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Monday, December 30, 2013

Slow Start



Not everything in life needs fast start! Some things in life are meant to have a slow start, like the tube filament! The inrush current surge is one of tube killers! 

This solution mentioned here are for DC filament only where one is using the very common 3-pin regulator, LM317 and the likes, to power the tube filament. 
LM317HV regulator
 
Picture above shows a normal / fast start LM317 regulator. As the reference voltage is available almost instantly, the output is programmed to the preset (R1/R2) value swiftly.  
LM117 with slow start
In this slow start configuration above, the inclusion of an extra transistor (2N2905), R3 and C1 makes it sure that the output of the circuit turns ON gradually after an input supply is applied. The output switch on ramp-up will depend on the value of R3 and C1. Increasing the values will produce higher time delays and vice versa. 

Almost all small transistors will work in this circuit. BC327 (pnp) was tried for LM317 and BC337 (npn) for LM337. For LM337, use npn transistor and turn all the electrolytic, instead of pnp transistor for LM317. 

What if I bought readily made modules without slow-start? Shown below is a standard module sold by some vendors online. It has to be modified slightly to have the slow-start function. 

R3 of 50k and pnp + npn transistors need to be added. D7, D8, C5 and C6 pins connecting to ADJ pins have to be disconnected and re-connect to gate pin of transistor. R3 should be added as per the original slow-start schematic. 

It’s not difficult to modify but the ROI is very high.
LM317 regulator without slow start

LM317 regulator modified with slow start

J&K Audio Design
30/12/2013

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* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.

* Our product lines are always improving and increasing. If you do not see what you want, contact us!

* Please email for volume discounts, distributor and OEM pricing.

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