Help wanted with delay on timer mystery - Page 1
Forum section: electronics / beginners

author: SteveSi
author_rank: Contributor
timestamp: December 03, 2019, 09:44:15 pm
content: I have designed and tested a circuit which is intended to power a relay approx 1 second after power is applied and to turn off the relay as the power begins to drop.This was designed because I have some DC-DC converter boards which give nasty large spikes of 4V - 10V of 100's of mS on the output when first powered on and when powered off and the input voltage to the DC-DC converter drops to 4V-2V.i.e. ATX PSU AC-DC source - 12V output - DC-DC buck\boost converter ---- (relay contacts) -- output terminals.^ - same 12V also powers delay circuit+relayI designed the on-delay circuit in LTSpice and built it on breadboard.It works exactly as expected. I use a 5V relay so that when power is switched off and decays, the relay will quickly turn off before I get the spikes from the DC-DC board which happens when the power rail gets to around 4V-2V. The Schottky diode is there to discharge the capacitor quickly so that if the power is turned on again quickly, the full time delay will occur again. Q2 and the Zener get a bit warm but not much above 60 deg C.Now it occurred to me that I could add a SPST On/Off switch across the capacitor to provide a manual method of turning off the relay without needing to switch off the whole circuit.This seems to work Ok at exactly 12V or below, but if I increase the source voltage slightly, shorting out C1 does not turn off the relay.Once the 5V relay is on and latched, when I short out the capacitor the relay does not turn off. The relay has approx 1V across it which is enough to keep it on. The Q1e to Q2b connection also has approx 0.6-0.8V.The relay ishttp://www.farnell.com/datasheets/1870236.pdfbut as far as I can see, the problem is that the transistors are still slightly on.Any help appreciated.

author: SteveSi
author_rank: Contributor
timestamp: December 04, 2019, 12:40:18 pm
content: Well, I tried a different type of Zener (1N4735A 6.2V 1W) which has changed things..The 'first' Zener was a 1N5339Bhttp://www.farnell.com/datasheets/1936352.pdf(5.6V 5W) (I used 11.4V instead of 12V for the supply to adjust for the different knee voltage).The Q2b now drops to 60mV when the capacitor is shorted out.Not quite sure why the previous Zener caused this issue, there was approx 6.2V across it so it was acting as a Zener - maybe it has a high reverse leakage?Can anyone explain what was causing this behaviour?

author: LateLesley
author_rank: Frequent Contributor
timestamp: December 04, 2019, 01:04:28 pm
content: Right, no expert here, but I do have a theory of why it may have been unreliable. Q1 can easily turn on Q2, but if you turn off Q1, the voltage on the base of Q2 has nowhere to go really, which may hold it on. I think if you has added a 100K to Gnd, it would help Q2 turn off when Q1 turned off, and make the circuit more reliable. The theory is, even if Q1 turns off (goes open) it's still going to leak something, and hold Q2 on. If you add a resistor, it just pulls that whole part of the circuit down to Gnd when Q1 Stops conducting, and reliably turns it off.

author: SteveSi
author_rank: Contributor
timestamp: December 04, 2019, 02:55:15 pm
content: Thanks for the reply. I did try various resistors between Q2b to 0V as I had the same thought. But even that didn't work!I tried again now a 50K and still get the same issue.I have now found it is source voltage dependent. If I raise the source voltage to above 12.9V then shorting C1 out does not switch off the relay with a 6.2V Zener (even though delay on and power off functions work OK). If I change the Zener for a slightly lower value (say 5.6V Zener), then shorting C1 does not work above 12.3V source voltage.The voltage\current drops but there is just enough current flowing through the relay to keep it on and there should be no current!It's very odd!

author: w2aew
author_rank: Super Contributor
timestamp: December 04, 2019, 10:08:31 pm
content: One other thing to keep in mind is that when your 12V supply turns off, it might be a high impedance, which means that the capacitor won't discharge through the schottky diode as you expect. It may work fine as you test it on the bench while turning the supply voltage down because it will still be a "stiff" low voltage (low impedance), but may not work if the 12V is turned off or disconnected.

author: SteveSi
author_rank: Contributor
timestamp: December 04, 2019, 10:18:50 pm
content: Thanks, actually I also connected a small 1k load resistor across the supply input to help with this.I still cannot understand why this relay wont turn off though when I short out C1 - the transistors seem to be slightly on and the current through the relay is enough to stop it from releasing. But why are they on and how can I turn them off. You would think that if you short the base to ground then they would be off - or have I missed something? The Q1e voltage is 0.6V even though Q1b is 0V - how come? Is there leakage through Q1c to Q1e and is that enough to turn on Q2 slightly?I will try a MOSFET version of this circuit soon, but I'd really like to try and understand what is going on with the BJT circuit!

author: SteveSi
author_rank: Contributor
timestamp: December 05, 2019, 04:13:22 pm
content: As an experiment, I powered up the circuit and waited until the relay energised.Then I disconnected the Q2 base wire.Then I connected the Q2b wire to gnd - and the relay was still on.So only Q2c and Q2e were connected and Q2b was 0V and there was still enough current to keep the relay on!Do you think the 2N2222s are within spec? I did try another 2N2222 from a diferent vendor and it behaved in the same way.Another curious thing is that when I un-short the C1 capacitor (with a normal circuit), the relay does then de-energise and then after a short delay it energises again. Its all very strange!

author: LateLesley
author_rank: Frequent Contributor
timestamp: December 05, 2019, 04:41:46 pm
content: I've had another thought. Have you tried it with D3 out of circuit. I'm wondering if it is going into reverse breakdown, and causing some current flow, holding the transistors on??

author: SteveSi
author_rank: Contributor
timestamp: December 05, 2019, 06:24:03 pm
content: Yes, I removed D2 and D3 as I had the same thought.I have ordered some different, higher power transistors to use with Q2 and will experiment some more when they come.Meanwhile...I have now used a MOSFET and it works perfectly.