[TowerTalk] STEPPIR QUESTION

[K4SAV]

My comments are interspersed with yours below:

K2XT: "Nice work Jerry.  I don't completely understand all, but it 
sounds to me as if you are saying something like the lines (i.e. pins 
1-2, and 3-4 going to the driven element) are balanced.  Meaning no 
current is returning on ground?".....

K4SAV: Yes, no current on ground.  Current up and back to the motors are 
only on the two wire pair.  The power supply is floating relative to 
case ground.  However if you use the cable which interfaces to the 
radio, the negative side of the power supply then becomes grounded.  
That is still OK with respect to signals to the motors, still no signal 
current on ground.  The wires to the motor are only connected to the 
controller, not to ground at any place on the antenna.

K2XT: "To provide some means of lightning protection on the control 
lines we need to consider two scenarios.  One is to protect the 
controller, specifically the driver chips.  The other is to "keep the 
fireball out of the house",  meaning keeping surges from getting to the 
controller where they can work their way to other devices such as a 
transceiver connected to the data port.  So, in the latter case, it 
seems to me MOVs across all conductors to ground, placed on a SPG at the 
house entrance, would serve that purpose the same as a rotor protector 
using MOVs, or gas tubes on the coax lines. "...

K4SAV: As far as keeping the fireball out of the house that should be 
done with proper grounding at the tower.  Anything that remains on lines 
to the shack should be small transients.   Placing MOVs on those lines 
may reduce the amount of transient energy going into the controller and 
into other devices like the radio and computer.  HOWEVER if a transient 
is large enough to trigger the MOV, the amount of reduction offered by 
the MOV is not enough to protect the controller.  Depending on what 
fails inside the controller will determine if the radio or computer also 
fail.  The MOVs may reduce the possibility of radio and computer failure 
because of lower transient voltages going onto the controller.  However 
if a failed controller puts plus or minus 39 volts on the line going to 
a radio serial port I doubt the radio will survive undamaged.  I'm not 
willing to test this theory and I don't have the controller schematic to 
research the answer.

K2XT: "The question is, do those MOVs protect the driver chips, due to 
their bridge configuration?"....

K4SAV: No they don't. The MOVs will not limit the current into the 
driver chips to 1 amp or less, which is the amount specified by the 
driver IC manufacturer as the maximum allowed before damage.  Notice 
this statement is in direct contradiction with that being given out by 
the people at SteppIR.

K2XT: "And if the MOVs should short out after doing their job, does the 
controller then destroy itself due to seeing a direct short to ground on 
one or more of the output lines?

K4SAV: Yes it does.  If you plug the controller into a line that is 
shorted by an MOV, the then driver chips will blow.  I have personally 
verified this.  I used to have MOVs on the lines, but then realized they 
were more of a liability than a help.  I disconnect everything when I'm 
not at home or storms are near.  With MOVs on the lines, should one fail 
while I am not at home, I might not suspect that one was shorted to a 
lightning event.  Then when I plug the controller back in, it will blow 
due to the shorted MOV.   I removed the MOVs from these lines.  
Disconnecting everything is the only solution I have found to protecting 
the driver chips.

K2XT: "What about MOVs across the motor windings (pins 1-2, 3-4, etc of 
the DB25)?  to protect the chips."....

K4SAV: If you mean putting the MOVs on at the antenna, that won't 
protect the driver chips.  It might help protect the stepper motors, but 
those are pretty robust and I haven't heard of anyone loosing a motor 
due to lightning.  If you mean putting the MOVs across the wire pairs 
(and not to ground) that won't help either.  The normal voltage 
excursions on these lines relative to ground is between a low voltage 
and something close to the power supply level (a little less).  Looking 
at the two lines differentially the voltage excursions are approximately 
plus and minus the supply voltage (a little less).  So the MOVs would 
have to limit the current, not limit the voltage.  MOVs don't work like 
that.

K2XT: "Incidentally, MOVs rated at 39 volts are available for 45 cents 
at Mouser.  They are rated for huge currents, 2000 amps if memory serves 
me correctly. "....

K4SAV: Yes you can buy MOVs of all sizes.  You have to pay attention to 
the capacitance of the large ones when used on circuits that switch.

Jerry, K4SAV



Rick Stealey wrote:
>
>   
>> Providing protection for the driver requires a circuit that limits the
>> output current on every line
>>     
>
> Nice work Jerry.  I don't completely understand all, but it sounds to me as if you are saying something like the lines (i.e. pins 1-2, and 3-4 going to the driven element) 
> are balanced.  Meaning no current is returning on ground?
> To provide some means of lightning protection on the control lines we need to consider two scenarios.  One is to protect the 
> controller, specifically the driver chips.  The other is to "keep the fireball out of the house",  meaning keeping surges from getting to the controller where they can work their way to other devices such as a transceiver connected to the data port.
> So, in the latter case, it seems to me MOVs across all conductors to ground, placed on a SPG at the house entrance, would serve that purpose the same as a rotor protector using MOVs, or gas tubes on the coax lines.  
> The question is, do those MOVs protect the driver chips, due to their bridge configuration?  And if the MOVs should short out after doing their job, does the controller then destroy itself due to seeing a direct short to ground on one or more of the output lines?
> What about MOVs across the motor windings (pins 1-2, 3-4, etc of the DB25)?  to protect the chips.
> Incidentally, MOVs rated at 39 volts are available for 45 cents at Mouser.  They are rated for huge currents, 2000 amps if memory serves me correctly. 
> Rick  K2XT
>
>
> _
>