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?
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?
An externally hosted image should be here but it was not working when we last tested it.
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
these are poping up prebuilt all over the place.....seems like a good idea if it brings the voltage up slowly.
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.
speakerfritz said:
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+.
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.
Not a Triac, an SCR.
which difference with a mechanical relay, electrically speaking , is not much, but without the hassle ..
regards
which difference with a mechanical relay, electrically speaking , is not much, but without the hassle ..
regards
zigzagflux said:
surely though it only fires once? Once it's on it's on, it only goes off if the overall HT fails, or is turned off at the switch? I would not have thought that 'transients' were a problem?
billr said:
Yes it only fires once, the reason that the SCR opens, can be by two reasons.
The ripple current is too high, or the current falls beyond a certain value.
on the first case, too much ripple, (besides a poor filtering) denotes an abnormal behaviour of the output tubes, such as going into distortion, or a change in the bias.
on the second case, this happens when you turn the amp off, which permits the board make a new delay period, once it is turned back on.
No transient have been denoted, the change of current and voltage starts not form zero.
Regards
rubli said:
Not sure what hassle you imply. A dry contact in this location is highly reliable and effective. Using the same rail for the timer and the relay adds no extra complications.
boywonder said:
But you also have two contacts in series, which increases the rating to 500V minimum. This would be safe to use on a 500-0-500 V secondary, good for just about any amp except the big 850V and up B+ supplies. Furthermore, the 250V rating is based on the INTERRUPTING rating of the contact while carry a specific L/R load. In this application, we are using the contact for make only, so the 250V rating is more than safe, and you could probably get away with even more.
rubli said:
Would this not be the case in a typical capacitor input filter, where the transformer supplies pulses of current for short durations, afterwards sitting at zero current? It seems to me this SCR will need to fire 120 times each second.
zigzagflux said:
Not sure if we are mixing up the replies...
The circut with an SCR does not need any power supply, a circuit with a relay, needs a power supply.
zigzagflux said:
Dunno what you mean. This delay pcb (the one with the SCR) switches the DC supply of any vaccum tube amplifier.
Well, not specifically to turn on, but it does need a supply for its timer, and for its gate trigger. Therefore, both circuits require a power supply. The relay method just uses the same supply from the timer for the coil. Still just one supply. Complexity is the same, IMO.
Got it. I was comparing apples to apples, where the switching occurs in the CT return to the power transformer. In that location, there would be regular switching of the SCR, which wouldn't be a good thing. This location is preferred by some since it is not at an elevated voltage.
In the DC path, I suppose either in the + or - rail, an SCR could work.
Triodeguy's amps use a IRFBG30 mosfet based slow ramp
Seems simpler and better than the timer+relay.
Triodeguy's 2A3 amp (pdf)
Seems simpler and better than the timer+relay.
Triodeguy's 2A3 amp (pdf)
quadtech said:
Nice design ! slowly increasing B+ on a SE, is the only way to go, any other switching device will give a thump on the speaker when turned on.
The mosfet is also acting as a regulator, haven't heard any good comments on regulated b+, but have never tried it, Maybe in this design due the (relative) low current consumption the supply impedance is more stable ?
Hi Zigzaflux,
I am interested in your time delay for B+. Thanks for your circuit. It looks great idea for my application.
I am using bridge rectification and have no center tap that can be grounded. I have recieved all sorts of conflicting information about what to do to delay the onset of B+.
My idea was to try your circuit between the ac secondary HV outputs and the bridge, switching both ac sources for the B+.
Alternately, I have seen only one leg of the secondary ac with a 10 resistor in series being switched out, while the other leg is connected normally.
Do you have any thoughts on the proper implementation of a delaying relay with a bridge rectifier?
I'd give the FET a shot but I'm not all that good with that stuff yet.
Thanks very much.
I am interested in your time delay for B+. Thanks for your circuit. It looks great idea for my application.
I am using bridge rectification and have no center tap that can be grounded. I have recieved all sorts of conflicting information about what to do to delay the onset of B+.
My idea was to try your circuit between the ac secondary HV outputs and the bridge, switching both ac sources for the B+.
Alternately, I have seen only one leg of the secondary ac with a 10 resistor in series being switched out, while the other leg is connected normally.
Do you have any thoughts on the proper implementation of a delaying relay with a bridge rectifier?
I'd give the FET a shot but I'm not all that good with that stuff yet.
Thanks very much.
I think it would work out just fine with the relay in one leg of the AC secondary. The circuit board I have designed places the two contacts in series. If you build your own board, you are free to use them independently to switch both legs, but I don't see any benefit to doing this. The contacts should be able to handle very high voltages, since they are only serving in low milliamp 'make' duty, not being requested to 'break' or interrupt current (the more damaging process).
The resultant circuit would be one that has no B+ for the initial time delay, then quickly charges to the final B+ when the contact closes. I have not experienced any problems (such as pops) when doing this. The 10 ohm precharge resistor won't work too well; with minimal current draw on cold tubes your B+ will still rise quickly. This circuit doesn't serve well if you want to have a slow start; it's either on or off. To be honest, I wouldn't bother with any complicated circuitry to use a slow start; my primary amp is damper diodes, which are the perfect time delay + slow start.
If interested, I think I may have another bare board to use. Parts are really cheap, like $15 or less.
The resultant circuit would be one that has no B+ for the initial time delay, then quickly charges to the final B+ when the contact closes. I have not experienced any problems (such as pops) when doing this. The 10 ohm precharge resistor won't work too well; with minimal current draw on cold tubes your B+ will still rise quickly. This circuit doesn't serve well if you want to have a slow start; it's either on or off. To be honest, I wouldn't bother with any complicated circuitry to use a slow start; my primary amp is damper diodes, which are the perfect time delay + slow start.
If interested, I think I may have another bare board to use. Parts are really cheap, like $15 or less.
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