (don't know what that is, didn't know to ask them, however ... thanks). We use their products a lot and have found them to be very reliable. Carriage/Post might be a cost problem if you are outside the UK, but worth looking at. The SW180 is probably the one to look at for single pole switching, but look through their site and see what suits you: http://www.albright.co.uk/ Peter I would ultimately reconsider a mechanical solution if one was available in DPDT and the specifications were favorable. What I've been seeing, is many of the manufacturers (I've talked to so far) are stating increased DC ratings of their lower current contactor relays. However, by the time you get to their higher current product lines (where they would support the 100 Amps I need, mostly with all contacts in parallel), they are actually de-rating the contacts at DC. Not sure why. My primary goal, cost aside, is to avoid facing a mechanical failure at an inopportune moment - so I've been trying to focus on a robust solid state solution. ANY solution I choose, however, must not generate too much heat (the biggest stumbling block to a solid state solution, no?). Does Albright offer a DPDT relay that can switch at least 100 ADC 450 times per minute through each contact (or 4PDT at over 50 ADC per contact)? If so, do you have any idea of the price range? Thanks Peter - your input is appreciated! J #### [email protected] Jan 1, 1970 0 PeteS said: What rise / fall times can you tolerate? Even assuming a non-inductive load, it's going to take some time to switch that amount of current. Any Note that even a non-inductive load will still have some inductance simply due to connectivity; very small admittedly on a good set of planes. Yes, load is "mostly" (per your observation) non-inductive. Thanks Pete L #### legg Jan 1, 1970 0 Hi all, After a half dozen re-writes, I realize I may be asking the impossible, but here goes... What I need to do is supply 100 ADC to a .001 ohm load and reverse the polarity to this load every four seconds. I'm seeking a (least expensive) solution that can be used in lieu of multiple relays because I believe most contactor relays within my budget would be short-lived. The most desirable characteristics in order of importance are: 1) Robust design (failure intolerant situation) 2) tolerate 50% duty cycle @ 100 Amps (100% if devices share the same case/heat sink?) 3) operate at as low a voltage as possible 4) Low thermal loss (lowest switching device saturation voltage) 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 contactor relays, 2 contactor relays + 2 power sources, 4 SCR's, 2 SCR's + 2 power sources, MOSFET(s), and power transistor(s). IGBT devices were abandoned because I could not find one with low enough saturation voltage, and DC SSR's due to cost. Optional control scenarios included programming the power source to zero current to turn off SCR's, and in the case of relay's, additionally firing an SCR across the p/s output to circumvent contact arcing (might not be possible if o/p voltage is under 1 VDC?). Obviously, an additional goal is a favorable MTBF of p/s('s) and component choice(s)... . I have some junk box parts, consisting of used contactors that will support 100 ADC with paralleled contacts (coils are 24VAC, high wattage...), 4 SCR's are on order to at least try (MITSUBISHI TM200DZ-H's X2, but I don't expect these to meet characteristic #4 above), a few SSR's suitable for p/s input or contactor drivers, and several large heat sinks. Can anyone recommend alternative device(s) (manufacturers & part numbers?!) or even a full switching/control scenario utilizing as much of my on-hand items as practical - that will generate the least amount of excess heat without eating up my two used power supplies that would cost me$4000+ each to replace if I had to do it
during a test cycle... .

You can get mosfets in the milli-ohm range.

Saturated switching is where these things can be used safely and
effectively in parallel. The more you use in your full bridge
arrangement, the lower the loss will be in the switch.

With four fets per switch arm, switch loss would be half the loss in

A full wave bridge at this voltage level could be driven directly with
15volt logic - though you'd want to keep the change-over slow, to
allow the current to reverse without resorting to body diode paths.

RL

J

#### Jamie

Jan 1, 1970
0
Hi all,

After a half dozen re-writes, I realize I may be asking
the impossible, but here goes...

What I need to do is supply 100 ADC to a .001 ohm load
and reverse the polarity to this load every four seconds.
I'm seeking a (least expensive) solution that can be used in
lieu of multiple relays because I believe most contactor
relays within my budget would be short-lived.

