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Protecting power supply from back EMF

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StaticBuildup

Mon Dec 25 2017, 11:56PM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
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I want to use a bench power supply to power projects like induction coils and flyback drivers, but there is a problem with back EMF. Induction coils are particularly bad, the arcing on the interrupter contacts shows how much inductive kick there must be. Most bench power supplies won't power them. I use some diodes in series with the coil, which reduces the problem, but does anyone know of a better way?

2Spoons

Wed Dec 27 2017, 12:08AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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An L-C T filter will help. Make sure you use a capacitor with a low impedance.

Patrick

Wed Dec 27 2017, 12:45AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
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well first i advise you to use a cheap PS like an ATX computer supply for this that way if you kill it no biggie.

second use zeners and or TVS diodes. across the outputs, and the 2spoons idea too.

StaticBuildup

Fri Dec 29 2017, 12:20AM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
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Thank you 2Spoons and Patrick for the replies. I looked up the L-C T filter and couldn't find an explanation of how it would be used to stop back EMF. Is there a formula for working out the values of L and C?

I tried the diode across the output and it greatly reduces the output of the coil, I think this is because the energy is dissipated through the diode instead of the secondary.

Patrick

Fri Dec 29 2017, 05:49AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
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oh yeah i should have thought of that. the TVS or zener would short out the reverse high voltage from a current interupted inductor based HV output. let me think.

2Spoons

Fri Dec 29 2017, 11:17PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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The rationale for using a T filter is based on the fact that the back EMF is generally a fast pulse : in other words it contains a lot of high frequency energy. So the series L in the T acts to block the pulse (high impedance at high freq) and the C in the T acts to shunt what gets through the L. So the input of the power supply sees a greatly reduced spike as a result. Its not perfect, and never can be. As for values : generally as big as possible - with the caveat that large inductors have more interwinding capacitance which renders them ineffective at high frequencies, so too with large capacitors having a higher ESL- again rendering them less effective at high frequencies. Since I doubt you have a scope fast enough to catch the EMF pulse accurately, everything comes down to guesswork.

WizardG

Wed Jan 03 2018, 03:02AM

Registered Member #43481 Joined: Tue Mar 11 2014, 06:15PM
Location: directly above the center of the earth
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There should be very little arcing at the interruptor contacts if the condenser is the correct value. Try a larger value capacitor.

StaticBuildup

Fri Jan 05 2018, 12:09AM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
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Thank you for the replies.
Is there a special ratio for the L and C values or will any reasonable values do? I tried using an online calculator but I had to enter a cutoff frequency so I did 1kHz. I guess the capacitor has to be non-polarised because the voltage is reversed during back EMF. This limits the capacitance value as the ceramic capacitors are usually low values.

I will also try a larger value capacitor for the coils. One coil has the original Visconol capacitor from the 1950's. I measured the capacitance with a multimeter and it seems OK, but maybe it is not functioning properly during use. The output of the coil is poor and there is a continuous blue spark at the interruptor contacts. There is sometimes more spark action at the interruptor than the output. I have tried adding another capacitor in parallel, but that didn't improve it, maybe because the original is shorting. I will try to disconnect it and put a new one.

hen918

Fri Jan 05 2018, 11:42AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

The voltage shouldn't go negative. When Back EMF occurs the EMF is positive, the same as the the voltage from the PSU (just higher), it's current that flows the wrong way. There is nothing wrong with using polarised electrolytics. I would go for a capacitance of a mF or so, and quite a low inductance. maybe 47uH. This gives a cutoff frequency of 1/(2*pi*srt(0.001*0.000047)) = 734 Hz, which is nice and low.

One thing to be aware of is that switched mode power supplies have control systems that are influenced by reactive components on the output. The designer knows how much capacitance to put on the output from the maximum ripple specification, and then designs the feedback system to take the capacitance into account. Adding reactive components like capacitors and inductors to the output changes how the power supply behaves and could result in instability (unwanted changes in output voltage, including periodic fluctuations and spikes).

In a well designed power supply, the above should not be a problem, as the designer should have given a large margin between the normal operating state and the unstable state, enough so you would need to put some very strange loads on it to cause anything bad to happen.

2Spoons

Fri Jan 05 2018, 09:10PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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hen918 wrote ...

One thing to be aware of is that switched mode power supplies have control systems that are influenced by reactive components on the output.

The usual fix for any instability is to add a damping resistor - in your case a few ohms in series with the PSU output would do. Same applies for driving reactive loads with op-amps.
Actually, adding a damping resistor to the T network is probably a good idea anyway - a few ohms in series with either L or C, or a k or so in parallel with either L or C. This provides a means of dissipating the energy in the EMF pulse, and stops the T network ringing. Again, you are going to have to experiment to find optimal values - this is where a scope comes in real handy.

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