12V vs 13.8V?

[JohnH]

How safe is using a 13.8V power supply to drive astro kit designed for 12V? The reason I ask is I use a Losmandy G11G and it is recommended to drive that at 13.8V (the voltage your car's "12V" circuits run at with the engine running) thus I have a 13.8V power supply.

I figured it is ok to run the dew heaters at this level, but what about my FW and focuser, it is the latter that makes me nervous - I do not want to risk the stepper in the Moonlite but likewise I would like to minimise the cable clutter - I already have to deliver 13.8V (mount), 5V (USB hub) and 3.3V (ccd).

[bojan]

"12V" is a name (label) for anything between 11 - 14.4V that can be found in cars.
So it is perfectly safe to use 13.8V.

[Zaps]

It may be "perfectly safe" to use a 13.8V supply for anything normally connected to an automobile's electrical system, but it's not always the case for other devices which aren't specifically designed to be operated from an auto.

Check with the manufacturer first if possible, JohnH.

[DavidTrap]

Try putting a multi-meter across your battery terminals. You will find the voltage can be quite a bit above 12V.

DT

[JohnH]

Hi Guys - yup I did the check with the multi meter and the PS that came with the focuser gave 12.2V hence my concern, I know auto parts are not to fussy but astro? Well they may be far less tolerant and can be far more $$$, so yes I wrote to Moonlite, their (very fast) response - the system is ok up to 15V. Better safe than sorry!

Thanks All!

[Hagar]

In general a small increase in voltage is perfectly safe for most common items. Generally the only thing that changes with a small increase in voltage is a similar small decrease in the current required to achieve the same desired result.
Even elctronic dvices which require a stable voltage generally have a voltage regulator built in to stabilise the voltage and protect from voltage spikes which happen every time a switch is switched.
This being the case electronics will be the most sensitive and more care should be taken but an increase from 12V to 13.8 shouldn't be of any concern. A fully charged 12V battery measures somewhat higher than 12 volts and as it is discharged it falls below the 12 V mark.

[Dean]

Fully charged 12v battery usually reads 12.7v -12.8v...
Some deep cycle batteries can hover around the 13.0v mark when fully charged but most sit around 12.7v full charge. Yep they are nearing complete discharge at around 12v, but true real full discharge is something around 11.5v (but if you get to that, you will likely damage your battery cells if it remains at that level for any period of time.

Keeping batteries in the upper levels of charge states extends their working life. Ideally a float charger is what you want and left on the battery continuously when not in use.

As for the 13.8v on a "12v" device... as others said... may be ok, but first check with the manufacturer as some devices are more sensitive than others.

[multiweb]

The G11 will run better if you pump in 14V and over rather than 12V but I agree some other components such as automated focusers might be less tolerant, although I assume the manufacturers would take precautions and still be able to operate within a reasonable range.

Talking about tolerant circuitry I recently had a regulator fail on my car alternator. Weird electrical behaviour inside the car. Warning flashing erratically, radio on and off, wind screen wipers in turbo mode and a foul smell (hydrogen fumes mixed with sulfur?) The voltage at the battery connections (engine running) was 18.5V :eyepop: . Got a new alternator, reset the computer and everything seems to be back to normal now. So there must be some tolerance in the circuitry.

[[1ponders]]

may be, maybe not Marc, some manufacturers (I won't mention Meade ...Ooops) did not have protection for their LX200 mounts if you installed batteries and put them in the wrong way round. Bzzz Fry. Stupid really. I hope they have improved that in later models. So if meade didn't protect a scope worth a couple of thousand buck from putting batteries in the wrong way maybe some other equipment is just as intolerant of too high voltage.

I don't know but JM2CW from someones who's only electrical experience is putting a screwdriver in a power point at 7 year old :lol:

[bojan]

14V and 12V are not that different from power point of view (36% more dissipation in the load for 14V).
All inside control electronics (microprocessor, logic and so on), must be powered from internally regulated 5V or 3.3V power supply rail anyway (otherwise they would not work at all).
And those regulators are all rated at 30V input (just have a look at data sheet - for example, MC78L05 or similar).
So, you see, there is a lot of tolerance for input voltage here ..
Of course, if the design of your equipment were bad from the start, then even 12v regulated power supply would damage it sooner or later.

[MrB]

Unbelieveable, reverse polarity protection is as easy as a 3 cent diode... or the ultimate is a ~50 cent P-chan MOSFET. :shrug:
Yet another reason I don't like certain manufacturers (the Ford and Holdens of the 'scope world)

[Steffen]

While I agree that some protection should be offered (there are always things that can go wrong, and doubly so in the dark), you have to keep in mind that a protective (silicon) diode causes a 0.7V drop in voltage. This means your car battery powered scope mount will stop working 0.7V sooner as the car battery discharges. Depending on the minimum voltage the mount needs to operate this can be a considerable amount of battery time you're losing.

Cheers
Steffen.

[MrB]

True, tho many of the fast diodes these days have forward bias of only 0.2 to 0.4v.
Even so, thats why I mentioned the 50 cent(maybe a dollar worst case) P-ch MOSFET, virtualy zero voltage drop reverse polarity protection.

[[1ponders]]

I'm not sure of the situation with a car battery Steffen, but this was with the D cell batteries that were inserted into one of the fork arms. Crazy really.

Anyway enough OT talk from me. :)

[Steffen]

Agreed. It is trivially easy to construct a battery chamber that won't allow this to happen. After all, an LX200 mount isn't a $5 torch…


Cheers
Steffen.

[Hagar]

An increase in voltage is a different matter to a small voltage increase.
Direct protection against reverse polarity must be provided on electronics to prevent an electronic BBQ where as a voltage increase in reasonable amounts won't even cause an increase in temprature of components. In fact a reduction in voltage which causes an increase in current is far more dangerous and is often refered to as browning out. The current increases to a point where heat is generated until insulation breakdown occurs and the result is another fryup.

[Grejoh]

Being an Instructor in Aircraft Electronics (Avionics) and an ex Radio Technician, the vast majority of electronic equipment labelled for use with 12 volts, with still work very well with 13, 13.8, 14 even up to 15 volts. The only exception to this would be VERY precise labratory equipment, which most of us wont have! As has been said, if you ever run a voltmeter over your cars battery while the engine is running, you will and SHOULD see voltages approaching 13.8 to 14.2 volts! If you not, your alternator charging circuit is defective and you will soon end up with a flat battery!

[Bassnut]

Ditto, for astro, dont worry about it.