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Old 18th October 2006, 03:21 PM   #1
jarthel is offline jarthel  Australia
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Default help with an ne555 delay circuit

I have the following circuit: http://www.diyaudio.com/forums/attac...amp=1156387690

the datasheet for the solid-state relay is: http://www.diyaudio.com/forums/attac...mp=1156212862. I must add that the relay that I have has no input LED.

The circuit's purpose is to delay the turn-on of the B+ supply. The DC supply used in the circuit is coming from the heater DC supply that I have. The transformer for this DC supply is connected diirectly to the IEC socket and mains switch (i.e. it is immediately
on after I flick the switch)

I thought that when the timer has reach the limit, pins 1 and 2 in the relay would short. Pins 1 and 2 is supposed to be connected to AC mains wire but it is not connected at the moment.

My problem is that nothing is happening. Pins 1 and 2 is not shortening.

some measurements:
VCC = around 10V
V47uF = reaches 8V at around 10secs and stabilizes to around 8.25V after 20 secs
- outpin pin (pin3) of the ne555 measures a constant ~17mV (with respect to ground).
- + pin (pin 3) of the relay is measuring a constant ~9.25V from ground

maybe I'm testing it incorrectly?

thanks for the help
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Old 18th October 2006, 03:30 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
have you downloaded the 555 datasheet?

Check your circuit against the schematic.
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Old 18th October 2006, 03:35 PM   #3
jarthel is offline jarthel  Australia
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Quote:
Originally posted by AndrewT
Hi,
have you downloaded the 555 datasheet?

Check your circuit against the schematic.

yes I did. looking at the fairchild's one, the circuit looks very similar to astable operation except that:

- pins 5 and 7 on my circuit is not connected
- there is no Rb in mine

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Old 18th October 2006, 04:13 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
pin 5 is a control cap that is usually only needed if false triggering happens. Most circuits show it connected to be safe.

Pin 7 is connected on EVERY circuit I have seen.

Your circuit is unusual.
You will have to debug it, by following through the hi/lows on pins 2/6, 3 and try to understand what the circuit is doing with pin 7 disconnected.

It may be right.
But I cannot tell without building it and testing it.
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Old 19th October 2006, 01:24 AM   #5
clem_o is offline clem_o  Philippines
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The circuit is correct. Pin 7 is the discharge pin, to reset the circuit - used when monstable or astable operation is needed.

In this case it (the 555) is only used to provide a switched delay. The capacitor charges via the variable resistor, and when it reaches 2/3 supply voltage, the output of the 555 (pin 3) goes low, and stays that way "Forever".

The action of the circuit as you've measured is correct. Check the voltage going INTO the relay coil itself - I note a 560 ohm current limiting resistor - you may have to reduce that value, perhaps the voltage is not enough to energize the coil for proper relay pull-in.

Better still, remove the circuit and use a variable voltage supply to check the pull-in voltage of the relay. If it's a 12V coil, usually needs at least 8V to do it - at maybe 80mA (for the clunky type relay) - based on that, the 560 ohm resistor is the culprit. If your supply is only 10V, short out the resistor altogether...

Cheers
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Old 19th October 2006, 01:50 AM   #6
jarthel is offline jarthel  Australia
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Quote:
Originally posted by clem_o


Better still, remove the circuit and use a variable voltage supply to check the pull-in voltage of the relay. If it's a 12V coil, usually needs at least 8V to do it - at maybe 80mA (for the clunky type relay) - based on that, the 560 ohm resistor is the culprit. If your supply is only 10V, short out the resistor altogether...

Cheers

I'll try that. thank you.
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Old 19th October 2006, 01:57 AM   #7
clem_o is offline clem_o  Philippines
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You can tell if it works by just listening for the 'click' of the relay...

Keep your finger on the 555, see if it gets hot while driving the relay on. Normally a 555 should be able to handle 100+mA, but I'm not sure how much your relay coil will really take in...


Cheers
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Old 19th October 2006, 02:01 AM   #8
jarthel is offline jarthel  Australia
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can I use a tantalum instead of an electrolytic for the cap?
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Old 19th October 2006, 02:14 AM   #9
clem_o is offline clem_o  Philippines
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Sure, why not...

Concerned with leakage from electrolytics?


If the pin 2 voltage rises to 8V, it means that it's ok...
(2/3 of 10 is only 6.7V)....


You can even use pin7 to pull the relay, come to think of it...
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