Re: Prof. Sommer's Controller Review [TANKS]

[Joe Sommer <anvilus9-at-adelphia.net>]

Frank and Chris,

At 04:56 PM 4/3/2005 -0400, Frank Pittelli <frank-at-rctankcombat.com> wrote:
>>>  BTW, if I was going to use 24V could I make the tri-pact control the 
>>> same as I would for 12V or would I need different components???
>>The Tri-Pact controller can be built to use 24V, but I would
>>substitute 24V relays.  24V relays have higher coil
>>resistance (360 ohms) than 12V relays (90 ohms).
>>http://relays.tycoelectronics.com/datasheets/VF4.pdf
>>Activating 12V relays with 24V will fry the coils.
>
>I think you guys may be speaking about different things.
>
>Chris, when you say "use 24V" do you mean using 24V for the "motor 
>voltage" or using 24V for the "control voltage" or both?
>
>I think Joe assumed that you wanted to use 24V for the "control voltage", 
>which would in fact require a 24V relay, because 24V will definitely burn 
>out the 12v coil in a standard automotive relay.
>
>However, if you were talking about 24V "motor voltage", then you can still 
>use standard 12v automotive relays, provided that you still use a 12v 
>"control voltage".  As I understand it, the "load" capacity of a relay is 
>more dependent on the amp draw then the voltage (within reason).  After 
>all, it's just a couple pieces of metal being pulled together.  As long as 
>the contacts are designed to handle the arcs generated by high loads, the 
>actual voltage of that load doesn't really matter (within reason).  So a 
>standard, heavy-duty automotive relay should be able to handle a 24V load 
>without a problem as long as the power requirements are roughly the same 
>(which is true in this case). Joe, do you agree with this?

I agree.  (See below for a technical digression about
"just a couple pieces of metal being pulled together".)

I have read the Tyco/P&B spec sheet for VF4 relays
http://relays.tycoelectronics.com/datasheets/VF4.pdf
several times and I believe that the contacts are the
same for their 12V and 24V relays.  The coils are
obviously different.

To use 12V relays for 24V motor operation in the Tri-Pact
controller shown at
http://www.rctankcombat.com/articles/speed-control/07TriPactSpeedControl-large.jpg
you can substitute +24VDC connected to the normally
open (NO) terminals 87.  However, you must still provide
+12VDC to the coil control terminals 86.

If you connect +24VDC to both NO terminals 87 and
to coil terminals 86, you must use 24V relays.

Technical digression about "just a couple pieces of metal
being pulled together" -
24V causes significantly more arcing than 12V when
contacts make and break.  Inspecting the "Load
Limit Curve" graph on the Tyco/P&B spec sheet
http://relays.tycoelectronics.com/datasheets/VF4.pdf
shows that the arc will always extinguish for 12V
across the contacts (Load limit curve 1).  For
24V across the contacts, the arc may not completely
extinguish during transit time for loads above 7A.
24V will not cause stationary arcs (Load limit curve 2).
Stationary arcs are VERY bad.

This prolonged arcing at 24V will reduce the life of
the contacts and may cause contact welding.
Unfortunately, I cannot find any contact life curves
as a function of switching voltage and switching
current.

Bottom line - give it a try.

Joe


**********************************************
Joe Sommer, Anvilus Machine Works
2378 Nantucket Circle, State College, PA 16803
814.234.4773     anvilus9-at-adelphia.net
http://www.anvilus.com