28-08-2019, 04:04 PM
Mike,
As I mentioned earlier, I'd be worried about thermal issues. The TO220 package is not the best at transferring heat to a heat sink, even with the best attention to detail. If you can get away without insulating the device from the heat sink, that helps a fair bit. Alternatively, I did suggest finding a s/h or NOS LM388K in a TO3 package...
But there is another way, and this allows you to make use of your existing "LM338" chips: simply add a PNP power transistor and resistor.
You might already know about this, and you might have already considered and dismissed the idea, but just in case, or for those who haven't come across the notion...
...the power transistor can be anything you like - it just has to be able to handle the power. The value of the resistor decides how the current is shared between the IC and the transistor - for example, if you pick 4.7 ohms, then the IC only sees around 130mA and the transistor handles the rest.
The basic circuit is shown in many books and datasheets - it's well-known, and works well. Here's an example, chosen at random: http://www.bristolwatch.com/ccs/LM317boost.htm
If you like, the regulator IC can be set to carry such a low current that it doesn't even need a heat sink.
You still get the pleasure of DIY rather than buying in a lump, and you can get away with a smaller heat sink that you would with a single LM338 as the junction in a transistor can run hotter than an IC junction. If you pick a power transistor in a TO3 package, the much-improved thermal conductivity between junction and heat sink would allow an even smaller heat sink again.
The only downside is that there is now no short-circuit protection - if that matters in your application, it's simple to include (just another transistor and a resistor or two).
PS: If you only have an NPN power transistor to hand, then you can use it if you include an additional medium-power PNP type to drive the NPN: https://www.eleccircuit.com/best-dc-powe...6v-9v-12v/
As I mentioned earlier, I'd be worried about thermal issues. The TO220 package is not the best at transferring heat to a heat sink, even with the best attention to detail. If you can get away without insulating the device from the heat sink, that helps a fair bit. Alternatively, I did suggest finding a s/h or NOS LM388K in a TO3 package...
But there is another way, and this allows you to make use of your existing "LM338" chips: simply add a PNP power transistor and resistor.
You might already know about this, and you might have already considered and dismissed the idea, but just in case, or for those who haven't come across the notion...
...the power transistor can be anything you like - it just has to be able to handle the power. The value of the resistor decides how the current is shared between the IC and the transistor - for example, if you pick 4.7 ohms, then the IC only sees around 130mA and the transistor handles the rest.
The basic circuit is shown in many books and datasheets - it's well-known, and works well. Here's an example, chosen at random: http://www.bristolwatch.com/ccs/LM317boost.htm
If you like, the regulator IC can be set to carry such a low current that it doesn't even need a heat sink.
You still get the pleasure of DIY rather than buying in a lump, and you can get away with a smaller heat sink that you would with a single LM338 as the junction in a transistor can run hotter than an IC junction. If you pick a power transistor in a TO3 package, the much-improved thermal conductivity between junction and heat sink would allow an even smaller heat sink again.
The only downside is that there is now no short-circuit protection - if that matters in your application, it's simple to include (just another transistor and a resistor or two).
PS: If you only have an NPN power transistor to hand, then you can use it if you include an additional medium-power PNP type to drive the NPN: https://www.eleccircuit.com/best-dc-powe...6v-9v-12v/
