Message 2691.1
From:
ANDREWL
Ok, it's getting on towards competition time, and a lot of you out there are going to be getting your Vantecs and powering them up
for the first times. Here's a few tips to help you make sure your $350 investment won't become an expensive cloud of smoke.
1: Check your Vantec out of the box. These units are hand-assembled, and sometimes defective out of the box. I have heard that
the controller board sometimes arrives with bad solder joints or bridges - if you know how to look for this sort of thing, you can
carefully pry the top cover off the Vantec and inspect the top board for defects. The little insulators on the FETs are sometimes loose
or misplaced, causing shorts between the H-bridge circuit and the case. Check between the battery inputs and motor outputs with a
multimeter. The +1 and +2 battery inputs should read high-impedence to the case or battery - terminals. The motor output terminals
should read high impedence to each other, and to the case and battery terminals. Don't power the unit up if anything is shorted -
that will destroy it.
If you do find anything obviously wrong, don't attempt to fix it yourself. These little units are very delicate and hard to repair without
damaging worse. Call Vantec for more advice.
2: Make sure you wire it right. Most early Vantec failures are caused by incorrect wiring. If you wire the batteries up to the +1/+2 and
- terminals backwards, you will destroy the unit. If you wire the bateries up to the motor terminals, or have a short between a motor
terminal and a battery terminal, you will destroy the unit. Check visually, check with a multimeter, and triple-check to make sure
everything's going where it should be before you apply battery power.
Vantecs have an unfortunate tendancy to totally self-destruct on the smallest insult. A single FET shorting out can cause a
chain-reaction that blows out other FETs, then feeds voltage surges back into the FET driver chips, destroying the logic board. I've
even seen cases where the back surge shorted out the logic power supply on the main board, which then destroyed the MCU driver
chip, PLCC, and other logic components on the controller board, rendering the unit a total loss. Fortunatly Vantec has a very
reasonable repair policy. Don't attempt to repair a Vantec yourself.
One thing that I've seen is when people use multi-stranded wires and feed them straight into the Vantec terminal block. Sometimes
a few strands escape off to the side, and cause a short between adjacent terminals. It's much safer and more secure to apply a
crimp spade or round end over the wire end, to insure a clean, mechanically sound, and short-free connection at the Vantec terminal
block.
3: Never, never overvoltage. We know that all the pros run thier motors at 1.5X - 2X the voltage rate to get more power. Does the
same apply to the Vantec? No - the voltage cap on the Vantec is a matter of the FET's breakdown voltage, not overheating!
The FETs uned in the smaller Vantecs have a 60V breakdown voltage. If you apply more than that to them, they will instantly fail and
short, even if they're not actually switching any loads! When you reverse a motor driven by an H-Bridge circut quickly, the FETs in the
circuit will very briefly see twice the max voltage. (If the motors were running at 30V full forward, and you suddenly slam the Vantec
into full reverse, inertia and armature inductance will still hold the 30V forward on the motor wires for a moment, causing the Vantec
to see an effective 60V difference between the battery and motor).
If you are very gentle with the throttle and never change speed very quickly, you might get away with running a higher voltage - but
why on the world would you want to, if it's going to impart that kind of limitation on you?
The danger with running right at that limit is that just-charged batteries can be 10-20% over their nominal voltages. (A 12V
just-charged battery can be at 13.2V. Just-charged NiCads can have as much as 1.5V/cell.). To be safe, you should keep your
batteries nominal voltage at 24-26V if you're using the 30V Vantecs. (20-21 NiCad cells seems to be safe. 24 cells is risking Vantec
self-destruct, even through that in theory only puts out 28.8V)
4: Stray bits of metal are bad. You'll notice your Vantec is a box open on only one side. Small pieces of metal that find there way into
the Vantec tend to stay in there, rattling around until they land across two traces, causing a short that totally destroys the unit as
described above. One of the first things you should do is devise some kind of cover to stop stray metal chips from finding their way
inside. A few strips of duct tape will do, although some kind of lexan shield will look nicer. You may also want to line the Vantec's
cover's inside with double-sided sticky tape, to act like flypaper to catch anything that does find its way in.
Many of the things you'll be doing to your robot's frame - grinding, drilling, welding - can generate small metal chips or dust. Even if
your robot's frame it totally nonmetallic, any metal bits your robot drives over can get picked up by its wheels and thrown inside its
shell. The BattleBots arena is full of metal chips, lost nute and bolts, and other small debris particles after just a few good battles.
Don't have anything exposed that can short in your robot.
5: Impact is bad. Vantecs are pretty rugged, but really hard blows can cause them to lock up or simply fail. You'll notice that most
Vantecs have threaded screw holes for mounting. DO NOT mount them straight to your frame. Use some kind of rubber or
wire-spring mounting system to mechanically isolate the Vantec from your frame.
If you're not worried about your Vantec getting hot, wrapping it in old mouse pads or packing foam and sealing it with tape is a
cheap way to solve the problems of impact and metal chips. Sealing your Vantec against debris and impact-protecting it while also
keeping it open enough for cooling air flow is a challenge, but by no means impossible.
Finally, for those of you who have read down this far, I will be at BattleBots Las Vegas, but not as a competitor. I will be bringing one
spare RDFR23, for use only by teams who destroyed their just before or during competition. You must supply reasonable evidence
that you actually had a Vantec of your own - this isn't intended to be an excuse for not buying a Vantec then claiming you blew it up.
If you blow mine up as well, you pay to get it repaired.
-Andrew Lindsey
To:
ALL
Oct-17 11:28 am
Edited 10/17/00 12:05:14 PM ET by ANDREWL