LC voltage spike app. note

Thank you Ryan, I found it in the trigger menu - single. Worked great!

I got into a problem by following the advice of adding a capacitor (electrolytic 44microF) between the ground and VIN of a
Pololu Adjustable Step-Up/Step-Down Voltage Regulator S8V3A
to protect against LC spikes.

The voltage regulator was powered from a 9V battery connected through a relay (Normally Opened):
Meder SIL05-1A72-71L
The relay switch support up to 200V at 0.5A (20W) so I thought will be no problem.
The relay coil was connected on and off to 5V.

But after some tests (with load or not on the output of the regulator S8V3A, doesn’t make any difference) the relay switch began to hung in ‘Closed’ position.
When relay coil wasn’t powered the relay switch didn’t opened anymore, it remained closed.

After some readings on the internet I learned that capacitor can cause high inrush current from the battery through the relay switch.
It will protect the regulator ok, but the inrush current still exist between the battery and the capacitor thus affecting the relay switch.
If I remove the capacitor or put a resistor in series with the relay (100ohm) the relay switch do not hung anymore in closed position when the relay coil isn’t powered.

So in this case the capacitor maybe solve one problem (protect the regulator) but create another (damage the relay).

Hi.

It looks like what you used was a reed relay, so I suspect that rather than “creating” another problem, what you did with the capacitor exposed another problem: that relay was never going to be a reliable part for switching power. Can you confirm that it’s a reed relay, and if so, why were you using one for power switching? (I think of them as for signal switching, but I have not ever used them.)

- Jan

With a relay, you cannot multiply the voltage and current ratings together to come up with some sort of “safe amount of power”, as you did. In fact, you are not allowed to exceed either the voltage or current rating, at any value for the other parameter.

You are probably correct in that the inrush current exceeded the rated 0.5 amps, which is very damaging to the contacts. As Jan suggests, that relay was never intended to be a power switch. In general, relays are very unreliable for switching DC power.

yes, it’s a reed relay.

the power rating of the relay was specified in its datasheet, I assumed I’ll never exceed either the voltage or the current.
The consumer connected at the output of the pololu regulator will never draw more then 20mA, but I got the same situation even without any consumer connected.

Any suggestion what should I use for this power switching?
In the complete circuit the relay coil receive 5V/10mA from a BC337 when a low current is received in the transistor base.
Should I eliminate completely the relay/diode + battery and connect the pololu regulator GND to BC337 collector and pololu regulator VIN to VIN +5V?
In this case do I still need the capacitor between GND and VIN at the pololu?

Thanks,
Razvan

There are many ways to switch power by electronics, and a relay is generally a bad choice. It would help if you tell us what you want to do and specified your circuit requirements. Are you aiming for the lowest possible power consumption while the rest of the circuit idles? Does part of the circuit have to be electrically isolated from another part?

pololu regulator power an IC555 whose output enter an buzzer. I use this variable regulator so I can change the intensity of the sound.
The base of the NPN tranzistor comes from HT12D decoder which decodes the output from an RF receiver.
Whole circuit consume 4mA at idle, when the signal is received the IC555 + the buzzer will consume around 10mA.
Yes, I would like the lowest power consumption as the whole circuit is powered by 6xAA batteries.

I don’t think the IC555 and buzzer needs to be electrically isolated from the RF receiver+decoder part.

We have released some much more efficient regulator products than the S8V3A design. If the maximum output of 8V can work for you, this is probably the best part:

Pololu Adjustable Step-Up/Step-Down Voltage Regulator S7V8A

It’s much more efficient when it’s on, and you can use the shutdown pin to put it in a low-power state, making the relay unnecessary.

If you really want a relay, you could use something like the unit on this basic relay board we just released:

Pololu Basic SPDT Relay Carrier with 5VDC Relay (Assembled)

However, relays like that draw much more for just the coil current than the target current you are talking about, so it would only make sense if you need your circuit on a very small portion of the time. And if your current really is so small (10 mA max), you could put the resistor in series to limit the inrush current and use a smaller relay, though 100 ohms seems a little too high.

- Jan

thanks Jan for the response.

I know the regulator I choose have smaller efficiency compared with the other one
but I need the larger range of output voltage.

If I will use a small resistor to limit the inrush current I will not need the capacitor right?
The resistor will protect also the regulator.

Right, you wouldn’t need the extra capacitor. You should also look at your input voltage with an oscilloscope so you can see how much of a problem you have (if any) and to confirm that you have fixed it. A nice bonus with the resistor is that if your circuit is battery powered and isolated from everything else, you can use your scope across the resistor to see exactly how high your current is getting when you turn on the relay.

- Jan

Hello.

I’m wiring up stepper motors using the tiny 4988 controllers. I’ve read the article on LC voltage spikes.

I know I need an electrolytic capacitor as close to the board as possible.

The motors are Vexta PK266-02A, 2A, 1.8 Ohm. I plan to run them at 12 volts, 1-2 A per coil depending on the required torque I wind up needing.

