Help with Water Pump

[alec_t]

I'm sorry, I hooked the timing part of the circuit back up

I know you're keen to get the show on the road Joe , but for initial testing we need to isolate the problem module as far as possible from external effects. We can't find the needle in the haystack if the haystack is hidden by fog

 

[KeepItSimpleStupid]

@alec
I believe Joe's 40 W lamp is a 120 V 40 W lamp, not a 24 V, 40 W lamp, but remember that the lamp's resistance is very non-linear.

 

[alec_t]

Just so. But that load should give an easily-measured voltage across the current-sense resistor and a good indication that the FET is 'on'. Without it there may be enough leakage current through the FET, when it's meant to be 'off', to light LED D8 which could be misleading. I'm concerned that, until the circuit has been adjusted, it could latch at 3A indefinitely ....which wouldn't be good for the pump. Better to use a dummy load for now. We can always simulate the effects of a higher current load by pressing the trip test switch.

 

[salty joe]

OK, power supply set at 22.5V-it's lowest setting.
One PDM-Mk10 is the circuit under test.
50K trimmer set at 25 or 26K.
R1 conected to 12V, and so is R4.
With the flat side with writing on it of Q1 facing me, I grounded the left pin, connected the middle pin to R3 and connected the right pin to D5-R4-trip test. I double checked that Q1 is a 2N3904.
The load is a 40W incandesant 120V light bulb.
R6 is all alone.
R3 is hooked back up.


The lamp does stay lit.
LED D8 stays lit.
V across trip test is 11.6.
V across R6 is 3.95.
V at D4/R7 is 11.93.

The only time I hear the alarm is when I push the trip test.
Hope these numbers make sense, thanks.

 

[KeepItSimpleStupid]

That actually looks pretty good. Can you take the lead to R1 that is connected to +12 and connect it to ground? See if the light goes off?

Schematic link: **broken link removed**

@alec
Is there an issue with the output of U1a? That part can source and sink current and it's not open drain.
i.e. Do we need to put a diode at the output of U1a to wire OR the signals? Probably U1 is dead?

 

[alec_t]

@KISS

Is there an issue with the output of U1a?

I don't think so. It can't source current, because of D4 and the base junction of Q2. Joe has confirmed that D4/R7 is at 11.93, which is what I would expect before the trip operates.

Probably U1 is dead?

Don't think so. When the trip test is pressed the alarm sounds, so U1a must then be pulling D4/R7 low as intended.

@Joe

Hope these numbers make sense, thanks.

Yes. Ok so far.
Now:-
(1) try KISS's suggestion above. Both lamp and D8 should turn off.
(2) reconnect R1 to 12V. Lamp and D8 should stay on.
(3) press and hold the trip test switch for ~ 1 sec then release it. Lamp and D8 should turn off immediately the switch is pressed and the alarm should sound after ~ 1/2 sec delay. The alarm should continue sounding for ~ 30 secs then reset itself. Lamp and D8 should then come back on.
(4) with trip switch released, measure the volts on sense resistor Rsense.

 

[alec_t]

Post deleted

 

[KeepItSimpleStupid]

@alec
OK, try again:
Suppose the control line is high and the output of U1a goes low. Can that happen? Doesn't that mean that an input of U1a is seeing 12V and the output of U1a is trying to pull it low at that time?

 

[salty joe]

When R1 is grounded, lamp and LED turn off immediatly. When R1 is removed from ground, lamp and LED turn on after about a two second delay. That is with R1 not connected to anything.

When trip test is pressed, LED and lamp go off immediatly and alarm sounds immediatly. I pressed the trip test for 1 sec., then for 2 sec., but as soon as trip test is released, alarm stops and LED and lamp go on.

I measured 0.03V across rsense. While pressing the trip test and causing the alarm to sound, I measured 0.00V across rsense.

Would this be a good time to use these giant resistors for the dummy load? Real big picture and specs about halfway down the page. I have them on hand. (Thanks for that link, KISS) **broken link removed**

 

[()blivion]

IMO, If you have a 100 Watt 120 Volt incandescent, you could/should use that instead of the 40 Watt you're using now. Or a 5~15 Watt 12 volt tail light as a dummy load. Something like these...