The most desirable characteristics in order of
importance are:

1) Robust design (failure intolerant situation)
2) tolerate 50% duty cycle @ 100 Amps (100% if devices
share the same case/heat sink?)
3) operate at as low a voltage as possible
4) Low thermal loss (lowest switching device saturation
voltage)
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 contactor
relays, 2 contactor relays + 2 power sources, 4 SCR's, 2
SCR's + 2 power sources, MOSFET(s), and power
transistor(s). IGBT devices were abandoned because I could
not find one with low enough saturation voltage, and DC
SSR's due to cost.

Optional control scenarios included programming the power
source to zero current to turn off SCR's, and in the case of
relay's, additionally firing an SCR across the p/s output to
circumvent contact arcing (might not be possible if o/p
voltage is under 1 VDC?). Obviously, an additional goal is
a favorable MTBF of p/s('s) and component choice(s)... .

I have some junk box parts, consisting of used
contactors that will support 100 ADC with paralleled
contacts (coils are 24VAC, high wattage...), 4 SCR's are on
order to at least try (MITSUBISHI TM200DZ-H's X2, but I
don't expect these to meet characteristic #4 above), a few
SSR's suitable for p/s input or contactor drivers, and
several large heat sinks.

Can anyone recommend alternative device(s)
(manufacturers & part numbers?!) or even a full
switching/control scenario utilizing as much of my on-hand
items as practical - that will generate the least amount of
excess heat without eating up my two used power supplies
that would cost me $4000+ each to replace if I had to do it during a test cycle... . Thanks in advance!!!! Stephen Kurzban Mercury Relay. S #### Spehro Pefhany Jan 1, 1970 0 Hi all, After a half dozen re-writes, I realize I may be asking the impossible, but here goes... <snip> Two 2-pole 100A mercury contactors would do the trick. IIRC, they take longer to break than to make, but if your power supplies limit smartly that might not be an issue (in fact, shorting might be the way to go). They are available with 120VAC coils. I'd expect around$500 total.
Durakool is one of several makers. Good for millions of operations.

Alternatively, you could use four of something like this:
http://www.st.com/stonline/books/pdf/docs/8220.pdf $35/ea. at Digikey. in an H-bridge configuration. You'll be burning up 90% of the PS output in the two MOSFETs that are on at once, in this case. The mechanical contactors will work too, but the noise might drive anyone in close proximity mad. Best regards, Spehro Pefhany J #### [email protected] Jan 1, 1970 0 Nermal said: How about mercury relays. Four spst NO relays will work. These relays have very good overload capabilities. They will emit a lot of RFI when they open on an inductive load. Thanks for the input Nermal Allied Electronics had a "mystery unit" listed I planned to follow up on (SRC Devices # MSS71A12). Just did - it's very low current... Magnecraft makes one I can parallel (WM60AA-120A) but preliminary searches Friday indicated four of these would be out of my price range. Two Tyco PRD-11AG0-120 relays might work, but are more than I want to spend (over$60 total) until I know if the SCR's I have coming
are going to work out - and - if a solid state solution would be
more reliable/cheaper, that would still be my first choice since the
Tyco unit might not stand up to the duty cycle long enough to make
it cost effective. Same with some Stancor's I just looked at ($84 -$120 for two SPDT, 100A or 200A at Mouser).

