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Relay to start AT(or AFTER) psu hits 24V exactly

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mrd

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Hi,

I am looking for some ideas for a very low parts solution to this problem I have.

I need my audio in a circuit to be grounded until power hits full 24V.
If audio turns on at 23V, it doesn't work.
The dc-dc converter feeding it is pretty precise so always reads 24V to possibly 24.4V

So, I'm thinking relay is my best choice?

However, I'm not sure on the best way to achieve this.

The relay I'm looking at starts at 18V it states (not sure how precise this would be?)
If it was precise each time and doesn't change, then maybe I could use a trim pot in series before it and drop the precise
voltage before it, so it only starts when 24V hits. Is this a stupid idea? Seems very hit and miss to me but I can but hope!

I mean, maybe if that isn't a good idea, I could consider a low current delay circuit to just start say 20 sec after turn on (Usually takes psu around 6 sec to reach 24V) would be sufficient.

Some other things I am trying to achieve is very low current. (as thats the reason for this problem)
Thinking about a 24V relay with 2880 coil resistance.

Also space is at a premium. Relay I'm looking at is tyco 10mm x 6mm x 5.65mm


Thanks for any help or inspiration you can give.

Nothing in life is simple hey!





PS. The reason for this problem, I have a mic pre which is running on DC converter with a very limited psu. So my
current available is minimal. Because of inrush current, if the audio is ungrounded it causes the converter to draw
more current and as such it never reaches full power. However, if the audio is grounded and switched on (with a temporary
switch for now) after power reaches 24V it works fine.
 
The relay will not be very accurate or repeatable. Do you need relay contacts, or could power be switched on using a transistor?
 
I don't mind using a transitor instead of relay but can't visualise it. I'm not delaying the power being switched on, instead I'm 'unmuting' the audio so to speak, but only when the power is full 24V.
 
Here is a circuit which is close to what you need. I did this for a different application and needed an adjustable trip point near 13V. It wouldn't take much to adapt it to what you need...

It is also possible to run the relay so it pulls in at say 15 to 18V and then drops out when the voltage reaches 24.0V.
 

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Thanks,

That looks like it could do the job.
I'm probably asking for the impossible, but I'd love to be able to work something out with fewer parts and draws less than around 10mA. (Probably setting my sights to high!)
 
Well, to get the current lower you will have to get rid of the relay. The PNP transistor shown is off while the voltage is low, and it turns on when the voltage reaches the set point. How do you propose to shut off the audio?
 
At the moment, I'm grounding it inbetween gain stages. Then ungrounding and letting it go on its merry way when voltage is up to full.

The relay I'm looking at is 24V / 2880 (coil resistance) so should be around 8mA if I'm doing the maths right?

Just having a think now and can't seem to get it down to less than 7 parts (incl relay)
 
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At the moment, I'm grounding it inbetween gain stages. Then ungrounding and letting it go on its merry way when voltage is up to full.

The relay I'm looking at is 24V / 2880 (coil resistance) so should be around 8mA if I'm doing the maths right?

Just having a think now and can't seem to get it down to less than 7 parts (incl relay)

Look at this: I modified the original circuit for your relay and 24V. Adjustable by trimpot from ~22V to ~25V.
 

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Thanks for that. Thats great.

One thing I'm thinking however, is that maybe relying on a circuit to only come on at 24V is a little bit too picky to work reliably. (eg. I'm not sure if it works if it comes on 23.5V, 23.8V, 23.9V)

Maybe I should be doing a circuit that only starts at say 22V, but then has a 2 sec delay to turn on (when 24V has been reached.)

Or failing that, maybe just a 20sec delay circuit would work. (Long enough to clear the amount it takes to go 0V - 24V + some clearance time)

For the previous option maybe something like this with some tweaks could work. Not convinced those values I've put in would work. (Sorry for poor drawing. Just have ms paint on this computer!)

https://i49.tinypic.com/2nki17b.gif

Maybe could work better if R1 = 10k R2=3k6 C= 680uF 6V
Not sure.

I'm thinking the zener would delay the transistor (2N4401)until 22V is reached. At which point the cap and resistor delay the turn by more time.

Do you think this circuit with the right values could work? Or am I fighting a losing battle with this circuit?
 
The delay is not long enough because of the low value resistors. This circuit using a NFET provides about 5sec after the input voltage reaches ~20V. I didn't have a 20V Zener model, so I just put two 10V Zener's in series. D4 is to discharge the timing network for the next cycle. The NFET is nothing special... Cost of a Zener is higher nowadays than the LM431...
 

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Ah that looks like a winner. Graphs look exactly like what I need. Low current drawn. I'm pretty sure I can squeeze it in. Small cap (probably 6.3V rating too would do?).

Great. Thanks for the help.

Do you think I can use a TO92 part instead of TO220? Maybe a 2N5457?
 
The 2N5457 is a depletion-mode JFet. The circuit I posted needs an enhancement-mode N-channel MOSFET. There are lots of those available in tiny packages. Just make sure it has a VDS rating > 35V. Look at Digikey. They have a selection tool which should help you find one...
 
Actually thinking about it, the 2N7000 has a lower threshold voltage so will turn on a little quicker than the IRF530,
so maybe I'll increase the cap a little to 47uF or higher.
 
Hi,

Just wondering if you could have a look at another section of the power supply.

I was going to use a transistor similar to the circuit I posted earlier but in light
of the fet circuit, I feel that would work better.

Not sure if this would work. Let me explain the usage.

I have +-16V (130mA) PSU and the module I want to power is +24V (110mA)
So I have decided to use a DC-DC converter. **broken link removed**

However the in rush is causing the regs to shut off.

So I'm gonna use a constant current source. And then thought that maybe it would
be better if the dc converter started after the rails were up near full.

So, am I on right track with this circuit? Or is there a better approach.

The DC converter starts at 18V it states. So not sure it will start if current is limited to 130mA. I guess it depends how much it tries to draw on turn on. (240mA maybe in which case I think the voltage will drop to around 16V)


Any opinions/ideas welcomed!

http://i50.tinypic.com/2uetkdj.png

PS. the RC pin is a switch (when open, converter is off. When shorted it to -Vin the converter turns on (0-0.4)

(Also, not sure the best way to simulate the dc-dc converter, but when I've just added a resistor drawing around 240mA, the circuit appears to be very picky in regards to which MOSFET works correctly.)
 
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