Re: Mach2 Slew Speed, Power, and Voltage Question #Mach2GTO


John Upton
 

Greg,

On Thu, Apr 8, 2021 at 04:40 PM, Greg McCall wrote:
While I've seen many discussions of the power bricks, I only do portable Astrophotography as I live in the centre of Sydney and must drive to a location. I currently use 12v (nominal) LiFePO4 batteries but was considering a 24v battery to power my Mach2. The alternative, a 12v to 24v booster (readily available from China) could be used, but I wonder about the quality of those available and the noise produced on the 24v output. This also seemed an obvious solution and wonder why AP has not suggested it leading me to think it may not be a good idea.
   One thing to keep in mind is that using a 12v to 24v DC-to-DC converter is that you might still have the same issue with running a charger during operation.

   My own LiFePO4 battery has a charge profile that requires up to 14.4 volts during the final absorption / equalization phase of the charge cycle. (The charger should then immediately switch to either float or monitoring mode.) The problem I would have is that the 12v to 24v power converter has a maximum input of voltage specified at 14.0v. Thus, charging the battery with the power converter attached could damage the converter unit.

   My own solution to this is to use my charger in "Supply Mode" where is becomes a fixed voltage float charger or "power supply" outputting a fixed regulated 13.6 volts. This allows some recharging of the battery while in operation without endangering either the battery or the voltage converter. Many of the high-end chargers have a "Supply Mode" setting -- both my CTEK and NOCO chargers support that manual form of operation.

   If you wish to use a normal Lithium charger on your battery during operation, you will need to carefully scan the specs for whichever voltage converter you use to ensure it remains in spec during a charge cycle of your particular charger.

   Regarding the output of the voltage converters, I found the one I use to be pretty good. (I am not taxing it at all. It is spec'ed for 24v @10A output and the highest consumption I have measured on the 12v side is about 6.5 to 8.5 Amps.) I did monitor the output both steady state and during a voltage transient on the input side. At steady state there is about 0.2v to 0.4v ripple on both input and output due to the five switching mode voltage converters (5v, 12.6v, 13.7v, 19v, 24v) contained in my field battery box. The voltage transient happens when I power up my NUC using another 12v to 19v converter as indicated in my second message in this thread. Due to the impedance of the wiring in my battery box wiring, I see a ~75 uSec drop to 10v on the 12v input side. (That is the cause of the motor stall issue I mentioned in my second post in this thread. This takes about 3 mSec to fully recover back to 13.3v. The output of the 24v lead to the Mach2 shows only a similarly short drop of about 1v and fully recovers within 150 uSec as seen on the scope trace below.



   In the trace above, the baseline 0v level is the bottom grid line. Each grid is 5v vertically and 100 uSec horizontally.

John

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