Voltage spikes in breaker circuit - what's the best approach to suppress them? - Page 1
Forum section: electronics / projects

author: alexthe5th
author_rank: Contributor
timestamp: May 28, 2020, 06:12:16 am
content: I've designed a circuit that intentionally trips circuit breakers. Yes, I know this is a bit bizarre so here's the background - these magnetic breakers are of a special type that are used in commercial flight simulators. They can be used to simulate the behavior of high-current breakers by allowing for a fast trip at relatively low current (<250 mA). The breakers have a built in resistance of 153 ohms, are intended to be driven from a 28V supply (so to trip them, all you need to do is short them to ground), and have two built-in diodes (one flyback, one forward-biased).This is the datasheet for the breakers.The circuit is very simple: The breaker is connected to a 5.1K resistor that's in parallel with a solid state relay for isolation, theAQY211EH. The SSR is normally off until it receives a signal from a microcontroller, which then turns on and bypasses the resistor with a short to ground, causing the breaker to trip.Here's a circuit diagram to help visualize what's going on.The circuit works well on the surface, but when I had a look at the behavior of the circuit in my oscilloscope, I came across theselarge transient voltage spikesthat occur roughly 10ms after the SSR switches on. The max and min of the spikes are roughly +14V and -14V, and occur several times -here's a more zoomed in viewat 20 us/div to show what's going on.Anyone have any idea what these spikes might be and what can I do to suppress them? This is my first time working with circuit breakers and SSRs, so I'm not well versed in their quirks.Thanks in advance.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 28, 2020, 07:08:14 am
content: Put a current sense resistor in the ground end of the SSR and view the current at the same time as the voltage - you will likely see the current increase as the SSR closes, and the current decrease as the CB contact opens. Perhaps isolate the 28V supply (ie. use a battery), and your scope (ie. use a battery powered scope) so there is no coupling to the SSR controller except for the single 0V node (ie. no ground loops or mains AC earth loops).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2020, 11:04:17 am
content: Put C or R+C across the breaker?Its resistance and speed sounds like it's basically a latching relay, i.e. it has considerable inductance. Whereas a conventional mains breaker has maybe a uH inside it, and operates magnetically in the kA range; though that plus wiring inductance stores a fair amount of energy at those currents. And, I don't know what proper aircraft breakers do, presumably something inbetween both in terms of inductance and fault current?If it's just a nuisance during testing, I'd suggest simply ignoring it. If you need clean waveforms, or it's triggering something else in the system, you may have trouble filtering it thoroughly because of common and differential mode coupling; sparking is quite pervasive and wideband. It is a lesson that your systemdefinitelywill need additional filtering in the real world, where sparking contacts are unavoidable!Tim