Is a 35V 330 uF capacitor appropriate? I have some larger capacitors in the 1000uF range. I also have some 50V 33uF capacitors. I’ve got some 16V rated capacitors in the 100 uF range. (I think I am describing the units appropriately, the symbol preceeding the F looks like a Greek “mu”, I assume it’s picoFerads but I’m way out of my depth here)

Is bigger better? How big is big enough? Does the initial transient spike mean I need a capacitor capable of dealing with the maximum voltage point of the highest spike?

Should I use one of your electronic switches close to the controller board as well, then energize the overall system, and only then use your electronic switch to supply power to the stepper board, hopefully avoiding the transients issue alltogether?

Is there a combination of these things that would be best?

Thanks for any help.

All the best,
David

Hi.

The 330 uF cap should be plenty. Even the 33 uF might be enough if your power wires aren’t really long. The 16 V cap is probably on the low end for your 12 V application. Part of what will help you is that you have quite a bit of margin between your 12 V supply and the 35 V maximum of the board, so even if you get a 15 V spike, you’re still safe. If you were trying to run at 24 V or higher, I’d be more concerned.

- Jan

Hi, David.

That symbol is the greek mu and it stands for micro (10^-6), not pico (10^-12). Pico is abbreviated with a “p”, nano (10^-9) is abbreviated with an “n”. The units of capacitance are “farads”.

- Ben

“We also see that the frequency of the oscillations is just under 100 kHz, which with a capacitance of approximately
9 uF indicates that the wire inductance is a few hundred nH, which is consistent with what we would expect for the
36” leads."

Just finished this article. Awesome read. For us aspiring electronics students, can someone please go through the calculations that were used to calculate the inductance as a few hundred nH? The math is what really interests me here. Which forumale were used to arrive at this conclusion. Thank you.

Hello.

I think the main calculation was based on f = 1/(2pi*sqrt(LC)) and the secondary calculation of wire inductance based on its length was just using some table lookup or online calculator of inductance of a wire. I tried the first few wire inductance calculators I found just now, and the results I’m getting now are 5x bigger (1-2 uH instead of few hundred nH).

- Jan

Hi Everyone,
Regarding the LC voltage spike, my project is in opposite direction. I want to create a voltage spike. I attempted a a simple equivalent circuit with 9v battery cell. I simulated the circuit on multisim and the result was voltage spike with a decaying oscillation after the closing the switch. However, when I build the circuit with the same values as in simulation, the resistor smoked. The resistor was capacitor, inductor, zener diode and resistor in parallel. (220uf, 1mH, zener 11V, 6.6ohm). I missed something in simulation.What it is? My goal was to use weak source of energy (4.5v, and low current in pulses) to charge battery. (about 10 ohm equivalent) That is why I created and equivalent circuit. My objective was to use that equivalent circuit (oscillator) beef up voltage passively with minimum loss of energy in process. Similar to the transformer except that transformers do not work and are dissipative. I appreciate more ideas on those LC voltage spike application notes and circuits.

If you programmed the simulation correctly, it would have oscillated for just a few milliseconds after switch closure. After that time the circuit settles into steady state in which the resistor just heats up, dissipating the battery energy.

If you want to charge a battery from a low-voltage source, you are on the right track, as either an inductor or transformer can increase the supply voltage (but not without losses due to internal resistance). However, you need to maintain the oscillations with an active device called variously a “boost converter”, “boost regulator” or “voltage step-up”. See for example pololu.com/catalog/product/799

Hello,
Thanks for reply and suggestions. I reviewed the specs of the voltage step up device and I believe it is not suitable even though it is doing what I am attempting to do. The problem is that my available current source is in 5mA at most at 1.5V or 4.5V. The source is charged up electrolytic capacitor of 100 uF.
The problem with my simulator is that it does not have ability to tell the difference between ceramic capacitor and electrolytic capacitor (ESD) and power rating for resistor.

I recommend the LTSpice schematic design and circuit simulator package, which you can download for free.

It is certainly possible to create a very simple voltage step up circuit, that will operate on very low currents and voltages (for example using a single 0.5V solar cell at 10 mA), but the efficiency will usually be poor. Basically any transistor oscillator working in the audio frequency range, connected to a small transformer, can be used to generate high voltages at low current. The “joule thief” is an example overunity.com/13175/25mv-jou … nergy-247/

Hello everyone, I need some electronics help with a problem that I think has to do with a voltage spike. I have a CFL backlight connected to the 12V rail of my HTPC and controlled by a physical switch:

The problem is, if I turn the backlight on while the PC is powered, sometimes the PC will hard reset. I suspect that powering up the backlight causes a voltage spike that will sometimes trigger short-circuit protection in the PSU.
I was told that adding a large capacitor BEFORE the switch may get rid of the voltage spike when the backlight is turned on. From what I understand it should be wired like this:

However, the spike still occurs sometimes. Does anyone here know if this is the right approach? The capacitor is not connected right next to the switch but a short distance away with jumper wires. Might that be the problem? Do I just need a bigger capacitor? Any help is appreciated, thanks!