Maker Pro
Maker Pro

scr latchup CMOS failure?

J

John Fields

Jan 1, 1970
0
after switching off, i mean. the main symptom apart from overheating is
the 393 can't pull down the A1 and B2 inputs of the 4538 any more - it
goes down to 10.63V and no further. With the 4538 disconnected, it works
fine, switching between 0V and +12V.

---
Dunno, but...

1. What are you using for Vdd?

2. Are you using 4538's or 74HC4538's?

3. You could be frying the 4538 because you don't have Schottky diodes
across the the timing resistors (cathode to Vdd) and the solenoid is
pulling the supply down, allowing the timing caps to dicharge through
the chip's protection diodes (yes, the dreaded parasitic SCR's rear
their ugly heads...)

BTW, did you get the circuit I emailed you?
 
A

andy

Jan 1, 1970
0
I've been testing the watering circuit i built:

http://skullcap23.tripod.com/daily-water3-bw.png
http://skullcap23.tripod.com/daily-water3-bw.jpg

It was working OK with an LED in place of the coil, and a 10 uF capacitor
instead of the 1000 uF one. I've just finished building the MOSFET switch
part of the system, which works OK by itself.

I just tried testing it with the coil and the 1000uF capacitor, and now
the 4538 is not working right and getting very hot when I switch the power
on. Switching it off and leaving it doesn't fix the problem. Could this be
a case of SCR latchup failure? The only way I can see that this could be
happening is if the fully charged 1000uF capacitor is discharging through
the 4538's A1 and B2 inputs. Is this likely to be the problem?
 
A

andy

Jan 1, 1970
0
I've been testing the watering circuit i built:

http://skullcap23.tripod.com/daily-water3-bw.png
http://skullcap23.tripod.com/daily-water3-bw.jpg

It was working OK with an LED in place of the coil, and a 10 uF capacitor
instead of the 1000 uF one. I've just finished building the MOSFET switch
part of the system, which works OK by itself.

I just tried testing it with the coil and the 1000uF capacitor, and now
the 4538 is not working right and getting very hot when I switch the power
on. Switching it off and leaving it doesn't fix the problem. Could this be
a case of SCR latchup failure? The only way I can see that this could be
happening is if the fully charged 1000uF capacitor is discharging through
the 4538's A1 and B2 inputs.

after switching off, i mean. the main symptom apart from overheating is
the 393 can't pull down the A1 and B2 inputs of the 4538 any more - it
goes down to 10.63V and no further. With the 4538 disconnected, it works
fine, switching between 0V and +12V.
 
A

andy

Jan 1, 1970
0
I've been testing the watering circuit i built:

http://skullcap23.tripod.com/daily-water3-bw.png
http://skullcap23.tripod.com/daily-water3-bw.jpg

It was working OK with an LED in place of the coil, and a 10 uF capacitor
instead of the 1000 uF one. I've just finished building the MOSFET switch
part of the system, which works OK by itself.

I just tried testing it with the coil and the 1000uF capacitor, and now
the 4538 is not working right and getting very hot when I switch the power
on. Switching it off and leaving it doesn't fix the problem. Could this be
a case of SCR latchup failure? The only way I can see that this could be
happening is if the fully charged 1000uF capacitor is discharging through
the 4538's A1 and B2 inputs. Is this likely to be the problem?

thinking about it, this is unlikely to be the problem - the capacitor
would also be keeping the supply rails high while it discharged, so it
wouldn't be driving the 4538 past the rails.
 
A

andy

Jan 1, 1970
0
a 12V 1.2Ah lead acid battery.
2. Are you using 4538's or 74HC4538's?

a 4538.
3. You could be frying the 4538 because you don't have Schottky diodes
across the the timing resistors (cathode to Vdd) and the solenoid is
pulling the supply down, allowing the timing caps to dicharge through
the chip's protection diodes (yes, the dreaded parasitic SCR's rear
their ugly heads...)

That could well be it - when the battery is low, the supply gets pulled
down to around 7V. Maybe need a bigger battery.
BTW, did you get the circuit I emailed you?

Yes, thanks. It looks like it would work OK, but I'd rather stick with the
design I have now.
 
J

John Fields

Jan 1, 1970
0
a 12V 1.2Ah lead acid battery.


a 4538.


That could well be it - when the battery is low, the supply gets pulled
down to around 7V. Maybe need a bigger battery.

---
Why? if 1.2AH will fire the thing under worst case conditions, day in
and day out, and you can be sure that the PV array will always keep it
topped off, then you should be OK. Just put some diodes across the
resistors. You might also think of replacing the lead-acid (SLA?)
battery with NiCd or NiMH for the lower internal resistance under
pulse load, but you should install the diodes in either case.
---
 
A

andy

Jan 1, 1970
0

I want to build the one I worked out for myself, not just copy
another design. I'm doing this to practice/learn how to, as much as to
have a working system.

and mine has 2 chips to your 3, so nerrr. ;)
 
T

Terry Pinnell

Jan 1, 1970
0
I assume you've double-checked that your 4538 (and other) connections
are wired up exactly as intended? And that the chip is inserted with
correct polarity! Have you swapped it for a fresh 4538, with same
result? Have you used the 4538 in a basic test circuit to check it's
not already zapped?

BTW, I got this message on trying that JPG link:
====================
Notification

Too Many Users

There are more clients using this WebWasher than your license allows.
Please upgrade your license to support more users.
 
A

andy

Jan 1, 1970
0
I assume you've double-checked that your 4538 (and other) connections
are wired up exactly as intended?
Yes

And that the chip is inserted with
correct polarity!
Yes

Have you swapped it for a fresh 4538, with same
result?

No - need to get another one.
Have you used the 4538 in a basic test circuit to check it's
not already zapped?

It was working before, so don't see the need for that.
 
J

John Fields

Jan 1, 1970
0
I want to build the one I worked out for myself, not just copy
another design. I'm doing this to practice/learn how to, as much as to
have a working system.
 
A

andy

Jan 1, 1970
0
---
Dunno, but...

1. What are you using for Vdd?

2. Are you using 4538's or 74HC4538's?

3. You could be frying the 4538 because you don't have Schottky diodes
across the the timing resistors (cathode to Vdd) and the solenoid is
pulling the supply down, allowing the timing caps to dicharge through
the chip's protection diodes (yes, the dreaded parasitic SCR's rear
their ugly heads...)

Is there any reason why they have to be schottky diodes? The only ones I
can find in Maplins are meant for power rectification. (1 amp plus max
current). I'm thinking of using germanium signal diodes instead.
 
Top