Hobbs switch needs a capacitor? - Third Generation F-Body Message Boards

esc · 08-07-2009, 11:21 AM

I have a Hobbs switch hooked up to my fuel system. The switch is normally open and closes when the fuel pressure meets/exceeds the set point.

That switch is hooked up to a relay which, when switched off (Hobbs open=low fuel pressure) turns ON a warning light.

The Hobbs switch begins cycling on and off very quickly, several PSI before the set point. I suspect it is reacting to pressure pulses in the fuel system as it does it worst just off idle and not at full throttle. This causes the relay to start buzzing like a beehive and the warning light will flash at inappropriate times.

I added a 20V 150 UF capacitor across the terminals of the Hobs switch. I was careful to check the polarity and hook it up in the right direction.

Initial testing shows that this completely fixed the problem.

It now exhibits a hysteresis of about 1 PSI.

I've never played with capacitors much and am wondering...

Is the proper way to use one?

Am I in any danger of blowing it up (the polarity is correct)?

Would a higher capacity capacitor (As I suspect) yield a larger hysteresis? Not that I need it, just curious.

Thanks,

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BigBadLou · 08-07-2009, 12:19 PM

Quote:

Originally Posted by esc

Is the proper way to use one?

Am I in any danger of blowing it up (the polarity is correct)?

Would a higher capacity capacitor (As I suspect) yield a larger hysteresis? Not that I need it, just curious.

Quick answers:
Yes
No
Yes

Hope this helps.
Lou

esc · 08-08-2009, 12:21 AM

Thanks!

So far it seems to have made a HUGE improvement. I'll test it more this weekend.

Without the capacitor, the Hobbs switch was useless. I'm surprised that a search didn't turn up anything else on the subject.