Designing a 555 Timer Circuit for 3-Minute SCR-Controlled 12VDC Load Under 1A

Hello Everyone,

I am looking for an electronics engineer type person who would be willing to help/assist/contract in the design of a 555-based timing circuit to build a small 3-minute timer that would activate an SCR device to switch a small 12 VDC load (less than 1A) until the unit is powered down. This is to replace an old Amperite time delay tube.

Thanks,
Jeff (w8cq at arrl dot net)

can you supply a schematic or more information to clear up various ambiguities, such as:

* Does the term "unit" refer to the "load" or to the delay tube it self or to some third device.
* Is it the case that the delay tube is off until the "unit" is to be powered down, in which case it does one of the following:
a. Switch 12 VDC to a load for 3 minutes and then open?
b. Connect the load to ground for 3 minutes?
c. Something else I haven't thought of?
* And when the delay tube isn't active, what is it's state?

Anything you can do to clarify this would get me(us) closer to providing a solution.

Hi Steve,
Thanks for the reply. Another way of putting this is below.

I need to design a 3-minute timer. The supply voltage for the timer will be rectified 6.3 VAC from a tube filament line. Once the 3 minutes has elapsed, the 555 will trigger an SCR switch top turn on a 12 VDC line. The current load on the 12 VDC line is low (a couple relay coils and lights, probably less than 1A).

The term "unit" refers to the piece of equipment that this will be installed into. The delay circuit is off until the unit is powered up. Then the 3 minute count begins. After 3 minutes the 555 will pulse or turn on a SCR or other appropriate transistor that will act as a switch to allow 12 VDC to go to other parts of the unit. Once triggered, the SCR or transistor will stay "on" allowing the 12 VDC to pass until the unit is powered off. In the off state (main unit), the delay module and SCR will be off.

Hope this helps.

Jeff

Something like this, perhaps

Or, actually, this is a little better...

And you might need to do it like this...

And, yet another improvement...

FYI, you will probably need to tweak R2 and/or C1 to get a 3 minute interval.
And, be sure to use a CMOS part that has a max voltage higher than the VDD supply (approx 6.3VAC * 1.4 - 0.7 = 8.1V) For instance, the 74HC4060 has a max voltage of 7V whereas the ON Semi MC14060B has a max VDD of 18V.

Also, the MOSFET will have nearly no voltage drop, even at 1A, whereas an SCR would have around 1V drop when conducting. If you were counting on that voltage drop, then you may need to add one or more rectifiers (such as 1N4004) in series with the MOSFET drain.

Hi Steve,
Many thanks for the info. I think I understand what the circuit is doing. I will have to do some searching for a current part for the MC14060 and IRF9Z34 as they are out of current production. Is the function of R7 to bleed off excess voltage? I assume R5 and R3 are current limiting resistors for the devices? What is the function of D2?

Thanks again!
Jeff

They're available at Digi-Key:

* The IRF9Z34PBF: IRF9Z34PBF-ND
* The CD4060BE: 296-2060-5-ND

R7 works together with C3 to reset the CD4060 on power-up (i.e. it's a "power-on reset" circuit).

R8 discharges the capacitors after power is switched off (since the other components can't be relied on to bring the caps to zero volts--at least within a reasonable time).

D2 blocks the path from 12V through R5. Without it, the current from R5 would modify the timing in the 4060's oscillator when Q2 is off--we only want an influence on the osc circuit when Q2 turns _on_.

Also, the attached diagram is yet another variation (using the 12VC supply instead of the 6.3VAC supply -- to reduce the parts count).

FYI: if I were designing this for myself, I would use a PIC microcontroller (such as the PIC12HV609) and a high side MOSFET switch (such as the BTS4175SGA), unless...

Couple more questions:

* Is there a concern about how much current this Delay Tube substitute draws from the 6.3VAC supply? If so, what is the limit?
* Can the 12VDC supply be used, instead of the 6.3VAC supply (as shown in the diagram attached to this reply)? And, what would be the limit on current draw there (i.e. current drawn by the Delay Tube substitute circuit).
* Is there any inductance in the switched load (i.e. the load that receives 12V after a 3 minute delay)?

Opps, forgot to attach the diagram...

Also, I picked the MOSFET after a quick search, on the following criteria:
* p-channel
* Rdson between 100 and 200 mΩ
* Vgsmax > 12V
* Vdsmax > 24V
* Pd > 500mW
* Price low

There are hundreds, if not thousands of other possibilities - and in fact there are probably lower priced candidates and parts that are much smaller [i.e. SMD].

Also, the PDip versions of the 4060 appear to be at near "end of life" so if you use one, I suggest buying some backup parts. Or, a better solution would be to use the SOIC package -- but that requires a bit more soldering skill (check out Schmartboard.com).

BTW: is this a "one-off" or will this be for production. If for production, [and especially if this needs to be made smaller] then there are other parts/package considerations. Also, PCB would need to be designed. All of which I do, but it will be expensive -- private message me if that is your intent.

Hi Steve,
I do have an intent for production. I have the main part of the whole board laid out. I would definitely like to speak with you about the addition of this circuit to my board. I don't know how to PM you. Please email me and we can figure a way to talk. w8cq at arrl dot net

Thanks,
Jeff