scr latchup CMOS failure?

A

andy

Guest
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?

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with
HTML: 
 or [attachment] in the subject line.
 
On Tue, 10 Aug 2004 22:11:09 +0100, andy
<news4@earthsong.free-online.co.uk> wrote:

On Tue, 10 Aug 2004 21:56:38 +0100, andy wrote:


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.

Is this likely to be the problem?
Click to expand...
---
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?

--
John Fields
 
On Tue, 10 Aug 2004 21:56:38 +0100, andy wrote:

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.
Click to expand...
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.

Is this likely to be the problem?
Click to expand...
--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with
HTML: 
 or [attachment] in the subject line.
 
On Tue, 10 Aug 2004 21:56:38 +0100, andy wrote:

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?
Click to expand...
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.

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with
HTML: 
 or [attachment] in the subject line.
 
On Tue, 10 Aug 2004 15:36:42 -0500, John Fields wrote:

On Tue, 10 Aug 2004 22:11:09 +0100, andy
news4@earthsong.free-online.co.uk> wrote:

On Tue, 10 Aug 2004 21:56:38 +0100, andy wrote:


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.

Is this likely to be the problem?

---
Dunno, but...

1. What are you using for Vdd?
Click to expand...
a 12V 1.2Ah lead acid battery.

2. Are you using 4538's or 74HC4538's?
Click to expand...
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...)
Click to expand...
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?
Click to expand...
Yes, thanks. It looks like it would work OK, but I'd rather stick with the
design I have now.

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with
HTML: 
 or [attachment] in the subject line.
 
On Tue, 10 Aug 2004 23:10:34 +0100, andy
<news4@earthsong.free-online.co.uk> wrote:

On Tue, 10 Aug 2004 15:36:42 -0500, John Fields wrote:

Dunno, but...

1. What are you using for Vdd?

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.
Click to expand...
---
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.
---

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.
Click to expand...
---
NIH?^)

--
John Fields
 
On Tue, 10 Aug 2004 17:21:29 -0500, John Fields wrote:

NIH
Click to expand...
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. ;)

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with
HTML: 
 or [attachment] in the subject line.
 
andy <news4@earthsong.free-online.co.uk> wrote:

On Tue, 10 Aug 2004 21:56:38 +0100, andy wrote:


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
Click to expand...
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.


--------------------------------------------------------------------------------
generated 11/Aug/2004 10:00:39 +0100 by bncfms04 (WebWasher 4.4.1
Build 1003)
====================

--
Terry Pinnell
Hobbyist, West Sussex, UK
 
On Wed, 11 Aug 2004 10:07:40 +0100, Terry Pinnell wrote:

andy <news4@earthsong.free-online.co.uk> wrote:

On Tue, 10 Aug 2004 21:56:38 +0100, andy wrote:


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

I assume you've double-checked that your 4538 (and other) connections
are wired up exactly as intended?
Click to expand...
Yes

And that the chip is inserted with
correct polarity!
Click to expand...
Yes

Have you swapped it for a fresh 4538, with same
result?
Click to expand...
No - need to get another one.

Have you used the 4538 in a basic test circuit to check it's
not already zapped?
Click to expand...
It was working before, so don't see the need for that.



--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with
HTML: 
 or [attachment] in the subject line.
 
On Wed, 11 Aug 2004 01:23:37 +0100, andy
<news4@earthsong.free-online.co.uk> wrote:

On Tue, 10 Aug 2004 17:21:29 -0500, John Fields wrote:

NIH

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.
Click to expand...
---
Fair enough...
---


and mine has 2 chips to your 3, so nerrr. ;)
Click to expand...
---
Yours has two chips with no lockout circuitry, while mine has four
chips with lockout circuitry. Remove the lockout circuitry and my
chip count goes to one... :)

--
John Fields
 
On Tue, 10 Aug 2004 15:36:42 -0500, John Fields wrote:

On Tue, 10 Aug 2004 22:11:09 +0100, andy
news4@earthsong.free-online.co.uk> wrote:

On Tue, 10 Aug 2004 21:56:38 +0100, andy wrote:


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.

Is this likely to be the problem?

---
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...)
Click to expand...
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.


--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with
HTML: 
 or [attachment] in the subject line.
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top