Electronics > Beginners
Question about start up current
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Electro Fan:

--- Quote from: ArthurDent on June 02, 2019, 03:29:20 am ---
--- Quote --- What got me thinking about it is was a GPSDO that I saw spec'd as having 2.5A inrush current but only needing about 0.3 - 0.6A normal operating current.
--- End quote ---

A lot of GPSDOs have OCXOs for the oscillator. When cold the oven draws a large amount of current that drops after a relatively short time, typically under 10 minutes, so this isn't what I'd call 'inrush' current. Inrush current is usually a few cycles or a second or so in most cases. That's why you may see your lights dip for a second when the fridge motor starts.

Most power supplies used for circuits like this are designed to handle what might be considered a temporary current overload until the oven reaches steady state. Power supplies with a high enough current rating aren't that expensive or physically much larger so get one with a current rating to handle the cold current rating and you can't go wrong.

--- End quote ---

Ok, thanks ArthurDent for the distinction in use cases between more conventional inrush current and the OCXO warm up use case.  That is very helpful.

Arthur - if a GPSDO could operate from 15 Volts to 18 Volts and would consume up to 3.5 Amps on startup and then less than 1 Amp after warm up, would you choose a power supply spec'd for 15 Volts and 4 Amps, or a power supply spec'd for 18 Volts and 3.33 Amps (if those were your only two PS choices)?  Why?  Thx!
radiolistener:
power supply current specification is ability to provide current. The same as your car can drive at 200 km/h, but you're don't need to drive it always at 200 km/h, you can drive it at any desired speed, but you know that your car limit is 200 km/h. The same thing with power supply. You're don't need to get 2 Amps from your power supply, you can get just 1 Amp, but you know that your power supply allows to provide up to 2 Amps and no more than that.
radiolistener:

--- Quote from: Electro Fan on June 02, 2019, 04:03:15 am ---if a GPSDO could operate from 15 Volts to 18 Volts and would consume up to 3.5 Amps on startup and then less than 1 Amp after warm up, would you choose a power supply spec'd for 15 Volts and 4 Amps, or a power supply spec'd for 18 Volts and 3.33 Amps (if those were your only two PS choices)?  Why?  Thx!

--- End quote ---

18 V 3.3A is bad choice. It cannot provide enough current. Power supply should have some margin for current. It should be able to provide for about 20% - 30% higher current, than your requirements. In your case 15V 5A will be good choice.

15 V 4A technically should works ok, but it has too small margin. So, it is possible that it can provide not so clean power. This may be critical for GPSDO.
ArthurDent:
radiolistener -
--- Quote --- 15 V 4A technically should works ok, but it has too small margin. So, it is possible that it can provide not so clean power. This may be critical for GPSDO.
--- End quote ---

That is correct. While I wouldn't use a power supply at its maximum current rating for a constant heavy load, a GPSDO using an OCXO will only draw the highest current for a short period of time and steady state the current drawn will be well within the supply's rating. I just checked a Trimble Thunderbolt and the +12 for the oven starts out at 0.7A and drops to 0.2A in 1.5 minutes. That +12 internally goes through a 78L08 for the oscillator and other circuitry so the OCXO package would operate properly probably down to +10 but would take longer to reach operating temperature. I'm not saying this is a good idea but in this case operating from a +12V/0.7A would work o.k.. Another consideration is the temperature rating on power supplies. most can output less continuous current at higher temperatures. Here is a photo of a Lambda linear supply with the rating on the case. 

So for many reasons it isn't a good idea to operate power supplies at their maximum ratings. What you do is determined by how well you understand the power supply you plan to use and the circuit you're powering.
Electro Fan:
Ok, so moving on from theoretical to slightly more specific.

A particular Star-4 GPSDO is spec'd as working from 15-24 Volts.  It is unclear depending on what day the seller specifies the current specs as to what the current requirements are.  Best guess so far is 0.3-0.6 Amps in normal steady state running conditions, but supposedly it could be anywhere from 2.5 to 3.5 Amps as recommended for "startup".  Setting aside the current specs for a moment, the question is, why is this (or any GPSDO) spec'd for a range such as 15-24 Volts rather than say 20 Volts?  Just easier to find a PS laying around that will work, or something else? 

Back to the the current.  Let's say the peak requirement for startup is 3.5 Amps.  Let's add 1 Amp for some margin (nearly 30% as suggested in this thread), so the total is 4.5 Amps.  4.5 Amps x 15 Volts = 67.5 Watts.  4.5 Amps x 24 Volts = 108 Watts.  For peak performance, it makes sense to go with the 24 Volt, 4.5 Amp, 108 Watt PS, yes?  Only reason to go with the 15 Volt, 4.5 Amp, 67.5 Watt PS is to save a few $ on the PS and the electric bill?  Or is there some other reasoning to be applied?  Which gets back to the question of why the GPSDO would be spec'd as operating at 15-24 Volts?  Is it to appeal to a user's sense of economics (ie save some $ at 15 Volts), or some other reason?  I guess if it's just economics, and the user doesn't care about how long it takes for the OCXO to heat up and for the GPSDO to lock, then the user might as well go for the lower voltage PS.  On the other hand it seems like both example PS's (15V and 24V) can handle 4.5 Amps, so why should the GPSDO operate differently with 15V vs 24V?  Unless maybe the resistance of the circuitry is such that the GPSDO would be stuck in constant current mode due to an insufficient voltage supply?  Is this like a car that runs ok on regular gasoline but will run somewhat better if the user is willing to spend more on premium gasoline?  (Only analogy I've come up with so far.) 

In summary, what is the best guess as to why the GPSDO is spec'd for 15-24V?  And, $ aside, would 24V/4.5A likely provide better performance than 15V/4.5A, or not necessarily?

(Disclaimer, I'd like to squeeze some extra performance out of the GPSDO if possible but I'm primarily interested in using this as a way to learn some electricity 101.  Thanks)
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