34VDC Delay Circuit Using Discrete Components?

Discussion in 'The Projects Forum' started by bullzai, Dec 22, 2016.

  1. bullzai

    Thread Starter Member

    Jan 19, 2015
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    I found the following circuit at http://darrenyates.com.au/electronics/archives/44
    My question is, how could I adapt this circuit to run at 34VDC? (More details below the image.)

    delay_circuit_example1.jpg

    The circuit needs to work with 34VDC because the power is supplied by a 24VAC transformer converted by a single diode and a 330uF capacitor. The circuit is needed to provide a short delay, maybe 1 second, before activating a 24VDC relay, then stay active until the power is cut. (The circuit should rarely activate, but I'm still a little concerned about 34V at the 24V relay coil. The good news is, it hasn't died yet!)

    Right now I have a different system set up, using a buck converter and an ATTiny85 microcontroller controlling the relay. The point of trying to use the transistor delay circuit is to keep things a little more simple and inexpensive. Ultimately I will need to make this circuit 52 times.

    One of my first thoughts is to use an 18V zener diode so the capacitor might only need to charge to less than 60% before activating Q1. The timing will also need to be adjustable. I'm guessing that adding a variable resistor before C1 would do it.

    Any ideas would be interesting to me.
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
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    r1/c1 work as a ramp.

    Q1+ZD1 as comparator that turns on at 0.7+5.6v.

    Q2 is a switch.

    as is, it works for 34v, in terms of being able to produce a delay.

    To redesign it for 34v depends on your timing + transistor parameters.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Yes that's a bit of an overload and could cause the relay coil to overheat and eventually fail. :eek:
    Suggest you put a power resistor or 10V zener (of sufficient wattage rating) in series with the coil to reduce the coil voltage to around 24V.
     
  4. bullzai

    Thread Starter Member

    Jan 19, 2015
    42
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    Ah, I did find that Q1 is rated for only 30V whilst Q2 is rated for 45V. The capacitor should maybe be 50V.

    I guess I need to study up on current flow through the transistors. I thought that increasing the source voltage might pump too much current through the transistor bases without changing some resistor values. For example, wouldn't the voltage at the base of Q1 eventually reach 34V- (0.7 + 18)=15.3V? Assuming a change to a 18V zener diode. (I've only been working with micro-controllers lately.)
     
  5. bullzai

    Thread Starter Member

    Jan 19, 2015
    42
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    The maximum rating is 110% of 24V, so, yeah! Thanks for the idea. I only recently learned that a zener diode could be used this way, but I didn't even consider using it to save the relay coil.
     
  6. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    No, the voltage at the base (with respect to GND), will be 18.6 V. That's ok because ther still is only o.6 V from the base to the emitter. The 15.3 V is across 200 K of resistance, for a base current of only 77 microamps. I recommend deleting R2. The base current still will be less than 0.2 mA, a very small value.

    1. Rule of thumb for long-term reliability: use components rated for twice the circuit requirements. In your case, 60 V transistors, 50 or 63 V capacitors, diodes and resistors running at a max of 50% of their power ratings.

    2. What is the relay coil current at 24 V? Without this, the circuit always will be in doubt.

    3. Increase the value of ZD1. Your current design has the trip point at 51.6% of Vcc. If ZD1 + 0.6 V = 63.2% of the power supply voltage, the delay will be equal to R1 x C1. This has the added benefit of decreasing the size of C1 for a comparable delay.

    4. Select R5 such that Q2's base current equals 5% of the collector current. The 10% rule of thumb for hard saturation is a carryover from the 50's when transistors sucked.

    5. Remember to add a diode across the relay coil.

    ak
     
  7. bullzai

    Thread Starter Member

    Jan 19, 2015
    42
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    Wow. I guess I could test at lower source voltages first, then see if I can blow something up at full voltage! Right now I mainly have 40V transistors to play with, but a few different 1W zeners; 3.3, 5.1, 10, and 18V.

    The relay is a Magnecraft 783XCXC, 24VDC. The datasheet shows 1.4W average consumption and 400 ohms coil resistance. Both seem to work out to about 60mA? (I wonder if the diode used across the coil is quick enough. It's a 1N4004 or similar.)
     