I can spend > $50 (more than I already have!) on a solution, but if I do, I'm hoping it would be reliable (permanent) and not run too hot - and I prefer to avoid mechanical solutions for that price unless there is no practical electronic one... L #### Lostgallifreyan Jan 1, 1970 0 However, by the time you get to their higher current product lines (where they would support the 100 Amps I need, mostly with all contacts in parallel), they are actually de-rating the contacts at DC. Not sure why. Are these parts specified for high volts (>200V) as well, i.e. switchgear for mains supplies? If so, it could be because DC at high volts and high amps can make an arc on breaking contact, that is hard to stop. Even with arc supression, I think there is more contact wear with DC. R #### reglarnavy Jan 1, 1970 0 Can anyone recommend alternative device(s) (manufacturers & part numbers?!) or even a full You might have a look at the Kilovac APX-150 relays at www.kilovac.com. They're now a division of Tyco. They can handle lots of current, have very low contact resistance, and are vacuum sealed, which significantly reduces arcing and contact damage. I think that they might even have an inert gas in them. I worked on a project years ago that used a similar Kilovac relay and they performed well. No idea what they cost. G #### Gary Reichlinger Jan 1, 1970 0 What I need to do is supply 100 ADC to a .001 ohm load and reverse the polarity to this load every four seconds. I'm seeking a (least expensive) solution that can be used in lieu of multiple relays because I believe most contactor relays within my budget would be short-lived. Take a look at some application notes for multi-phase buck regulators. International Rectifier (www.irf.com) and others offer parts for this approach. I would envision that you could use 2 regulator configurations (one for each polarity). By leaving the ground side mosfets on during the time the other regulator array is active, you would complete the circuit. The multiphase approach allows the use of multiple mosfets without the need to parallel them. M #### Mook Johnson Jan 1, 1970 0 Hi all, After a half dozen re-writes, I realize I may be asking the impossible, but here goes... What I need to do is supply 100 ADC to a .001 ohm load and reverse the polarity to this load every four seconds. I'm seeking a (least expensive) solution that can be used in lieu of multiple relays because I believe most contactor relays within my budget would be short-lived. The most desirable characteristics in order of importance are: 1) Robust design (failure intolerant situation) 2) tolerate 50% duty cycle @ 100 Amps (100% if devices share the same case/heat sink?) 3) operate at as low a voltage as possible 4) Low thermal loss (lowest switching device saturation voltage) 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 How much loss can the switch have? is the supply a current source or a 0.1 V voltage source? My first thought would be a TO-247 Mosfet H bridge with 2 - 4 0.002Rds FETs per switch (8 to 16 total FETs.) with 4 FETs per switch your looking at .5mohms per switch so thats .001ohms through both the top and bottom switch in the bridge. At 100 amps your looking at .1V of drop and 10 watts of Pd. That would mean your source would need to provide .2V to get 100 amps through the load. You will need to drive fets with mofet drivers located close to the FET gates. I'd use at least 12V for hard saturatinon. If you need lower contact resistance than 0.001 ohms you will have a diffiult time. J #### [email protected] Jan 1, 1970 0 Spehro said: <snip> Two 2-pole 100A mercury contactors would do the trick. IIRC, they take longer to break than to make, but if your power supplies limit smartly that might not be an issue (in fact, shorting might be the way to go). They are available with 120VAC coils. I'd expect around$500 total.
Durakool is one of several makers. Good for millions of operations.

Don't have that kind of money for the relays, especially when there is no DPDT
version (requiring the purchase of at least two..). I did look at these from
Magnicraft (sp) though.
Alternatively, you could use four of something like this:
http://www.st.com/stonline/books/pdf/docs/8220.pdf $35/ea. at Digikey. Hmmm - missed the$35 each, spent over an hour looking these up and seeking
alternatives when the one price I found was \$40.00. Very nice solution, but in
this incarnation, more than I can afford when you add up the need to purchase 4
(eight?) of them. I did see they were much higher voltage rated than I need
though - and went to the manufacturer's site to see if I could identify another
cheaper alternative device at lower voltage rating. Unfortunately, the site
done, I came away with no alternative devices and no pricing...
in an H-bridge configuration. You'll be burning up 90% of the PS
output in the two MOSFETs that are on at once, in this case.

Astute observation about the 90%! I am thinking with the right devices, and
several paralleled, perhaps that can be dropped down to 50% - but even at 90%
its a win over mechanical switching.

I wonder if anyone knows of a MOSFET switching device at low voltage and low
cost...

THANK YOU FOR THE HELPFUL INPUT!!!!!!!!!!!

Best,
Steve K.

P.S. I will be calling ST MicroElectronics on Monday.

J

#### [email protected]

Jan 1, 1970
0
Thanks - will telephone them to check pricing, but I'm betting it's too
high - and, still targeting a solid state solution.

J

#### [email protected]

Jan 1, 1970
0
I struck out on searching for info on "multi-phase buck regulators" ...
do you have an info link?

Thanks!

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