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Author Topic: Diode for Battery Bank Merge Solenoid  (Read 10227 times)
DrivingMissLazy
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« Reply #30 on: November 07, 2007, 02:45:45 PM »

below  is a schematic diagram for the contactor an suggested (and correct this time - I promise) TVS diode installation.

Cheers!

-Tim

Tim I am having a problem understanding your diagram.
It appears that the TVS diode is just connected to itself (short circuit) via a set of normally open contacts. You really need to explain things good for us ole' WV hillbillies. You probably know we are "dumber than a sled track".

Richard
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« Reply #31 on: November 07, 2007, 03:13:19 PM »

Richard,
    Tim and I finally agree, the diagram is correct.   The TVS diode is in parallel with the normally open contacts.  What the TVS does is prevent arcing of the contacts WHEN they are OPENED.  An arc is possible because there may have been considerable current flowing in the wires to and from the contacts and the magnetic field the current created around the wires contains considerable energy.  The TVS allows the energy stored in the wires to be dissipated in a path around the contacts.  Tim's diagram also shows an ordinary diode to protect the switch that controls the solenoid's coil.
Regards
Jerry 4107 1120       
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DrivingMissLazy
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« Reply #32 on: November 07, 2007, 03:22:17 PM »

Richard,
    Tim and I finally agree, the diagram is correct.   The TVS diode is in parallel with the normally open contacts.  What the TVS does is prevent arcing of the contacts WHEN they are OPENED.  An arc is possible because there may have been considerable current flowing in the wires to and from the contacts and the magnetic field the current created around the wires contains considerable energy.  The TVS allows the energy stored in the wires to be dissipated in a path around the contacts.  Tim's diagram also shows an ordinary diode to protect the switch that controls the solenoid's coil.
Regards
Jerry 4107 1120       

OK. if you guys say so.
I just thought it should be in series with a connection between the two battery banks. I only see one battery banks.
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« Reply #33 on: November 07, 2007, 03:25:27 PM »

Richard,
    Tim and I finally agree, the diagram is correct.   The TVS diode is in parallel with the normally open contacts.  What the TVS does is prevent arcing of the contacts WHEN they are OPENED.  An arc is possible because there may have been considerable current flowing in the wires to and from the contacts and the magnetic field the current created around the wires contains considerable energy.  The TVS allows the energy stored in the wires to be dissipated in a path around the contacts.  Tim's diagram also shows an ordinary diode to protect the switch that controls the solenoid's coil.
Regards
Jerry 4107 1120       

OK. if you guys say so.
I just thought it should be in series with a connection between the two battery banks. I only see one battery bank.


I went back and blew the diagram up and it showed the other battery bank. When I just clicked on the diagram, the right bank was cut off. The drawing was so large I needed to scroll sideways to see it all.
Richard
« Last Edit: November 07, 2007, 03:27:19 PM by DrivingMissLazy » Logged

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« Reply #34 on: November 07, 2007, 03:28:41 PM »

Wow - I think I may know where all of the confusion came from (i.e. ME).  Some times it's a really bad idea to chime in to a topic when I'm not really reading the thread  Roll Eyes - or even putting forth the effort to logically proof the concept I'm attampting to explain before hitting "post".  Allow me to redeme myself while actually adding to the clarity of this topic (wouldn't that be nice...)


But first off, my most sincere appologies to the board... Jerry L here is correct and his instruction is going to saving you a bit of headache.


I realize I have above instructed you to connect the diode between the Alternator side of the contactor to the ground.  This is correct for clamping the colapsing field of the contactor coil - almost... actually, not even really close Lips Sealed.

Below is a schematic diagram for the contactor an suggested (and correct this time - I promise) TVS diode installation.

Cheers!

-Tim

Ok, Tim, I mostly understand the diagram.  So, you depict  a diode on the solenoid coil, and you depict back to back diodes on the house bank side and alternator side.  You said "TVS".  Is that something we haven't discussed previously?   I'm symbol challenged so the backwards S on the cathode side of the two back to back diodes means what?   Are they hooked together cathode to cathode?  

Also what is a TVS diode and where do I find it?  The link Kd5fl gave didn't help in the search.  

Thanks for your patience.

David


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Jerry Liebler
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« Reply #35 on: November 07, 2007, 03:35:19 PM »

All,
    Sorry I had a brain fart.

Regards
Jerry 4107 1120
« Last Edit: November 07, 2007, 03:49:35 PM by Jerry Liebler » Logged
Jerry Liebler
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« Reply #36 on: November 07, 2007, 03:42:12 PM »

David,
    Tim's symbol is that of 2 zener diodes in series (back to back). This is the symbol for the device I led you to earlier, it conducts in either direction above the specified voltage but negligible current flows through it below that voltage.  TVS is an acronym for Transient Voltage Suppression, which is the function of the diode.
Regards
Jerry 4107 1120
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Tim Strommen
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« Reply #37 on: November 07, 2007, 05:56:35 PM »

All-right, a little more clarity:

There are two types of TVS diodes made: "unidirectional" (DC), and "bidirectional" (AC).  I think this should be fairly self-explanitory, but I do know that there are some non-electrically endowed people on the board.