  8. crutschow

    Expert

    Mar 14, 2008
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    That diode is fine.
    The turn on speed of even slow power rectifiers is quite fast, and that's the main parameter of interest in suppressing the coil inductive spike.
    It's just the turn-on that's slow with such a rectifier and that doesn't hurt anything here.
     
  9. dannyf

    Well-Known Member

    Sep 13, 2015
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    that's true. the original design has the threshold voltage set to 50% of the supply (=50%*12v=6v). at 34v, it takes shorter time (due to higher charge current) for the circuit to get to 6v, but the same time for it to get to 50% of the higher supply (17v). a higher voltage zener yields longer delays.

    to change the timing, you can change the time constant of the rc network - with a modern transistor, going to 330K is no problem and likely 1-2M.

    I would put a led in serial with the zener or the collector resistor as an indicator.

    I would also put a diode (optionally in serial with a small resistor) in parallel with R1 (anti-supply) so the delay is in force if there is a short loss of supply voltage. as is now, the delay will not be armed.

    the 100K base resistor isn't needed.
     
  10. dannyf

    Well-Known Member

    Sep 13, 2015
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    the current through the zener is minimum -> 3ma max at 34v rail. So the power dissipation on the zener is minimum as well.

    40v transistors should work.

    the 24v relay can be a problem. you will need to drop some voltage over it. diodes, resistors, or power transistors or even voltage regulators.
     
  11. dannyf

    Well-Known Member

    Sep 13, 2015
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    the only rating matters for that capacitor is its leakage current.
     
  12. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Not really. To over-current the base with 200K in front of it, the source voltage would have to be at least 2000 V.

    Also, a 10 V zener with 60 mA of relay coil through it dissipates 0.6 W. A 1 W part will be warm, but fine. To help it, gap it up off of the perf board by 1/4" to let air contact it all around. Also, the longer leads act as heatsinks.

    Relay coil (and 1N4004) to GND, then the 10 V zener, then the Q2 collector.

    ak
     
    Last edited: Dec 23, 2016
  13. dannyf

    Well-Known Member

    Sep 13, 2015
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    having built the discrete version, i'm not sure if you save much vs. the attiny version. certainly not on part counts. For simple tasks, it is very difficult to beat a mcu for cost, simplicity and flexibility.
     
  14. bullzai

    Thread Starter Member

    Jan 19, 2015
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    True. The ATTiny solved the problem quickly and easily. The thing is, it also requires the buck converter...unless yet another zener diode could easily drop 27V! From there it could use a linear regulator. (My prototype is Digispark with a built-in regulator, but the production design is just the chip.) Also, it just seems like too much processing power and delicate circuitry to solve a simple problem. It does work well though!

    The way I'm using the ATTiny, it debounces a switch in code, delays the activation of the relay, then it reads a pot to determine run time. (I thought about using another switch to end the run cycle, if possible, but the timer works fine with less wiring.)
     
  15. bullzai

    Thread Starter Member

    Jan 19, 2015
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    I'm really impressed with the people here. Thanks for all the responses!

    BTW, I've seen other circuits that place the zener at the base, and maybe also use a Darlington pair. Any big advantage or disadvantage to that compared to the posted circuit?
     
  16. dannyf

    Well-Known Member

    Sep 13, 2015
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    your avr consumes a few ma when it is running and ua when in sleep. so you can use a resistor + zener (or even a led which also acts like an indicator).

    or if you want, the avr can acts as its own switching regulator, with the help of external switches + coil, etc. with the avr doing the adc + adjusting pwm duty cycle.

    high rail voltage isn't too difficult to solve with a mcu.
     
  17. bullzai

    Thread Starter Member

    Jan 19, 2015
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    Does this look correct? Assuming Q2 is saturated.

    zener_voltage_reduction.jpg
     
  18. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Yup.
     
  19. dannyf

    Well-Known Member

    Sep 13, 2015
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    the key is to calculate the power dissipation on that zener. 60ma * 10v = ...

    BTW, there is a way to save that zener, at the cost of more power dissipation on the pnp and one extra resistor - which can be done by rewiring R5.
     
  20. bullzai

    Thread Starter Member

    Jan 19, 2015
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    2
    I don't know how I managed to do this, but here is a simulation that I ran.
    Green is the capacitor voltage. Blue is Q1's collector voltage. Red is Q2's collector voltage.

    delay_circuit_simulation.jpg
     
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