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Old 9th November 2012, 01:35 AM   #31
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Quote:
Originally Posted by AndrewT View Post
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.
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.
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Old 9th November 2012, 09:20 PM   #32
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I hope you put the common mode choke upstream of the filter capacitors? anyway they are pretty much a waste of time with 50Hz supplies their main purpose is to break the ground loop caused by high frequency noise acting on stray capacitance to ground, which is an SMPS problem.
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Old 12th November 2012, 08:15 PM   #33
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Increase the input cap. to 220 mf or 470 mf even 1000 mf after the cm choke and it should settle down as it the will not react to the inductive front end . Also after that measure the voltage drop across the resistor and regulator at start up . Voltage drop across the res. is 1.25 volts
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Old 27th November 2012, 09:01 AM   #34
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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
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Old 27th November 2012, 10:06 AM   #35
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Sounds like you've got parasitic oscillation - somewhere the reg is seeing too much inductance - probably on its input or on the gnd. When you say 'lifting the ground' do you mean removing 0V from mains earth?
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Old 27th November 2012, 10:36 AM   #36
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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?
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Old 27th November 2012, 10:42 AM   #37
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That would depend on what you are feeding, and how that is supposed to be grounded. Generally, the chassis is safety ground (connected to the incoming mains ground) and should have one and only one connection to signal ground. All other ground connections should be to signal ground.
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Old 27th November 2012, 10:51 AM   #38
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Quote:
Originally Posted by CSlee View Post
The 0V is connected straight to the chassis, is that correct? Or should I connect to signal ground?
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.
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Old 27th November 2012, 01:09 PM   #39
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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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Old 27th November 2012, 02:12 PM   #40
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Might over heating be a possibility? I have been monitoring it for an hour now and everything was working fine, but the PS has got very hot and the oscilltion has started again.
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