dc coil surge supression 11

[electricpete]

I was asked to provide a “quick” (i.e. not my real job…helping someone else out) general recommendation for dc coil suppression for relay coils powered from 125vdc. . There are about 30 different coils in the cabinet, all powered by 125VDC and all drawing about 0.25A or less. Various problems have been experienced that are believed attributable to voltage spikes from coil switching. Response time is not critical in this application… 1 sec delay would not hurt anything. It is important for relays to change state reliably and even more important not to short out the dc power supply if the surge suppression device fails.

I did a quick search and it has been discussed many times on eng-tips.
Also I found:

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3264.pdf

http://relays.tycoelectronics.com/kilovac/appnotes/fig48.asp

http://www.sprecherschuh.com/library/techdocs/get/TECH_Surge_Suppression_109.pdf

I’ve read through the above and formed my own conclusions, submitted for your comments.

I think the 2 most common discussed options are:
1 – flyback diode
2 – varistor.

The first link especially seems to push the option of varistors. They highlight a concern that flyback diode can make the coil be so sluggish that it might not even operate. Also apparently when it operates slowly, it’s output contacts can be degraded. I have to admit I have not heard much about these concerns before (other than time response).

I really don’t like varistors in this application. I think every time the coil switches open there can be fairly high current at high breakdown voltage drop and these things degrade. Sure there is a rating, but they use a little life every time they cycle. If they ever short circuit, life is not good.

So I like the flyback diode. Diodes can short, but then again I have an easy solution: put two diodes in series…. Makes me feel a lot better. Either diode can fail short and not a problem. Also I’m thinking I would put a resistance about equal to the coil resistance (R = 125V/0.25A = 500ohms) in series. That should tend to minimize concerns about effect of slow field collapse upon the relay discussed in the first link. I picked 1 times coil resistance since when the full coil current switches into flyback loop the voltage is limited to the original 25VDC voltage (if I had double the resistance I could have double the voltage).

In summary I am thinking about two reverse-biased diodes (during normal operation) and a resistor 1x coil resistance... all connected directly in parallel with the relay.

What do you think? Am I grossly overlooking anything?

By the way, any tips for diode selection?

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(2B)+(2B)' ?

 

[electricpete]

Attached is corrected version of the previous spreadsheet, with new comparison graphs.
(I tweaked the inductance higher to compensate for change in permeability, and resulting graphs look roughly the same as before).


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(2B)+(2B)' ?

 

[electricpete]

I have posted one more correction to my spreadsheet (attached)

The reason: There was actually a typographical error in Equation B of the textbook that I referenced and cut/pasted into my tab “model” (a textbook typo, not my typo, although I should have noticed it)
The induced voltage terms of Equation B can be derived as follows:
Lambda = 2*w*d*mu0*N^2*i/(g+x) (equation for ideal gapped inductor with gap g+x, area 2*w*d, and turns N)
L0 = 2*w*d*mu0*N^2/g (definition of L0)
Lambda = (L0 * i) /(1 + x/g) (combines two previous equations)
dLambda / dt = dLambda/di * di/dt + dLambda/dx * dx/dt (chain rule for differentiation, considering that x and i are both functions of t)
v = dLambda / dt = L0 /(1+x/g) * di/dt + L0*i / (g*(1+x/g)^2) * dx/dt
The factor of i above was not included in textbook equation B and therefore I omitted it from my previous spreadsheet…..
But I have now added it into my spreadsheet (bottom of model tab, and slope routine), and updated the comparison tab with new plots.

The general result of the change is that the hump seems smaller in proportion to the decay portion (compared to previous simulations using incorrect model). Although the parameter values used are still SWAG’d.


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(2B)+(2B)' ?