View attachment 67111

But what you already have will work well enough to be sure.

 

[KeepItSimpleStupid]

So, basically things works as they are supposed to except "but as soon as trip test is released, alarm stops and LED and lamp go on."

Joe:

How easy is it to do this modification in the pic? You can easily short out the diode without removing it to put it back the the circuit back the original way. Use whatever D4, D5 or D6 is.

Is u1 at all warm at any time, if you touch it with your finger?

Is it only one driver that's behaving badly?

 

[alec_t]

@KISS

Suppose the control line is high and the output of U1a goes low. Can that happen? Doesn't that mean that an input of U1a is seeing 12V and the output of U1a is trying to pull it low at that time?

Not sure what your 'control line' is? If the input (both input pins are strapped together) of U1a is hi then yes, the output of U1a is lo; but the output is not connected to the input so I (and LTSpice) don't see any conflict. U1a output connects only to D4/R7/U1b input. Are you perhaps interpreting a wire cross-over in the schematic as a junction? Putting a diode where you suggest would prevent the trip working, since U1a output would then not be able to pull D4/R7 lo as is necessary to switch the FET off and sound the alarm.
U1a is acting as a bog-standard Schmitt inverter oscillator. U1b and U1c are there simply to ensure that the oscillator can be gated (via U1d) and be in the appropriate quiescent polarity state.

 

[alec_t]

@Joe
Check you have the polarity of D4 correct.

Edit:

When R1 is removed from ground, lamp and LED turn on after about a two second delay. That is with R1 not connected to anything.

With R1 not connected to anything all sorts of weird results may arise. The 2 sec delay implies gate leakage is causing the FET to turn on. One possibility is oscillation of the FET. The left end of R1 should always be connected to something so that the gate isn't 'floating' in an indeterminate state. If you have a SPDT switch to hand you could use that to switch R1 selectively to +12V or ground while we're testing.
Re changing the dummy load, let's get the trip timer sorted first then try different loads.
Further checks:
Do you have 12V at U1 pin 14?
With the test switch held closed what is the voltage on C3?

 

[salty joe]

D4 polarity is good.
I got 11.94V at pin 14.
C3 with trip test held down showed 6.92-6.94V.

I hooked up a SPDT and the LED and lamp turn on immediatly when switched from ground to 12V.
Thanks guys!

 

[alec_t]

Ok.
More checks, with R3 switched to the 12V rail:
1) With power off, what is the Ohms value of R9 when measured in circuit (a) with the meter probes one way round and (b) with the probes reversed?
2) With power on, what is the voltage at the R9/trimpot junction (with/without the test switch held closed)?

 

[salty joe]

With the DMM set at 200K, R9 measured 2.2 both ways.

R9/trimmer junction with trip test held down is 6.61-6.91V.
R9/trimmer junction without trip test held down is 0.15V.

 

[alec_t]

Now we're getting somewhere.
R9 should be 2.2 Megohms. Your measurements suggest you have only 2.2k.

 

[salty joe]

Really good catch.
I asked about that awhile ago and thought the answer was 2meg2 was another way to say 2k2. So the good news is I installed the part that I intended to install. The better news is I did not fry everything and the 2m2 is on the way. Sorry for that misunderstanding and thank you. When the 2m2s get here, I'll replace R9 in all the drivers and try again.

 

[alec_t]

The better news is I did not fry everything

Good news indeed . With only 2k2 U1 is busy oscillating at ~14Hz, but not doing any mischief.

Edit: Just to confuse things, "2m2" is LTSpice-speak for 2.2 milliohms! Hence the 2meg2 notation

 

[salty joe]

Well, that is sorta confusing. Would you mind taking a look at this and tell me if I ordered the right part?
https://www.ebay.com/itm/50-x-Resis...876?pt=LH_DefaultDomain_0&hash=item3cb99d7d04