My system has a 28VDC supply which I need to step down to 18VDC (~5A). Does anyone know of a regulator that will do the job. I have no preference on type, switching, linear, etc. Almost anything will do the job. Thanks.
A National LM138 linear regulator will do the job for you. It's currently stocked by many of the parts houses. Get the datasheet from
-- Dave M MasonDG44 at comcast dot net (Just substitute the appropriate characters in the address) Some days you\'re the dog, some days the hydrant.
the
Using a linear regulator for this would hardly be the best choice. At
5A, it will have 50W dissipation (and I don't know of a LM138 rated at that, even with a heat sink less than the size of a small box). Apart from that, it's very wasteful.There are some commercial POL converters on plugins (although 18V is not exactly a standard size, but I am sure one might be adapted).
Failing that, Maxim, TI and Linear tech have a nice line of controllers (TI has some really nice ones with the compensation internally that works for most cases).
Do you need a plugin solution or are you willing to lay out a switcher (which is not that simple a matter)?
Cheers
PeteS
The T-package (TO-220) has a thermal resistance of 4C/W from junction to case - which would be a 200C temperature rise at 50W, so you can't use that.
The K-package (TO-3) is a lot better at 1C/W, but you've got to limit the junction temperature to less than about 125C (otherwise the LM338 will do it for you) so your total thermal resistance from junction to ambient has to be less than 2C/W, which isn't easy to achieve - I'd be looking at a 0.5C/W heatsink, like Farnell's150-019, which costs about $25, stand 2"" high , 5" wide and 6" long.
Paralleling a couple of linear regulators might save you money and heat-sink volume, or you might put a beefy power transistor in series with a low-drop-out linear regulator, mounting it on the same heat-sink as the linear regulator to exploit the linear regulators thermally driven current limiter.
-- Bill Sloman, Nijmegen
"DaveM"
** Easily fixed by adding a series resistor of about 1.5 ohms.Won't change the total dissipation, but shifts most of it off the LM338 so it runs cooler and avoid approaching thermal shutdown. Means you can use the cheap TO220 version OK.
....... Phil
I've used that trick - though not for many years - and it works really well. Make sure that you don't mouint the reistor on the same heatsink as the regulator - resistors are typically designed for a peak junction temperature of 250C, and the heat sink can (and should) be sized to get pretty hot (since resistors are cheap and heat-sinks are relatively expensive).
-- Bill Sloman, Nijmegen
How high can the nominal 28V go? If this is for aircraft, you'd better defend against at least 50V spikes.
How low can the 28V go and do you need to maintain regulation under that condition?
-- -- kensmith@rahul.net forging knowledge
There might be a SimpleSwitcher (R) that would work (National)
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