B+ time delay circuit - your comments

[zigzagflux]

Have been thinking for a while about building a timing circuit, something more simple than what I have been using in the past. The idea is to time delay a set of dry contacts, placed in the center tap of the main secondary winding. Also wanted to keep the cost down.

Came up with the following, your comments are welcome. My thoughts are as follows:

Can be fed with either 5V or 6.3V AC, taken from a heater winding. For a ST70, for example, using SS rectification, the 5V winding is unused anyway. To use 6.3V is no problem, as the current draw from this circuit is negligible. Selection of R1/R2 will help the relay coil operate in its nominal range, while limiting crest factor on the current draw.

Timing is independent of supply voltage, so whether you use 5V or 6.3V, your delay will be the same. No need for regulation, which is sort of required with 555 based timers.

Timing can be set from 5 to 60 seconds with R3.

Automatic reset should the amp be briefly shut off, forcing a new timing cycle.

Cost w/out PCB is $12, of which $5 is for the relay, and $4 is for the timing cap. The design requires a low leakage cap for best performance, so I chose a Vishay 150D solid tantalum cap. You could go cheaper if you want, I suppose.

Small footprint, at 2.35" x 1.7", will fit almost anywhere.

C5/R8 can be removed if desired, or tweaked for the specific power transformer for quiet RF operation.

Two contacts in series should be more than adequate for make only operation. Break operation is nearly impossible.

Could offer a group buy if anyone is interested, but to test the circuit first I've ordered 6 pieces. If interested, let me know.

Thoughts?

[dueeffe]

Good job!
Very cheap.

Normally I use a vacuum tube diode.

Fabio.

[AnalogLism]

My friend made simple timer delay for me. Use 555 timer too.
- Size pcb is 46 x 53 mm.
- 7-120 S Delay time set by DIP Switch
- Use 6.3V to 12.6V AC

[speakerfritz]

these are poping up prebuilt all over the place.....seems like a good idea if it brings the voltage up slowly.

[zigzagflux]

Got the prototype done. Got the values for R1 and R2 tweaked in such that it works fine with either 5V up to 7.5V AC in. No need to adjust values for this range. Nice and small; can tuck this away just about anywhere near the power tranny.

[speakerfritz]

how about a high current Thermistor . pros is that it starts out passing low current on power up, then gradually passes full power in about 20 seconds.

[zigzagflux]

Originally posted by speakerfritz
how about a high current Thermistor . pros is that it starts out passing low current on power up, then gradually passes full power in about 20 seconds.

Depends greatly on the design and transformer. In the case of a ST70, for example, heaters and B+ come from the same transformer. Putting a thermistor in the primary will not only limit the B+, but heater voltage as well. The goal is not to limit heater power during startup, just B+.

[rubli]

take a look at

http://nervenet.info/HT_delay/

Regards

Alex

[zigzagflux]

take a look at
http://nervenet.info/HT_delay/

No question it would function properly, but putting a triac in my audio gear's B+ would not be my first choice. Good chance of switching transients.

[rubli]

Not a Triac, an SCR.

which difference with a mechanical relay, electrically speaking , is not much, but without the hassle ..

regards