Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

help needed on using LM317/LM350 in Multisim...

Status
Not open for further replies.

liamk23

New Member
Hi, I am building a power supply and intend on using either of these IC's, the LM317 or LM350 voltage regulators, I however wish to do a simulation of the complete circuit before i order components and build the circuit. I have searched the internet and posted questions in various forums with no results as yet. I require a 2A rated current output and therefore wish to carry out a simulation with the LM350.

My problem is i can not locate the SPICE model to import the data for this chip anywhere on the net, If anyone can get me the details it would be greatly appreciated, i would love the LM350 SPICE model details but even the LM317 SPICE model would be of some help, preferably the LM350 though :)

The more direct the import into Multisim 9 - the better, but any info posted i will try to work with, Thanks very much for your help...
 
The LM350 is just a simple (but very smart like an LM317) voltage regulator. Everything about it is in its datasheet. You can probably make a much better power supply yourself because Multisim never seems to know if it is coming or going.

3A? The little TO-220 package cannot get rid of much heat. The big TO-3 package cools better. Multisim won't know that.

3A? Look at the spec's for its current-limiting. It is guaranteed to provide 3A if the voltage across it is 10V or less. If it has 30V across it then its guaranteed current is only 250mA. Then it is difficult to use in a variable voltage 3A supply. Multisim won't know that.
 
hi, thanks for your reply - what would you recommend for using in this specification then if you think an lm350 TO-3 package would not do the trick - i only require a max 20v output from variable supply,

it appears in a lm317 using a SPICE model i found, simulation in multisim shows the circuit to be operating as required, although i haven't looked into current operating stats yet. Voltage operation is as required though. A pointer in the right direction of a voltage regulator of use to me would be great, i will be adding digital led display to this circuit at a later date
 
For 20V at 2A you need an LM350. Its minimum input voltage is 3V higher than its output voltage so it doesn't dropout. You allow some ripple voltage to keep the size of the main filter capacitor to be reasonable, then the average input voltage is 25V.

Then you gamble about how much or how little its current-limiting will give you when its output voltage is turned down. With a 5V output it will have 20V across it which is exactly at the typical dropout point of a graph in its datasheet. Some will work fine and others won't.

With an output voltage of 5V and a current of 2A then the LM350 will try to dissipate 40W. With a perfect huge heatsink and a high velocity fan, the TO-220 package will get too hot and cause shutdown above 25W. Even a TO-3 package would need either a huge heatsink or a pretty big one plus a fan because its max dissipation is 67W.

If you short the output then it will try to dissipate 50W? It doesn't have a max current-limiting rating. It will continuously overheat and shut down, cool then start again, overheat and shut down again etc which causes non-recommended thermal fatigue. Where can you buy a bottle of liquid nitrogen?
 
Thats a great help in understanding the situation, I am trying to get hold of a LM350 TO3 type as i feel this is the one thats going to prove best and allow maximum range. I am starting to wonder if there is another way of doing this as i am not that experienced i am using any knowledge i have of anything to put something together. Looking at the possibilty of transistor type regulation now although i don't know much about this.

I will carry out some tests with the TO3 type, although it maybe a while till i get this chip, in the mean time do you think i should be looking at another way of doing it as oppose to a voltage regulator chip?

I can see the heat dissipation problem, although i can use a pretty large heatsink if required so this maybe the way forward. Its sole use is as a bench power supply so its size isn't important to an extent. Am i right in thinking that you think a TO3 should be fine if cooled properly as it dissipates 67W and the max i should experience is around 50W when short-circuited?

Thanks once again for your help.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top