This is the first LT regulator I have built so I was following the schematic and building instructions to the letter, I guess it's a case of MJ says jump...
The supply chain is transformer - bridge - reservoir - cm choke - regulator.
It must work though, so it will be interesting to make it stable with the chokes and find out where the construction mistake are.
The supply chain is transformer - bridge - reservoir - cm choke - regulator.
It must work though, so it will be interesting to make it stable with the chokes and find out where the construction mistake are.
My guess is the regs don't like seeing inductive reactance between the input and the reservoir cap(s). Normally datasheets for 3 pin regs say things like 'input cap needed if reg is more than 6 inches from res cap' implying input reactance is an issue for stability. By putting a CM choke in there you'll get a fistful of leakage inductance (tens to hundreds of uH typically) - potentially equivalent to putting the res cap in a different room from the reg. Like DF96, I can't see any point in having a CM choke there.
If you try AndrewT's suggestion of turning the regs into CCS, I've found 317s can go unstable in those configurations too.
The application hints in the datasheet say that when caps are used on adjust and/or output that bypassing at the input is needed to keep the regulator stable. Abraxalito's explanation seems plausible since you have none between choke and input and you do have 10uF on the adjust.
Personally, I'd just leave out the CM choke.
If you don't like the idea of current peaks powering up cold heaters and the CCS configuration might be prone to instability, then perhaps the slow turn on config I linked to in my previous post is the best solution. Here's a video showing the slowly rising output voltage of a slow turn on regulator. From the comment below the video: "Slow turn on - adjustable voltage regulator - 1.5v to 35v - 1.5A ideal for tube filament supply 6.3v", I' wouldn't be surprised if it's also based on an LM317.
Personally, I'd just leave out the CM choke.
If you don't like the idea of current peaks powering up cold heaters and the CCS configuration might be prone to instability, then perhaps the slow turn on config I linked to in my previous post is the best solution. Here's a video showing the slowly rising output voltage of a slow turn on regulator. From the comment below the video: "Slow turn on - adjustable voltage regulator - 1.5v to 35v - 1.5A ideal for tube filament supply 6.3v", I' wouldn't be surprised if it's also based on an LM317.
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a CCS 317 is just two components: 317 + resistor.
There is no connection to ground. Therefore no need for decoupling of either the input or the output. There is no complex circuit.
It just works.
The Tube/valve Members have used them and there are some good reports for this style of heater drive.
BUT !!!!
each CCS can only drive heaters with the same current requirement. The rules on heater to cathode voltage differential still apply.
There is no connection to ground. Therefore no need for decoupling of either the input or the output. There is no complex circuit.
It just works.
The Tube/valve Members have used them and there are some good reports for this style of heater drive.
BUT !!!!
each CCS can only drive heaters with the same current requirement. The rules on heater to cathode voltage differential still apply.
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That's my pick, too. Those regulators are happy with something like 100nF from input and output to ground, physically close to the IC itself. Did you do this?
There are different heater current requirements here so I guess a CCS is not an option this time.
I have a 1uF tant cap on the output as recommended in the datasheet, but actually I have only a 10nF and not 100nF on the input, so maybe this is the reason it is misbehaving.
Regulator circuits of this type are quite new to me, so learning a lot. Thanks for the help.
Edit: ditched the chokes, if it works without them...
I have a 1uF tant cap on the output as recommended in the datasheet, but actually I have only a 10nF and not 100nF on the input, so maybe this is the reason it is misbehaving.
Regulator circuits of this type are quite new to me, so learning a lot. Thanks for the help.
Edit: ditched the chokes, if it works without them...
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It hasn't always just worked in my experience, I often have to twiddle something. What got my last circuit to work was swapping an LM337 into an LM317 but reversed in polarity. I'd had a normal voltage reg configuration with positive and negative supplies and decided to change this to two CCS. The positive side one worked fine but the negative one only put out around 2/3rds the current of the positive one despite the same resistor value being used for both.
Moral of this story - don't use an LM337 as CCS, swap to an LM317 put in backwards.
To get LM317s as CCS reliably stable I add some series resistance to the output and input (10R or more) and often an inductor on the output too. This is running at currents under 100mA (opamp and similar kinds of circuits) into TL431 shunts, perhaps at higher currents this isn't necessary.
I just ditched the cm choke altogether as it worked fine without.
However, the regs have developed a problem - for want of a better term they seem to be pulsing. The voltage is a steady 6.5V then jumps briefly, it seems to be worse when I put a probe anywhere near it.
I thought it may be a faulty cap as it looked like a cap was discharging/charging, but I replaced them and it is still there. The problem does go away when I lift the ground, and actually lifting the ground with a resistor the problem disappears and the regs work fine. I am not sure whether this is technically approved or not though.
Can anyone offer suggestions for the cause and solution to this?
Thanks
Charlie
However, the regs have developed a problem - for want of a better term they seem to be pulsing. The voltage is a steady 6.5V then jumps briefly, it seems to be worse when I put a probe anywhere near it.
I thought it may be a faulty cap as it looked like a cap was discharging/charging, but I replaced them and it is still there. The problem does go away when I lift the ground, and actually lifting the ground with a resistor the problem disappears and the regs work fine. I am not sure whether this is technically approved or not though.
Can anyone offer suggestions for the cause and solution to this?
Thanks
Charlie
Yes, removing the 0V from mains eath. Obviously that is no good but it stops the oscillation. When lifting the 0V with a high resistor to ground the oscillation stops, but there must be a reason and solution to this.
The 0V is connected straight to the chassis, is that correct? Or should I connect to signal ground?
Edit : Connected 0V to signal ground and the oscillation is gone. So a ground loop was the reason?
The 0V is connected straight to the chassis, is that correct? Or should I connect to signal ground?
Edit : Connected 0V to signal ground and the oscillation is gone. So a ground loop was the reason?
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As DF says, just the one connection to chassis from 0V (how is that different from signal gnd?) and the chassis to earth. You could try signal ground to chassis via your CM inductor (wire both halves in parallel) - it provides safety grounding for the RCAs and speaker outs but provides a high impedance path (giving some isolation) from HF ground currents.
I thought that as the heater circuit is independent from the signal circuit it would not make a difference, but it clearly does. With the heater regs sharing the same ground as the HT the pulsing stopped, however it appears again after a short time. So I am now wondering whether there are a number of issues here...
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