HV regulated supply with soft start

[rankot]

I wanted to build high voltage regulated power supply with soft start, so I found an interesting tech note http://www.ti.com/lit/an/snoa648/snoa648.pdf LB-47 by TI. There is also similar design by P. Millet derived from that, and can be found here: http://www.pmillett.com/HV_reg.html



I also wanted to make it start gradually, to protect tubes from starting spikes, so I added a simple soft circuit. This is my circuit below. I don't use parallel diodes in rectifier, they are there just to provide space for different diode types.

http://docdro.id/0mQtR8C

So I built it and, to my surprise, it really works making gradual rise of output voltage, but not more than 53 Volts! I really have no idea why. I have tested this circuit in LTspice and it works fine there, running smooth until output voltage gets to 260V and stays steady there. I used LM317T (LM317H/HV is not available in my country), but everything else is the same as in first two circuits. I tried to change few things, but output voltage stays 53V.

I have measured input voltage and it is OK.

I have one more idea, to put few Zener diodes between R33-R36 and ground (and change them and R29 accordingly), so I can have elevated adjust pin of LM317. Will it work? Did anyone tried this or something similar? Like this:

[rankot]

I've found a problem - somehow I reversed 20V Zeners. After fixing that, everything works fine.

[thomasha]

Nice circuit,
have you tried running a tube amp with it?
I'm interested to see if there is a large voltage drop under load.

Could you also post some pictures? Just to give an idea of the size of the circuit...

Do the transistors get hot? I think this is the main problem with the usual circuits (other than the noise)

[anotherjim]

The tube era answer is a thermal time delay relay in a tube like glass bottle. Available for any of the socket types. The heater obviously runs off the tube heater supply.

With projects running HT from back to back transformers, I've noticed it's a slowish soft start anyway, but it still gets to peak before the tubes are running and the HT can drop under some load.

[rankot]

Amplifier and preamp are still in assembly state, once I finish them I will know if there is a drop or not; however, I wanted to make sure my PS works at the first place. It shall work w/o problems for loads up to 32mA.

[rankot]

For those who wish to build this, there's a PCB design file, with fixed Zener diode orientation: http://docdro.id/b9xPkAX. I still didn't test it under load, so stay calm until I report it works as expected.

I used 4x100k resistors cause I wanted to have 260V, and 25k resistors are not available, but you can use any value you want. There's a good on line calculator for LM317 regulators here: http://www.reuk.co.uk/wordpress/electric-circuit/lm317-voltage-calculator/.

[rankot]

Do the transistors get hot? I think this is the main problem with the usual circuits (other than the noise)

Transistors shouldn't get too hot, IRF740 has a peak of 4W dissipation in the first two seconds, then settles at 200mW. It's nothing. Regulator (500mW dissipation) and BF472 (650mW peak, 0mW stabilized) shouldn't be hot, too.

And I also wanted to have regulated DC without use of chokes, before they are not easily available here where I live.

[rankot]

Below: mid gain preamp from Merlin's book (small PCB) and HV soft start PS.

[PRR]

> The tube era answer is a thermal time delay relay in a tube like glass bottle. Available for any of the socket types. The heater obviously runs off the tube heater supply.

The tube-era answer was, almost always, to just BANG the tubes on. Certainly on small tubes, bang-on does no harm. (Over 1,000 Watt rigs pushed to their limits do need a start-up.) All this slow-start is a modern infatuation.

I'd also say that regulation is a new invention. Yes milli-Volt DC meters and large analog computers ran to regulated voltage. In most audio, supply voltage is NOT critical.

BTW: many-many of the thermal delay relays were 115V and run from the line. Why not? 6V ones were special order. ALL these parts are now scarce. And they still banged-on, just delayed, not ramped.

[rankot]

I did this because I wanted to have soft start for heaters, so if I soft start them, then it is logical to soft start B+ too. Or not? Never mind, I'm quite happy with this.

[thomasha]

Thanks for the pics,
I had some bad results with the IRF740. It get's really hot and drops to 100v.
Since then I'm using the IRF644 for half the on resistance, but lower HV. I guess my heat sink was too small. (I'm running the SMPS from 12v)

How high is your input voltage? Are you pluging it direct to the wall, or are you using a lower dc supply?

[rankot]

I am actually using AC220V from back to back transformer, no SMPS.

[rankot]

I've tried it with 33k load (10W resistor) and it is still stable. Voltage drops a little, to 245V.

[thermionix]

EZ81

[rankot]

As I said in the beginning, I don't want to use tube rectifier because I need well regulated PS without using of a choke. And considering price, all the parts I need for this PS cost less than 5 EUR in my local store, while EZ81 alone cost at least 8 (on a flea market, probably used).

[rankot]

Finally I have tested it with my preamp and, although the voltage is stable, there is some kind of very strong hum. Preamp is definitely good, it works really fine with usual PS, so there is some kind of problem here.

I don't have 2.7 ohm resistor (R6 on original schematic, R7 on mine) installed, just 1u capacitor there. Could it be source of a problem?

[anotherjim]

Just to answer Paul earlier, I have definitely seen old gear with thermal soft start timing, and they don't bang-on and the thermals definitely had 6.3v heater. The soft start is a single step of course. Usually the thermal only has one contact, so its used in conjunction with a normal electromagnetic relay. Cold start has a fixed resistor in series to drop the initial voltage or uses a lower transformer tap. When the thermal contact makes, the em relay picks, disconnects the thermal heater, maintains its own coil via its own contacts (since the thermal starts to cool and opens its contact) and bypasses the dropper resistor or switches transformer voltage tap up.
The Thermals I've seen were numbered 6Nxxx where the xxx number was the time delay in seconds? Like 6N030.

[rankot]

I have already built "classic" soft start circuit similar to one you described and it works fine, but I wanted to combine soft start together with HV regulation. Here is the schematic for that one:

https://docdro.id/qHpg5BM

[anotherjim]

Has anyone looked at the LR8 high voltage regulator? I only just noticed its existence. It's like a TO92 LM317 with the voltage set by a pair of resistors, but works up to 450v! Only 20mA limit, so preamp supply only I suppose.

[printer2]

Has anyone looked at the LR8 high voltage regulator? I only just noticed its existence. It's like a TO92 LM317 with the voltage set by a pair of resistors, but works up to 450v! Only 20mA limit, so preamp supply only I suppose.

Keep in mind the power dissipation limit.