Dealing with excessive current draw

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dkpiper

New Member
I am building a device containing a neon sign transformer, rectifiers, and a .1uf oil-filled capacitor. A pulse generator powered by a 120v to 24v power supply module drives reed switches to turn on/off the high voltage. The issue arises when the pulse generator closes the reed switches. The current draw from the neon transformer is so high that it causes the 24v supply to drop significantly, resetting the pulse generator.

I'm seeking a solution to maintain the 24v supply voltage. I'm using a 24v LED supply, although I can substitute a different supply or other components such as capacitors as recommended.

Thanks for the advice!

Dave
 
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1) Do you know what the startup current demand is for the reed switches ?

2) What is the rating of the 24 V DC supply ?

3) How much does V drop ?

4) If reed switch transient is causing 24V to droop, a bulk capacitor on the
load side of the 24 V supply may hold up the V during transient. You can calc
its size as follows :

Q = C x V

I = C x dV /dT or

C = ( I x dT ) / dV dT in secs, length of transient, dV how much you will
allow V to droop during dT, I current needed during transient (amps)

C will be in farads.


Regards, Dana.
 
Last edited:
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Hey Dana, thanks for your suggestions!
The 24v power supply is rated at 15w. The 24v supply only powers the pulse generator.
The reed switches are driven by a separate 5v power supply.
By the way, the device HV output is connected to two probes inserted into damp soil. The soil resistance is around 50 to 80 ohms between the probes, so the current draw from the neon transformer is significant. There is a 50Kohm ballast resister in line with the HV output.

I'll try the capacitor.

Thanks again!

Dave
 
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I work on a small industrial estate, in unit 3, and we've recently taken an extra unit (unit 6), and we have a small desk top spot welder (that plugs in a 13A socket) for making up battery packs. When we use it, the lights flicker in unit 3 - and now we also have unit 6, we noticed the lights also flicker in unit 6 when we use it

So I suspect it's likely that you have the same issue, with the mains fluctuating - as danadak suggests, adding extra capacitance on the output of the power supply should help to prevent it resetting.
 
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If rated for 15W and transient load is say 20W in 1 ms the extra 5mJ at 24V would be E=CV^2/2=5 then C~ 17 mF which in low ESR will cost more than a 65W universal laptop supply.
 
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Adding capacitors in parallel to the existing .1uF oil-filled capacitor can help stabilize the voltage by providing additional energy storage. Experiment with different capacitance values to find the optimal combination.
 
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