TL783 linear voltage regulator. - Page 1
Forum section: electronics / beginners

author: sureshot
author_rank: Regular Contributor
timestamp: September 08, 2018, 12:27:27 pm
content: After having used the LM317 in the past with good results, i was looking to put together a higher voltage power supply. The transformer i had in mind was around 45 V To 55 V. I have some LM317HV regulators, but the maximum input voltage is 60 Volts. After rectification and filtering the input voltage would be a bit above 60 Volts. I was concidering the TL783, with a 1.25 Volt to 125 Volt range, whilst i don't plan to go nowhere near 125 Volts, i could really use the higher input voltage. This higher voltage regulator has or can deliver a maximum of 700mA current. I would plan to use multiple pnp series pass transistors to achieve more current. Around 5 Amps would be ideal, so i was wandering if there are any real issues using the TL783 in the same way as an LM317 voltage regulator. I'm only going a shade over 60 Volts on the input to the regulator. Any help appreciated thanks.

author: iMo
author_rank: Super Contributor
timestamp: September 08, 2018, 05:59:42 pm
content: What would be your output voltage?

author: Zero999
author_rank: Super Contributor
timestamp: September 08, 2018, 08:26:21 pm
content: Why not use a switching regulator?The LM317 can be used at much higher voltages with a cascode.This circuit limits the voltage across the LM317 to 39V, which is a little high. I'd go for a lower voltage zener diode.The problem with using BJT pass transistors at such high voltages is the safe operating area becomes a limitation. The TL783 overcomes this by using a MOSFET pass transistor. You could try an LM317 with a cascode and a combination of BJT and MOSFET pass transistors. It will also work with the TL783 without the cascode. The downside of MOSFETs is they have a higher gate threshold, so the drop-out voltage will be higher, than with a BJT.

author: sureshot
author_rank: Regular Contributor
timestamp: September 09, 2018, 09:45:18 am
content: My output voltage would be variable from the 1.25 Volts reference up to the maximum the transformer can deliver. I was hoping to keep it simple in terms of layout and parts count. The only thing with power supply circuits is the limits you can breadboard up to in terms of current, and testing first. Before building it that is. Wish there was a high current breadboard on the market, one that can take thicker component leads, and wire gauges. Thank you for your replies. Usally i just build the circuit as breadboard layout isn't practical for high current testing. I've wanted to try mosfets in a power supply circuit for some time now. Most of the higher voltages i use from any supply circuit would be for series circuits testing.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 09, 2018, 02:18:27 pm
content: Hi,have you tried the design by ChangPuak ? It has the following advantages:- Range from a few 10Vs to several 100V- external MOSFET pass transistors to cover the power requirementsFor HV use, you could try IXYS linear MOSFETs.- Short circuit protection- Stable, low noise.The Changpuak page is here:https://www.changpuak.ch/electronics/VB408.phpI used this in a lot of designs and I like it.https://electronicprojectsforfun.wordpress.com/power-supplies/high-voltage-lab-power-supplies/

author: exe
author_rank: Unknown
timestamp: September 09, 2018, 03:58:34 pm
content: With linear design, watch out power dissipation when output voltage is low.BTW why do you need such a high voltage?

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2018, 04:07:54 pm
content: Quote from: sureshot on September 09, 2018, 09:45:18 amThe main downside with the TL783 is the huge dropout voltage, which has a positive temperature coefficient. See figure 10, on page 7 of the data sheet. Even at 100mA, it's over 5V and at 700mA, it's nearly 20V, at high temperatures. Definitely use a combination of a PNP BJT and N-MOSFET pass transistor to keep the current low and the it shouldn't increase the dropout by more than a volt or so.http://www.ti.com/lit/ds/symlink/tl783.pdf

author: sureshot
author_rank: Regular Contributor
timestamp: September 09, 2018, 10:14:20 pm
content: Thanks for the information, I have a copy of the data sheet, I should have looked it over more closely so thanks for that. Why do I need such a high voltage ? Well about 60 Volts would be as far as I would use it. But for the most part between 5 Volts and 15 Volts. Having the ability to go up to 60 Volts is kind of handy.I'm hoping the series pass elements will deliver the current needed, I don't expect no where near as much at 60 Volts as say 13.80 Volts. I would expect that current to drop off the higher I go.

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2018, 10:55:16 pm
content: Does the transformer have a centre tap? If so, the voltage could be taken from that, at low output voltage settings. There's no point in burning off more power than necessary.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 10, 2018, 08:18:55 am
content: Hi,you could use a transformer tap switching scheme or a preregulator so you can use the full voltage range but you need only a fraction of power to dissipate with low output voltages, like here:https://electronicprojectsforfun.wordpress.com/power-supplies/preregulator-circuits-for-linear-power-supplies/

author: sureshot
author_rank: Regular Contributor
timestamp: September 10, 2018, 10:55:46 am
content: I did think about finding a transformer i could switch different windings in and out for a selected voltage. I'm not sure its suitable for the TL783, i think minimum input voltage is quite high. But yes, burning up large voltage gaps is a bad idea.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 10, 2018, 12:07:39 pm
content: ... if you have no two-tap transformer, you could still use another (maybe SCR) regulator to keep heat losses down.

author: sureshot
author_rank: Regular Contributor
timestamp: September 10, 2018, 09:59:02 pm
content: Yes that's something I will look into.Thanks again for all the replys and advise.

author: tvmanmarks
author_rank: Newbie
timestamp: August 25, 2020, 08:58:09 pm
content: i"m a late comer to this topic but i hope to get a reply.my test equipment's power supply may be 150vdc max. i need a simple 125 vdc regulator. i see the TL 783 max input isn't high enough to use in this old tube test equipment, are there other 3 terminal regulator devices than can handle 150 vdc on the input ?barry

author: Ian.M
author_rank: Super Contributor
timestamp: August 25, 2020, 09:31:06 pm
content: Maybe Supertex (now Microchip)'sLR12?It very much depends on how much load current you need as the main issue is dumping the heat.

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2020, 07:34:28 am
content: Quote from: tvmanmarks on August 25, 2020, 08:58:09 pmHow much current do you need? Will the output ever get short circuit? Does the 125V output have a capacitor across it, if so how big?The TL783 and LM317 are floating regulators. This means there's no absolute maximum output voltage rating, only the difference between the input and output voltage. You need 150V in and 125V out, which is a voltage drop of just 25V, so even the LM317 will do, as long as the output doesn't get short circuited.