GPSDO feature creep (how to enable/disable OCXO from Micro) - Page 1
Forum section: electronics / projects

author: staze
author_rank: Frequent Contributor
timestamp: May 24, 2014, 02:52:47 am
content: Related to the previous post, generically, I'm working on a GPSDO based on jrmiller's design:http://www.jrmiller.demon.co.uk/projects/ministd/frqstd0.htmBut, being a computer guy by trade, I realized that if I've got a GPS running and serial out from it, I've got the source for a stratum 1 NTP server... but, I don't need the OCXO and PLL loop running for that, just the GPS (which shaves off about 200mA of power usage, plus heat).So, I'm already going to put an arduino in the enclosure to give me an LED indication of when the GPS locks, as well as provide output to an LCD screen of date/time, and number of satellites in use. All that code I have working.My question is, how (from the arduino) can I turn on/off the OCXO and PLL circuitry? The Arduino outputs max at about 40mA I believe, which isn't nearly enough to fire up the OCXO, which takes about 600mA or so on startup (it's an Isotemp 131-100, 5V CMOS OCXO). Maybe just a simple FET? Or should I look at something like Dave's latching soft switch design (http://www.eevblog.com/2012/03/30/eevblog-262-worlds-simplest-soft-latching-power-switch-circuit/)? A relay seems overkill for just switching on and off 5V.Thanks! Though, I guess this question probably means I should draw up a schematic...

author: jpb
author_rank: Super Contributor
timestamp: May 24, 2014, 02:03:42 pm
content: I'd think you could switch a relay or relays, possibly using a single transistor as a driving stage and perhaps a diode across the coil. You can get some pretty small and neat relays now-a-days and as they are used even in hand held DMMs (see Gossen Energy teardown video) it seems to me to be the neatest solution.

author: staze
author_rank: Frequent Contributor
timestamp: May 25, 2014, 02:52:08 am
content: Yeah, lot of discussion online seems to indicate a tip120 or a MOSFET to do this type on thing with lights/motors. A relay just seems overkill for only 5V/1A.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 25, 2014, 04:42:37 am
content: A logic level P channel MOSFET is all you need.

author: staze
author_rank: Frequent Contributor
timestamp: May 25, 2014, 04:52:54 am
content: Oh. Well, I have some FQP30N06's which are n-channel logic level. Could I not just put those on the low side? Or should I really look for some p-channels?

author: gxti
author_rank: Frequent Contributor
timestamp: May 25, 2014, 05:32:43 am
content: Low-side switching would not be appropriate because current might still be able to flow out of the oscillator output. Best to switch it on the high side with a P-FET, and if needed, a N-FET to "level shift" your control signal to the high side voltage.

author: staze
author_rank: Frequent Contributor
timestamp: May 25, 2014, 05:40:06 am
content: Yeah... Just realized that. Plus, I don't have the logic level version of that FET. Grr.Kk. Anyone have a favorite logic level p-FET?The Fairchild ndp6020p looks promising. So at 5v (logic high) it would be off and at logic 0V (logic low) be fully on (looks lime Vgs is only a max of -1V!). Though, that doesn't mean I can't go lower than that, I would assume, that's just it's transition threshold from on to off?

author: jeremy
author_rank: Super Contributor
timestamp: May 25, 2014, 12:28:27 pm
content: My goto p-fet is the fdn360p.

author: dannyf
author_rank: Super Contributor
timestamp: May 25, 2014, 01:14:34 pm
content: Run the clock signal + the control signal through a AND/NAND gate.

author: staze
author_rank: Frequent Contributor
timestamp: May 25, 2014, 08:26:05 pm
content: just realized another option...a 5V regulator with a disable pin (TO-220-4 package)... =P of course, that would require feeding the whole thing higher than the dropout voltage... so not something I'm looking at, but an interesting option. =)