12V linear power supply for OCXO - Page 1
Forum section: electronics / projects

author: jpb
author_rank: Super Contributor
timestamp: June 27, 2013, 10:44:57 am
content: I have just bought a rather nice OCXO which I've been powering from my bench supply. I want to get an inexpensive linear supply module for it.The problem is that when the oven is heating up it draws up to 1.5A.When the oven has reached temperature it only draws about 300mA.Conrad seem to do a nice module but it only goes up to 900mA.I don't want to get a switched-mode supply as I'm trying to minimize electrical noise and at 3.6W I'm not excessively concerned with efficiency.What I'd like to do perhaps is limit the current to 900mA and just allow the oven to take longer to warm up. I'd thought of using a relay to switch in andout a 5 ohm series resistor.I guess if I didn't do anything the power supply might destroy itself when asked to supply 1.5A or would the voltage level just drop?Any ideas?

author: digsys
author_rank: Unknown
timestamp: June 27, 2013, 11:00:58 am
content: So you're looking for 12V at 0.9A. Solution is easy - use a LM317 or any equivalent and add current limiting to it.Usually it's just another resistor + transistor. The OP Voltage will drop, as you say, so check that the heater will work at say ~8V.The LM317 etc will have to handle a little more heat, that's all. All the Linear regs have application circuits on all that.If you need a suggested circuit, just ask, but it's all easy peasy stuff.

author: jpb
author_rank: Super Contributor
timestamp: June 27, 2013, 11:45:55 am
content: Quote from: digsys on June 27, 2013, 11:00:58 amThanks, I'll look into this. I wasn't particularly looking for 0.9A, that was just the limit on the ready-made module from Conrad which would save me building my own. I'd guess that the Conrad module already uses something like the LM317. If I do my own I could probably up it to 1.5A as the higher current will only flow for a relatively short time.

author: digsys
author_rank: Unknown
timestamp: June 27, 2013, 12:59:52 pm
content: Great. IF you don't really mind a slightly longer heat up time, maybe go for a current limited ~0.8A design anyway,especiallysince youonly need 300mA for most the time !! Reason is - less heat, smaller parts, more efficient and IF somethingdoesgo wrong - LESS magic smoke :-)

author: mariush
author_rank: Super Contributor
timestamp: June 27, 2013, 03:52:21 pm
content: Look for an adjustable LM1117 ... or *1117, it's made by lots of companies. 800mA linear regulator with 1.2v drop ... it will self limit to 0.8-1A.

author: edavid
author_rank: Super Contributor
timestamp: June 27, 2013, 04:14:15 pm
content: The Conrad module will most likely work. If it is advertised as being short circuit proof, it won't destroy itself, or at least you can ask for a refund if it does.The only problem would be if it has foldback current limiting (you can look that up). If so, the voltage may never rise to 12V with the OCXO load connected. It's probably not specified well enough to know this without trying it (you could have made things easier for us by providing the catalog link).If you build your own supply with an LM317, there's no need to do anything special, since it's rated for 1.5A.