LM317 Question - Higher Voltage Wanted - Page 1
Forum section: electronics / beginners

author: jpc6204
author_rank: Contributor
timestamp: April 15, 2012, 09:51:21 pm
content: Discussion on the forum and data sheets for the LM317 talk about putting several chips in parallel to increase current output, but what about increasing voltage output. I am interested in making a power supply with an output of 2.5 - 65 V and a current limited to 1 A. Is it possible to put two LM317s in series and power them with something like 70 V to achieve this?Jeff...

author: sacherjj
author_rank: Frequent Contributor
timestamp: April 15, 2012, 10:02:09 pm
content: This data sheet might help for high voltage circuits:http://www.national.com/ms/LB/LB-47.pdf

author: T4P
author_rank: Super Contributor
timestamp: April 15, 2012, 10:51:55 pm
content: 2 LM317 is more expensive then just a LM317 and another TO-3 or TO-3P transistor .http://www.bristolwatch.com/ele/lm317.htmThe classic 2N3055 or the TIP41C can be used .Or if you can live with just 57V output you can use this one :http://www.futurlec.com/Linear/LM2594HVN-ADJpr.shtmlBut be very careful , if the input voltage is 70 and your output voltage is 1 , the power dissipation can hit 69W .Switcher or not up to you .http://www.futurlec.com/Linear/LM2575HVT-ADJpr.shtmlis a 57V 1Amp switcher

author: ivan747
author_rank: Super Contributor
timestamp: April 16, 2012, 06:46:35 pm
content: You typically use separate supplies for that, otherwise you get too much power dissipation at low voltages.

author: jpc6204
author_rank: Contributor
timestamp: April 17, 2012, 01:19:10 am
content: Thanks to all. I'm gonna do some research and draw up a schematic. Stay tuned...Jeff...

author: jpc6204
author_rank: Contributor
timestamp: April 27, 2012, 01:36:25 am
content: Quote from: AcHmed99 on April 15, 2012, 11:58:10 pmI like AcHmed99's suggestion of using this chip (TL783C, see link above). In the data sheet (http://focus.ti.com/lit/ds/symlink/tl783.pdf) is shows a nice schematic (Figure 20 on page 10) of a Adjustable Regulator With Current Boost and Current Limit. I am looking for parts to build this and am trying to find the TIPL762 transistor. Mouser has them (http://www.mouser.com/Search/Refine.aspx?Keyword=TIPL762), but I need to buy a high quantity (>100). I tried looking on Digikey for something that will work and came up with these:http://search.digikey.com/us/en/products/BUJ403A,127/568-6739-ND/2119437http://search.digikey.com/us/en/products/2SD1047/497-11320-5-ND/2674471http://search.digikey.com/us/en/products/STWH13009/497-8815-5-ND/2035449http://search.digikey.com/us/en/products/BUL743/497-12455-5-ND/2214337Do you guys think any of these would work?Many thanks in advance!

author: Flávio V
author_rank: Regular Contributor
timestamp: April 27, 2012, 06:39:39 am
content: Good ones are the said above TL783C and the LM317HV...The LM317HV is a 1.2-57V 1.5A linear regulator...so you lose 8 of your desired volts but that might not be very important...I would't make a linear power supply were the regulator gets something near 50W heat...because it is too hot and expensive to get it at normal temperatures(60ºC)

author: jpc6204
author_rank: Contributor
timestamp: April 27, 2012, 04:28:14 pm
content: Ahhh, the importance of switching power supplies.For the price of the heatsinks I can build a 1.25 - 30 V supply. I can use the TL783C for 30 - 60V. This is what I get for that operating range:Worst case highline of 40VDCSA= Tjmax-TA/PD –(JC/2 –CS)= 125-60/40 –( 0.7/2- 0.24)~ 0.5C/WThis is a little better. Hmmmmm...

author: Flávio V
author_rank: Regular Contributor
timestamp: April 28, 2012, 08:57:00 am
content: Other way to not use fans is to use a peltier.....The peltier is a small device than colds the air around it(+- -10ºC,some reach -60ºC...but -10ºC is the regular for 18ºC ambient)and in the back of the peltier it comes hot air....so you only need a heatsink in that part....but it is better....a 40W one might be able to do it....

author: Rerouter
author_rank: Super Contributor
timestamp: April 28, 2012, 10:06:40 am
content: another way is to simply use tap switching, if you have 70V its normally a 35V center tap transformer, if so when you reach ~30V, you switch with a relay to the lower voltage coil, that way you almost cut your maximum wattage in half, though really a switching pre-regulator is probably prefered,personally i am using the common 317/337 for +-72V, at 1.5A, though i have been slowly trying to implement a switcher / tap switcher,

author: Zero999
author_rank: Super Contributor
timestamp: April 28, 2012, 02:57:02 pm
content: Yes, it agree, a tap switcher is the best way to do it. All you need to do is monitor the output voltage of the regulator using a comparator which switches the taps.

author: Unknown
author_rank: Unknown
timestamp: April 28, 2012, 03:34:22 pm
content: Quote from: Flávio V on April 28, 2012, 08:57:00 amNote that you still need to get rid of the same amount of energy, plus the energy dissipated by the Peltier device (efficiency is not terribly high). The only advantage is that cooling might require less airflow because of the higher temperature on the hot side, but I doubt that this compensates for all the extra energy used by the Peltier device. Don't expect it to replace a fan.