Rectifier diodes (like the classic 1N4001) are designed to allow voltage/current to pass though in one direction after the Positive/Negative gap has been saturated, but block voltage/current in the opposite direction (also known as "reverse biased").  You have really high current rectifier diodes in your alternator which turns the AC signal into a DC output with ripple - and those of you who have battery isolator devices have two rectifier diodes (one to each battery bank) in one package.  Even these come in two basic configurations: "general" and "Schottky".  General is designed to block reverse bias throughput, but allow forward biasing with a moderate forward "saturation voltage" (this is the voltage diference that must exist at the anode/cathode for it to begin to condict, this is usually between 0.7-Volts to 2-Volts). Schottky is designed to do the same thing, but with a much lower saturation voltage (typically around 0.5-volts or less - they are also typically more expensive).

Zener diodes will do the same thing, but they are also designed to allow voltage to pass in the "wrong" direction (reverse bias) once the voltage reaches a certain level (but only for a limited current throughput) known as the "avalanche" or "breakdown" voltage.  This function is very useful if you want to limit the voltage being applied to a voltage sensative device when the supply current is very low (like to a microprocessor drawing less than 10milli-Amps).  The point at which the Zener begins to conduct in reverse bias is very programmable by the process at which the diode is built (you can get voltages at commonly used points, like 3.3-Volts, 5-Volts, 9-Volts, 12-volts, 15-Volts, etc.).  With appropriate circuit design, this "regulation" of voltage can be done continuously, but this is usualy done by a linear voltage regulator or other type of task-specific power-supply (or DC-DC converter), as the regulation effect is through "dissapating" the excess energy as heat (this is why it's only every used for very low current applicatoins - as it would litterally burn up if the dissapated power at the silicon exceeded what the [typically] glass-package could transfer to the air).

Transient Supression Diodes are designed to do exactly what it sounds like they are designed to do - absorb a quick, high impulse power surge of energy.  They are not designed to continuously absorb a steady voltage.  The Uni-directional TVS diode (meaning one-way) is a variation to the Zener, in that it will conduct a forward voltage, but dissapate a reverse biased pulse over its avalanche voltage.  Since it can't sustain the current of a forward biased rectifier role, designers will typically place a general or Schottky rectifier diode in series to prevent forward conduction through the TVS (to block any signal except a transient pulse).  With an AC signal, this would mean doubling the devices (one set of TVS and rectifier for each direction of flow of the alternating direction of current).  Silicon manufacturers logically placed two TVS diodes in series within one package but cathode to cathode in order to reduce the parts count for AC applications (by 3x).  This is what we call a bidirectional TVS - it will only absorb a transient in any direction above the avalanche voltage for the part, while blocking voltage/current in any direction below the avalanche threshold.

Since you are using a pair of battery banks, where logically one would want to plan for each one potentially having a higher charge, (Jerry recommended and) I used a bidirectional TVS across the Contactor switched pins, to absorb any impulses above the expected system voltage regardless of which battery bank has the higher potential/charge (electrons don't care which way they go, so you don't want to have a TVS diode that does Wink).

Below are the two schematic symbols for the two TVS variants.


As an asside (somewhat related, and since we went into this we might as well go all the way) - companies like Tyco Electronics, and Siemens Electronic Controls, suggest that DC arcing can sometimes be encoraged (and damage done to the contacts) by having a reverse biased general rectifier diode across the coil.  Their research appears to suggest that the diode may slow slightly the movement of the contact armature as the diode and reverse collapsing pulse may create a small eddy-current (the same effect that Temla electric retarders are based on), resisting the pull of the return spring until the coil dissapates the remaining field as heat - all the while and arc might be sustainable in the "slowly" opening gap.  They recommend replacing the general rectifier diode, with a TVS diode and series general rectifier diode to clamp the immediate pulse (protecting the switch, electronics), but allow the mechanics of the relay to operate the contact opening as designed.  This may be something to consider before you just put a general rectifier diode reverse biased across the contactor coil terminals.  I've attached a Tyco App-note I had in my collection as refference.

Cheers!

