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Driving the relay causes large ripples in PSU.

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rainman1

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My PSU (LNK306) has quite low ripples in its two output regulators (5V & 3.3V) - 20mV ripples.

The problem is that when the relay pulls 200mA (short pulse of 30msec), the PSU outputs large ripples and spikes that reach 1V, and it causes problem in the other ICs, like MCU.

Is the problem pulling suddently a 200mA short pulse?
Or is it because the relay suddenly changes its COM state from conducting 0A to conducting 16A.

Will placing an inductor and "protecting againts voltage drop" capacitor between the Relay Driver and the PSU will help reducing the ripples? (since the inductor will slow the current change in the PSU, while the capacitor will charge the relay driver).

Addition:
It also happen when I dont use a relay but manually connect and disconnect loads to my PCB, meaning i do the job of the relay

Thank you.
 
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Sceadwian

Banned
Are you using a diode on the relay primary to keep the inductive spike down?
 

Sceadwian

Banned
I don't know the specs on your power supply, but a 16 amp pull is gonna cause a dip in just about any power supply, so that may well be it. A capacitor for the control electroncs may help, but it dpeends on how long the dip lasts for.
 

rainman1

New Member
Thank you fellows.

Are you sure that the problem is with the power supply?
Because even when i dont need the 200mA for driving the relay, meaning when I dont use the relay for switching the loads, but my own hands, then i also have these ripples and spikes.
 

Mike_2545

Super Moderator
Thank you fellows.

Are you sure that the problem is with the power supply?
Because even when i dont need the 200mA for driving the relay, meaning when I dont use the relay for switching the loads, but my own hands, then i also have these ripples and spikes.
Say again? Your own hands?:confused:
 

Sceadwian

Banned
He means physically triggering the relay mike.

What is on the 16amp side? Is it part of the same power supply circuit as your regulator for your low power electronics? And I don't know what the specs on your power supply are. But you're suddenly throwing a 16 amp draw (maybe much much more if it's a motor) on your circuit, yeah the voltage is gonna dip =)
 

rainman1

New Member
You know, I took the plug of the 16Arms consumer and connected/unconnected it to/from my PCB which has a socket connected to it specially for consumers.
 

rainman1

New Member
He means physically triggering the relay mike.

What is on the 16amp side? Is it part of the same power supply circuit as your regulator for your low power electronics? And I don't know what the specs on your power supply are. But you're suddenly throwing a 16 amp draw (maybe much much more if it's a motor) on your circuit, yeah the voltage is gonna dip =)
Not exactly physically triggering the relay, but hold the consumer's plug and connecting it to my PCB, and then disconnecting it.
The 16A consumer is provided with the AC mains.
My PSU and the consumers are both provided with the same AC mains (230Vac).

You think that the voltage drop on the AC mains causes the ripples in the PSU?
 

Sceadwian

Banned
Depends on your PSU I have no idea what an LNK306 is, if that's a commercial power supply or an IC you have your own components are, and if so what values etc.. more info about your power supply is needed, but yeah it sounds like you're switching a motor or some other inductive load and you're getting way more than 16 amps at the start so the PSU dims.
 

rainman1

New Member
inputs about LNK306 -
Vinput = 230Vac, 50Hz.
(Half wave input - using a 1N4007 input diode).
Iinput is very low ~1mA.
around 70% efficiency.
Voutput = 7.5V, that goes to 5V regulator.
I output = 60mA normally.

The consumers I drive are regular consumer, like heat-spreader, and such.
I just connect a few in parallel to reach 16A.
 
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Sceadwian

Banned
Halfwave input??? I hope it has a lot of buffer caps. You may need to add some really big electrolytic caps to the output to keep the voltage steady on dips. You never did say what the load you were switching was, you may need to current limit it to keep the starting current down.
 

rainman1

New Member
Halfwave input??? I hope it has a lot of buffer caps. You may need to add some really big electrolytic caps to the output to keep the voltage steady on dips. You never did say what the load you were switching was, you may need to current limit it to keep the starting current down.
Yes.
What is bad with using a diode to cut off the negative half of the 230Vac input?
Does it contribute to dips or ripples?

I use 100uF & 0.1uF in the PSU output.
 
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Mike_2545

Super Moderator
If you are having that kind of problem I would try a bigger cap on the powersupply, like say 1000mf. See if that smooths things out.
 

Sceadwian

Banned
Because you're not using half of the power you have available from a full wave rectifier, and yes it would significantly contribute to ripple, and make dips more probable. Go with what Mike said, use 1000mf cap and see what happens. It may help things more if you put the big cap on the input to the regulator instead of the output.
 
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rainman1

New Member
I cant use a 1000mF because its too big a capacitor for my PCB.

I want to understand the source to the problem.
The relay is obviously not the problem, the problem (i think) comes from the fact that 16Arms wires are going through my PCB or that there is a voltage drop in the AC mains.

What do you think?
Would love to hear more potential sources to the problem.

THank you!
 

Sceadwian

Banned
The source of your problem is you're using a half wave rectified power supply with insufficient filtering and expecting no voltage sags when a 16 amp inductive load is switched on. How big is the filter cap you have before the switching regulator? This would really be simplified if you would post the schematic for your entire circuit and all it's componenets so we don't have to keep tradeing back and forth over trivial details.
 
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rainman1

New Member
Hey, I'm sorry for the delay in my response.
Here is the design that I use:


The input rectifier that cuts off the negative half of the wave is 1N4007, is it ok?

Moreover, The input capacitor is 10uF, 400V rating.
Also, the output capacitor is 100uF and I also added 0.1uF in output to get ripples to ground.

Could you also explain please why half way is not good?
What are its disadvantagous?
I really want to know about it.

I thank you!
 

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