NTC THERMISTOR as SOFT START for TUBES

[Wavebourn]

Originally Posted by simon7000

Which is very close to optimal, as amplifiers that do this seem to last much longer with less service issues.

The relay does not see much current so it does not need to be large but should have a flash suppressor capacitor across the contacts from the turn off surge as the inrush limiter will be cold at that point, so there can actually be more voltage across the contacts at turn off.

When using a relay you can also use a resistor, but high surge current rated resistors are not common and a bit pricey.

The downside of the less expensive inrush limiter is that they do fail and sometimes with a flame, so only put them in metal chassis away from other parts.

I have a big box of 20 Ohm 12W wirewound resistors. They are old, made in USA. Still, no one failed.

[Tyimo]

Thank you to everyone!
I will use the NTC in the primary side of the transformer.
Only one question remains:
Which NTC is good for this job? (CL-60, CL-150 are on hand)
Maybe CL-70 16R/4A, CL-80 47R/3A or CL-90 120R/2A would be even better?

Tyimo

[M Gregg]

Originally Posted by Tyimo

Thank you to everyone!
I will use the NTC in the primary side of the transformer.
Only one question remains:
Which NTC is good for this job? (CL-60, CL-150 are on hand)

Tyimo

As long as they can handle the current you can experiment.

Remember you can series them. Try two CL-60s in series.

Just remember that the more volt drop across it the hotter it will get.

Also if you series them you can share the heat between them.


Regards
M. Gregg

[Tyimo]

Yes, thanks!
Theoretically how many resistance would be optimal?
For my SS power ampliefiers the CL-60 works perfect.
Greets:
Tyimo

[M Gregg]

Originally Posted by Tyimo

Yes, thanks!
Theoretically how many resistance would be optimal?
For my SS power ampliefiers the CL-60 works perfect.
Greets:
Tyimo

Again this is difficult to say,

It depends on the primary inductance / type ie torroid or E/I Tx.

I will give an example.

I built an amp with 8X EL34 when I powered up it could weld the power switch closed due to the inrush. With 10 Ohm NTC it was OK however you could still hear the inrush in the TX so I put another in series - no more problems.

I have never had to use more than 20 Ohms.
Also I used MOV across supply to stop back EMF from the TX. YMMV.

Remember to put the NTC so its not in contact with anything due to the heat!
( I use two mounted on stand off's in series in wooden chassis with no problems)

Just a thought remember my mains supply is 230V so if you are on 110v then one CL - 60 may be optimal.

Regards
M. Gregg

[M Gregg]

Just a thought,

With no inrush control...You may have to have higher fuse ratings so it does not blow at power up!

With inrush control you may be able to fuse closer to running current!

So with NTC in the mains you may be able to have closer protection for the power TX.

Regards
M. Gregg

[Tyimo]

Thanks Gregg!

greets:
Tyimo

[tim614]

Newbe here.
So how do you install the thermister I'n the primary?
One on each leg of the ac?

[M Gregg]

Originally Posted by tim614

Newbe here.
So how do you install the thermister I'n the primary?
One on each leg of the ac?

If you put it across the supply it will explode! Or blow the fuse in the AC!

The thermistor must be in series with the load. It is only a low value of resistance so you put the live to one side of it and the Tx to the other.

live cable -thermistor-Tx-neutral.

Link:

http://www.thermometrics.com/assets/images/cl.pdf

----------------------------------------------------------------------
(A MOV goes across the supply to remove transients).<<<<this is not a thermistor and correct type must be used! <<get this wrong and it will explode! Possibly in your face.

Link:

http://www.epcos.com/web/generator/W...plications.pdf


Regards
M. Gregg

[richwalters]

Originally Posted by M Gregg

Just a thought,

With no inrush control...You may have to have higher fuse ratings so it does not blow at power up!

With inrush control you may be able to fuse closer to running current!

So with NTC in the mains you may be able to have closer protection for the power TX.

Regards
M. Gregg

You'll have not much fun on a "brown out"......or serious intermittant interruption of the AC with a low thermistor hot resistance. Where I live there are alot of bad interrruptions and a thermistor wouldn't survive long.

richy