-Tim

P.S. Since we're working with DC, a trick used in the industry to break high current arcs in battery disconnect switches is to place a permanent magnet near the arc area - as arcs are magnetically influencable.  By pushing the arc with a strong permanent magnet, you can litterally "push" the arc until it is too long to sustain itself on the conductance of the air.  For sealed contactors this is not controllable, but it's interesting and usefull to note if you upgrade to one of the larger non-sealed electric vehicle contactors, or knife switches (as you can place a magnet which will push an arc to parts of the contactor that are not used for the primary conduction while closed, or edges of the contact points which will encorage the arc to break sooner due to distance). -T
« Last Edit: November 07, 2007, 08:50:26 PM by Tim Strommen » Logged

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« Reply #38 on: November 07, 2007, 06:46:38 PM »

Tim,
     Thank you for an outstanding tutorial on Zener TVS devices.
    A very good point about the diode across the coil slowing the opening of the contactor, thus increasing arc issues on the contacts.   I would suggest that it would be simpler and more reliable (because of fewer connections) to use a bidirectional TVS across the coil.  The coil TVS shouldn't need to be very big, say a 1500W pulse rated device at twice the system voltage.
Regards
Jerry 4107 1120
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David Anderson
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« Reply #39 on: November 07, 2007, 07:26:47 PM »

Tim,

Thanks for the primer on diodes.  I've learned more in the past two days about electomagnetic circuits than I've known in my lifetime.   The sad thing is I'll probably forget most of it when I wake up tomorrow morning. Undecided 

It appears the 15KP17 CA bipolar diode from Little Fuse that Jerry referred to should do the trick.  The VR is 17 volts, VBR 18.9 volts, and Ipp rating up to 512 amps.  I should be able to print that reference page and go to some place in San Antonio TX and find the diode I need.  Not sure where to look, but at least have a reference to ask someone. 

Using your diagram I'll wire the diode from lug to lug on the contactor's switched loads (house bank & alternator) 

Just for clarification, should I put a unidirectional diode on the coil terminals of the contactor?  The 15KP17A  Little Fuse diode should suffice in this position.

David
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Tim Strommen
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« Reply #40 on: November 07, 2007, 08:01:39 PM »

Well Dave - that's the great thing about this board - You can always come back to the thread Grin.

For the coil, it's probably easier from a parts locating point of view to just get another of the same bidirectional TVS diodes since you're probably running at the same system voltage for both the chassis/house batteries, as you are the contactor coil control voltage.  When companies build electonic products (Jerry apparently can confirm this per the PM he sent me with some of his background), EEs will try to use the same part in many places to ease the purchasing and qualification process - and take advantage of the price-breaks due to the "economy of scale".  It also makes locating and maintaining a "spares" inventory less complicated and more compact.

If you can't find a local shop, Digikey will take your money without complaint Wink.  I buy from them, Mouser, Future Electronics, Avnet, etc. all the time for personal projects.

If none of them have it, you can always try to purchase sample quantities from the manufacturer directly (they'll usually support this, but some manufacturers can be pricks...)

For the contactor coil, simply replace the general rectifier diode in the schematic with the TVS diode part (if you get the bidirectional type like we recommend, it won't matter which way you hook it up across the coil).  Just remember to keep the leads insulated to prevent shorts.

Cheers!

-Tim

Again - My appologies for my stupidity earlier in the thread Tongue -T
« Last Edit: November 07, 2007, 09:03:41 PM by Tim Strommen » Logged

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« Reply #41 on: November 07, 2007, 08:56:13 PM »

All,
    For these little electronic parts it's real hard to beat Digi-Key.  http://www.digikey.com/
They take small orders online with a credit card, have incredible stock, ship promptly and offer decent prices.  The Littlefuse 15KP17CA is a very good choice to protect the merge solenoid's contacts but it's a $8.91 part ( use 2 in series for 24 volt systems).  That's way more than we should spend for use across the coil.  For all the coils I'd suggest a 1.5ke30ca.  This is a 30 volt device, it will limit arcing on the switch without slowing the solenoid much and it's much more reasonably priced at $0.27.
Regards
 Jerry 4107 1120
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David Anderson
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« Reply #42 on: November 08, 2007, 02:54:53 PM »

Thanks guys,

I won't have time to do this for a while.  I'll post back when it's done.  It may be a couple of weeks.  This has been a great learning experience for me.

David
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« Reply #43 on: November 08, 2007, 04:27:24 PM »

I think my golf cart has one of those TVS diodes across the contactor terminals. ( it keeps the contacts from arcing/welding ) when you press the pedal the solenoid clicks to apply power to the controller.

Sorry.. The thread just refreshed my memory about the diodes function.

Dave.....
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David Anderson
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« Reply #44 on: November 27, 2007, 07:39:43 AM »

Sorry about bringing this back to the top, but I finally ordered this from Digikey.  They didn't stock the 15000 watt diode so I could only get the 5000 watt.  5Kp18CALFCT-ND.  I ordered two.  I hope this will work.

David
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