Can I safely parallel the three pins of two adjustable regulators to increase the current handling limit?
Up to about 3 amps, you can use a single pass transistor. If a positive voltage regulator, use a 2n3772 or mj15024 npn. Use resistor between the regulator out & base to limit base current, say 5.6 ohms. Transistor will drop about a volt below the regulator output. Plus to collector, emitter to load. Make sure you have enough heat sink.
If you can't drill 3 holes to fit TO3 package, single hole packages like MJL4281 or MJL21194 can stand about 2.5 amps.
If you can insulate the heat sink from touch & chassis ground you don't have to use an insulator under transistor. If not, use one. If mica, use heat sink compound, if silicon rubber, not needed.
#4-40 or 3 mm screw & nut will fit the holes even on the non-TO3 packages.
If you can't drill 3 holes to fit TO3 package, single hole packages like MJL4281 or MJL21194 can stand about 2.5 amps.
If you can insulate the heat sink from touch & chassis ground you don't have to use an insulator under transistor. If not, use one. If mica, use heat sink compound, if silicon rubber, not needed.
#4-40 or 3 mm screw & nut will fit the holes even on the non-TO3 packages.
Yes you can but you need to add a diode in series with the output of each reg before joining them together.
Two LM7812 voltage regulators connected in parallel for a maximum output current of 2 Amp | Belajar
Two LM7812 voltage regulators connected in parallel for a maximum output current of 2 Amp | Belajar
Thanks, this is a start. Ive had regulated filament supplies kicking around my head all week. I may not do this parallel arrangement but sometimes you just want to try something to learn something. Series stringing like tubes and doubling the voltage will probably be enough to keep my currents lower. But knowing this circuit adds another ingredient to the fun.
Several things can happen if you parallel voltage regulators.
They may be slightly different voltages (very likely) in which case one does all the work and
you've not helped anything.
They may oscillate violently and fry the load circuit (more likely with fussy low-dropout designs) - a voltage regulator is a high gain amplifier using negative feedback so the conditions for stability are complex.
They may be slightly different voltages (very likely) in which case one does all the work and
you've not helped anything.
They may oscillate violently and fry the load circuit (more likely with fussy low-dropout designs) - a voltage regulator is a high gain amplifier using negative feedback so the conditions for stability are complex.
You can put a resistor from the voltage input of the regulator to the output. Size it so that it will provide 80% of the current required at the operating voltage. Then the regulator will provide the rest to reach the target voltage. You will need to use at least a 10 watt power resistor, but can go much higher. When using a square ceramic white power resistor derate the wattage rating of the resistor by at least half.
This looks like the best route, to put alike tubes (or tubes with the same v/a rating) in series each on their own current-limited regulator. The LM1084 can pass 5 amps. I'd limit the current to that of a single tube in the string, so the current limiter drops out of the circuit after they are warmed up, only kicking in during the cold filament inrush. Since I am using a separate heater transformer I can choose any voltage needed for the series strings (12v or 24v transformers, bridge, filter, hopefully I will still be above the LDO of the LM1084, If not Antek has a good selection of 15v and 28v transformers).
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LT1083 will handle 7.5 amps.
Consider a soft start to protect you heaters - google for 317 soft start and you’ll see some simple add-ons that will allow you to gently ramp up the voltage.
Consider a soft start to protect you heaters - google for 317 soft start and you’ll see some simple add-ons that will allow you to gently ramp up the voltage.
Figure 9.3.11 in the application notes actually does parallel 3 regulators... A different mfg didn't present a circuit for this. But appears to be possible. But the Lt1083 with 7.5A should be plenty.
http://www.ti.com/lit/ds/slvs044y/slvs044y.pdf?HQS=slvs044-aaj&ts=1589414013034
http://www.ti.com/lit/ds/slvs044y/slvs044y.pdf?HQS=slvs044-aaj&ts=1589414013034
I have designed a few valve amplifiers and in every case with noise it was never due to AC heaters. Even after making heaters DC the same noise was still there.
50Hz is pretty slow and doesn't radiate too badly from a wire.
Also, DC heaters get a very strong power up surge as the heaters are virtually a short when cold.
AC heaters cold current is limited by the transformer.
That's my experience of valve/tube heaters.
If you are using DHT then that's a different matter.
> can I parallel voltage regulators?
Yes. The highest voltage one will take ALL the load until it cuts-out. If it current-limits it *may* "sag" enough so the next lower-V regulator takes some load, and so on. It's like three columns in your cellar, 8'1" 8'0" 7'11". With very light load only the tall pole carries load. If the loaded house sags a couple inches, all poles carry load, but not equally. Poor support.
With 0.2 Ohm resistors. This makes the output saggy. It will drop about 0.25+V at 4A. If the regulators are within a tenth-volt of each other, current share *may* be acceptable. But there's that sag! Why did we buy three costly chips to get a saggy supply? Their answer is to sell us a fourth chip to jack-up the right amount.
Fig 9.3.12 is a different answer. The "added chips" is a big dumb transistor. But that part does not have current or thermal limiting.
Yes. The highest voltage one will take ALL the load until it cuts-out. If it current-limits it *may* "sag" enough so the next lower-V regulator takes some load, and so on. It's like three columns in your cellar, 8'1" 8'0" 7'11". With very light load only the tall pole carries load. If the loaded house sags a couple inches, all poles carry load, but not equally. Poor support.
With 0.2 Ohm resistors. This makes the output saggy. It will drop about 0.25+V at 4A. If the regulators are within a tenth-volt of each other, current share *may* be acceptable. But there's that sag! Why did we buy three costly chips to get a saggy supply? Their answer is to sell us a fourth chip to jack-up the right amount.
Fig 9.3.12 is a different answer. The "added chips" is a big dumb transistor. But that part does not have current or thermal limiting.
It's on Pintrest, that must be a good electronic circuit reference, right???
I don't like it. If one leg gives higher voltage and thus puts through more current than the other (as it surely will due to resistor tolerances), the associated diode will warm up more, lowering its voltage drop and causing even MORE current to go through that leg. I don't like it.
Did I say I don't ...
I like that idea.
The 3-regulator circuit will surely work, but it's got a transistor and op-amp added, looks like extra stuff. Might as well use a LM723 and a couple big pass transistors.
Be sure to figure out your maximum input voltage (with say, a high line voltage of 130/250V) and calculate power dissipation for the pass device(s), and provide adequate heat sinking so they don't get too hot. You might want to check for dropout at low line voltage and such, but with tube filaments I think running them a bit low would be no big deal.
In the 1980ies ELEKTOR published an article about a bench supply, featuring five LM317's. They were paralleled using current balancing resistors, and the voltage regulation task was done by an additional opamp (741, me thinks). What an effort!
Best regards!
Best regards!
Thanks all so far. I agree there are probably better solutions without adding this complexity. I have flexibility because the filament X former is separate, I can buy any voltage I need. So I like the idea of series stringing like ma 6 volt tubes at 12.6 volts in pairs, then paralleling the pairs, to get the total current down by half making everything else simpler. It's interesting to know how versatile these regulators are though.