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It is currently 16 Oct 2019, 10:57

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PostPosted: 13 Jul 2019, 11:33 
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Heater board finished,

PITA.. :D
You know when the spec circuit on a data sheet should work but doesn't.. :up:
Anyway after a few different attempts I ended up using the 317 soft start with the bypass diode and polyswitch fusing.

The diode bridge is snubbed with ammobeads built on kite board.
I used a tantalum timing cap with the hope it might last.

For some reason the data sheet circuit values just wouldn't work (why?) anyway after a fault find I ended up reducing the 47K to 22K and increasing the ramp cap to 68uF 25v. Perhaps its using the BC557 as a replacement to spec device.
I'll probably use a dropper in front of the board to take the load off the regulator.

So its now working and fits the space available.

The ramp time should also give a mute release with a relay, but we'll see how it goes.
The aluminium bracket will be compound coated and bolted to the copper chassis.
The BUZ11 ramp would probably have been better but it meant using two devices.

Regards
M. Gregg


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PostPosted: 13 Jul 2019, 16:32 
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Theres very little point on a slow start on valve heater.
Since they are heater like littratley a piece of metal glowing red. High peak currents will not damage them, but make them heat up faster intially.


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PostPosted: 13 Jul 2019, 17:00 
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ILoveHiFi wrote:
Theres very little point on a slow start on valve heater.
Since they are heater like littratley a piece of metal glowing red. High peak currents will not damage them, but make them heat up faster intially.


Hi,

Tube heaters are very low resistance on start up resistance increases as they heat up and current drops, some early tube heaters "Flash" at start up which is not the best of situations.

The surge can loosen EI laminations and cause toroids to buzz at start, its possible to control it with inrush suppressors.
However with small regulators they go into current limit and shut down then just sit there.
I have controlled it in the past with timers shorting inrush limiters.
With controlled surge you can fuse closer to the running current and have better protection.

Just found a youtube video with someone playing with the circuit:
https://www.youtube.com/watch?v=nazf9PwCTC8

Just to give an idea of how it works.

Regards
M. Gregg

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PostPosted: 14 Jul 2019, 07:06 
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Heater board fitted in place,

Just some photos for interest.
As you can see there are support posts that take up some space so it had to be a build around transferring between sides of the board.
More of the slow build :sleep: :D

Regards
M. Gregg


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PostPosted: 14 Jul 2019, 18:51 
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Looking good. Could be a candidate to be on display at Tate Modern.

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PostPosted: 15 Jul 2019, 05:39 
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Woodo wrote:
Looking good. Could be a candidate to be on display at Tate Modern.


LOL,

I could do with a time machine, If I travel 50 years into the future I might get to see the power up and test. :D
Just before the first parts fitted crumble to dust from old age and corrosion.

Regards
M. Gregg

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PostPosted: 16 Jul 2019, 13:23 
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Toroid fitted,

I cut the mounting plate because I'm not happy with it tilted and putting pressure on the tail out connections.
Burden resistor fitted 2.2 ohm 3 watt.
Toroid mounted and wired then in situ test of soft start.
Bottom pic is start up then increase to 12.6V.

For anyone interested I had a faulty BC557 when initial testing the circuit the fault showed as a ramp that stopped at mid voltage then shut down again with the ramp capacitor discharging. Transistor replaced no further problems. The transistor is fitted in a socket so its just plug and play.

Regards
M. Gregg


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PostPosted: 16 Jul 2019, 17:01 
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Faulty transitors are extremely rare, unless the circuit has potnetial for startup transients which over the maximum power ratings, or overly high operating voltages or power.

I've used more than 600pcs of chinese cheapest TO-92 transtiors like arround 2-3usd for 400pcs and they never broke on me.

Fake is not rare, but knowing the transitor would work in current circuit 100% and fail on you because of bad batch is extremely rare, I've never seen it before.


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PostPosted: 17 Jul 2019, 10:43 
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ILoveHiFi wrote:
Faulty transitors are extremely rare, unless the circuit has potnetial for startup transients which over the maximum power ratings, or overly high operating voltages or power.

I've used more than 600pcs of chinese cheapest TO-92 transtiors like arround 2-3usd for 400pcs and they never broke on me.

Fake is not rare, but knowing the transitor would work in current circuit 100% and fail on you because of bad batch is extremely rare, I've never seen it before.


I agree,

I'm wondering if the Emitter / Base voltage has some how been exceded. Its very strange.
I'm still undecided on this I think it might be time for another look.
But the circuit is all over the internet in one form or another.
EG https://www.diy-audio-guide.com/soft-start.html

The circuit is correct and seems to work every time since transistor change.
Its such a simple circuit but as you say strange, taking into account its more or less straight off the data sheet LM317.
NB the "faulty" transistor isn't dead, it just fails intermittently and is not reliable. :confused:

Regards
M. Gregg

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PostPosted: 17 Jul 2019, 14:50 
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I have no idea, but I think maybe you got something else wrong or you've connected something the wrong way.

Unless your being silly delibratley, otherwise I've also never seen transitors break down from excessive base emitter voltage


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