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Solid State Talk all about solid state amplification.

I would like a few ideas..
I would like a few ideas..
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Old 3rd May 2015, 05:08 PM   #1
M Gregg is offline M Gregg  England
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Default I would like a few ideas..

Here is the scenario,

I am looking for a simple circuit to recreate this I have had a few ideas however I though I would ask the question.

The problem is a tube based situation, however the answer is SS.

Simple circuit that works.

This is feeding a tube heater circuit that have low resistance before warm up. The power supply trips on current limit and requires a soft start. Easy.

OK.

Existing is an NTC at 160 ohms that is shorted by a relay after 10 sec.

The tubes are in series so require 12.6V at .36 A.

Now the issues.
The voltage from the PSU is 12.6V so I cannot have any voltage drop after ramp up. The current increase needs to be exponential to stop the PSU tripping. Ramp time 10 sec.

The problems:
If not enough current is drawn the heaters don't warm up enough to increase resistance before relay short is energized. Then PSU goes into current limit.

I need to create a simple circuit that will emulate the NTC circuit without depending on temperature. Like the NTC.
The reason is the NTC even though shorted after start up cannot repeat its function until the equipment cools down, so repeat restarts cannot happen within a short space of time<<problem is repeatability.

NB you are switching onto almost a dead short at start up.


Any thoughts?

Regards
M. Gregg

Last edited by M Gregg; 3rd May 2015 at 05:12 PM.
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Old 3rd May 2015, 05:37 PM   #2
M Gregg is offline M Gregg  England
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The only conclusion at the moment is a ramped CCS with relay short.

Regards
M. Gregg

Last edited by M Gregg; 3rd May 2015 at 05:41 PM.
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Old 3rd May 2015, 06:11 PM   #3
Elvee is online now Elvee  Belgium
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A simple PWM circuit would be much more effective than a CCS: at the beginning, it will generate short, high current pulses into the ~shorted filament whilst keeping the average current to an arbitrarily low value, 0.36A for example.
The rms current into the heater will be much higher, making the heating so much faster and efficient and when the process terminates, no need for a relay: the 20milliohm or so of Rds will provide a nice short.

A MOS, a pinch of low cost components for the control, and there you are: no heatsink, no complication, can be made for 1€ or 2 (or £)
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Old 3rd May 2015, 06:20 PM   #4
M Gregg is offline M Gregg  England
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Quote:
Originally Posted by Elvee View Post
A simple PWM circuit would be much more effective than a CCS: at the beginning, it will generate short, high current pulses into the ~shorted filament whilst keeping the average current to an arbitrarily low value, 0.36A for example.
The rms current into the heater will be much higher, making the heating so much faster and efficient and when the process terminates, no need for a relay: the 20milliohm or so of Rds will provide a nice short.

A MOS, a pinch of low cost components for the control, and there you are: no heatsink, no complication, can be made for 1€ or 2 (or £)
Sounds interesting,

I haven't done any design on Pulse width modulation so any further information would be useful.

Regards
M. Gregg
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Old 3rd May 2015, 08:07 PM   #5
M Gregg is offline M Gregg  England
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Just had a quick look,

So something like a 555 driving a FET..how would you turn off the PWM and use the FET in the supply?

If the PWM circuit was between the supply and the heater elements the voltage across the PWM would reduce.. (turning off the PWM)

if the pulses were negative the FET would remain on..

Is this the idea you were thinking about?

Set the duty cycle for max current of PSU without current limit..

Regards
M. Gregg
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Old 3rd May 2015, 08:23 PM   #6
Mooly is offline Mooly  United Kingdom
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I would like a few ideas..
I have a working circuit for a soft start which could perhaps be adapted. Its repeatable and operates following even the briefest of on/off cycles (milliseconds). Although intended for mains it could be also used on the secondary side using two FET based solid state relays to switch your heater current, or two conventional relays come to that.

The logic uses only one CMOS quad gate and works well.

Edit... or one relay even, because you don't need the on/off function.
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Old 3rd May 2015, 08:28 PM   #7
jcx is offline jcx  United States
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I don't see that a linear soft start is guaranteed to work depending on the warmup curve/resistance of the heater and PS current limit margin over running level

maybe dumping a enough stored energy from a (slow)charged cap would get the filament up to temp

in general switching power conversion may have a better chance but you will have thinking to do since most controllers/chip literature will be for creating relatively constant output V

Last edited by jcx; 3rd May 2015 at 08:44 PM.
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Old 3rd May 2015, 08:35 PM   #8
vzaichenko is offline vzaichenko  Russian Federation
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I would like a few ideas..
Hi M.Gregg,

Not sure if you like the idea (), but here is the thread I have started some time ago, related to the micro-controller based control board - comprehensive soft-start plus protection. It's extremely flexible and its very easy to add PWM there - just a small piece of code in the firmware (I can help with the code if you ever decide to go this way). The design is progressing - Jeff Wilhelm is doing a great job at the moment, see the end of the thread, the board is built many times in many versions - really good piece of hardware (and software).

Cheers,
Valery
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Old 3rd May 2015, 08:54 PM   #9
M Gregg is offline M Gregg  England
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The heaters are being fed with DC from a DC to Dc converter.

I was looking for fun at this..
http://www.instructables.com/id/Very.../Building-it-/

With a 220k feeding the gate of a FET and the PWM across the FET ..

Don't know the pulses will probably shut the 555 down..don't know..

If someone could model it ...I don't have the SS spice..

It would save some soldering..

Regards
M. Gregg

Last edited by M Gregg; 3rd May 2015 at 08:56 PM.
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Old 3rd May 2015, 08:55 PM   #10
M Gregg is offline M Gregg  England
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Quote:
Originally Posted by Mooly View Post
I have a working circuit for a soft start which could perhaps be adapted. Its repeatable and operates following even the briefest of on/off cycles (milliseconds). Although intended for mains it could be also used on the secondary side using two FET based solid state relays to switch your heater current, or two conventional relays come to that.

The logic uses only one CMOS quad gate and works well.

Edit... or one relay even, because you don't need the on/off function.
Can I have a look at your circuit I assume you are switching the AC?

Regards
M. Gregg
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