AVR-Chat

Thank you all for the replies.

I normally put a 1N4004 diode across the inductor and a BZY18 Zener diode
across the 12 volt supply with a 2Amp fuse before the Zener diode on my own
designs.

I have not designed these boards myself and have only seen a couple of the
boards not the installation, there is a diode across the driver FET on the
board but I now suspect that there may not be any across the solenoid coils.

I think that perhaps I had better go and have a look at the installation
take my Tektronix THS720 scope with me and do a few measurements.

Just done a test with a 12Volt SLA battery and a 12Volt relay coil to prove
the point, 20Volt spike across the battery with a 1N4004 diode across the
coil, 600Volt spike across the battery without the diode, looks like the
original designer may have been expecting the battery to absorb the spike!!
Or ???

 

Thanking You,

Owen.

  

 

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-----Original Message-----
From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Roy E. Burrage
Sent: Thursday, October 05, 2006 2:12 PM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] Timer Counter

 

You might want to go back and add diodes across those solenoids, Owen.  

Transients on the power line can cause you all sorts of problems that a 

3 cent diode will solve.

 

We once had a customer who had us modifying some control modules that 

they sent all over the world.  They had a problem with something letting 

all the smoke out of the power drivers and warranty repairs were killing 

us.  I went over to their place and did some system evaluation and found 

the problem to be coming from transients generated by a clutch/brake 

assembly.  With a high voltage scope probe and Tektronix 465 scope, all 

I could see of the transient was about 1,000 volts...and it was way off 

the display so I never could see what the peak values were.  These 

transients were also several microseconds in duration.  A 1N4004 across 

the coil of the clutch/brake cured the problem...and we made money from 

then on.

 

The clutch/brake was drawing less than half an amp at 24 volts DC so it 

doesn't take a lot of current or voltage to generate humongous 

transients.  All it takes is a lot of ampere turns.  Diodes should be 

physically mounted as close to the solenoids as possible.

 

 

REB

 

 

 

OWEN-A wrote:

 

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>>I am trying to modifying a PIC program to get a shift register to work

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>>properly, toggle the clear pin on the shift register to force all outputs 

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>>to

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>>"0" to prevent it from holding the data for too long and burning out an

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>>expensive solenoid.

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>>The shift register is a 74HC595 and will occasionally lock an output on 

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>>for

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>>a long enough period to destroy a solenoid.

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>>These solenoids are $350-00 each and they burn out around 40 to 50 a year 

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>>on

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>>the system.

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>>Have tried a PIC group but have not received any replies.

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>>I normally use AVR's with a C compiler the more that I see of the PIC the

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>>more I dislike them!

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>Sounds like a problem with a transient on the supply. Something like an AVX


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>TransGuard will cure the problem.

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>Leon 

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>These units are in the field on 12 volt battery supply with solar panel

>charging system (no mains supply) the 12 volt solenoid's are driven by

>VNP5N07 Fully protected power mosfets     (ST Microelectronics devices)

>switching one end of the solenoid to V- the other end of the solenoid is

>connected the battery V+ .

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>The 1N4004 protection diodes are across the mosfets "not the solenoids" (no

>diodes across the solenoids).

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>I had not noticed this before but perhaps the internal resistance of the

>batteries may be high enough that the ringing of the solenoid coil could be

>causing a spike on the supply??? 

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>Sorry for the HTML!

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>Thanking You,

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>Owen.

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>Yahoo! Groups Links

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