Relay Question

Hello everyone!

1.) I am looking to use a relay that will switch between using two different power sources for my system and that can handle up to 20A with the voltage of 10.5-30 VDC.

The input voltage that I have available to active the relay can be in the range from 10.5 VDC to 30 VDC. (Anywhere in this range)

Most of the relays that I am coming across have a voltage rating of either 12V or 24V. The problem is that for my application I need to be able to handle a large range of input voltages.

2.) I found a part that I like here but the problem I am having is the large operating voltages that I need for my application. If I am feeding in 30V to turn the relay on that is rated for 12V I can see a problem happening.

3.) Here is the 12V part that I am looking at as an example:

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4.) Are there any cleaver way to make this work? Or a different part you can recommend?

Thank you so much!

-Robert

Reply to
robertxmillerusa
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12v relays work fine on 10v. Add a basic constant current circuit and you're done.

NT

Reply to
tabbypurr

Assuming a 12 volt relay will work with a 10 volt input , how about using a 7812 three terminal regulator to keep the activating voltage to 12 volts max.

If that will not work, then use a 6 volt relay and a 7806 regulator.

Dan

Reply to
dcaster

It is possible to use discrete FET devices to do this. That is what's in a solid state relay. I was going to suggest a solid state relay because they work over a wider range of input voltage, but for your specs they are a lot pricier than a coil relay. Rolling your own wouldn't be so expensive and would save the power waste of a relay.

The relay you selected pulls in at any voltage above 7 volts. You can use the LM7812 voltage regulator to drop your input voltage to provide no more than 12 to the relay. It should pass through the input voltage to the output with little drop when it is not in regulation (Vin < 13 volts). Be sure to use a diode across the relay coil to absorb the inductive kick back spike when it turns off.

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Rick
Reply to
rickman

I was thinking along those lines... but how much does a 7812 drop when driven by

10.5 volts? .. 7809? At 30V and ~100mA it will need a heat sink.

George H.

Reply to
George Herold

Why not PWM? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Ouu, Ouu... one of those three terminal SMPS voltage regualtors. My Cui ~7815 drops about 1.5 volts when stimulated with less than the needed input. But (depending on your circuit) you could run it upside down.. -15 (or -12V) output and that works down to ~7 volts in or so. The coil shouldn't care which direction the current goes. And no heat sink!

George H.

Reply to
George Herold

As in...

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

OK.. or one of these,

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upside down. Maximum of only 32 volts which is close to the edge... (I think I ran them up higher than that while testing...)

George H.

Reply to
George Herold

That has a problem with minimum voltage in. The 12 volt version is only specified down to 15 volts in. At the low end of 10 volts input the regulator might not even function. The 7 volt pick-up voltage only leaves 3 volts head room which is just not enough to use any of these switchers according to their data sheets. Maybe they could work, maybe not.

The 6 volt switcher could work with a 6 volt relay. I see one rated for

20A but the voltage is not clear. Most seem to be rated for 12 volt operation. I'm not familiar enough with relay specs to know how they would stand up to switching 30 volts. Maybe Mouser has more units to choose from. Digikey only has 3 types in stock.

I do *not* like the way Digikey has put the search parameters in a separately scrolling window within the page. I keep trying to scroll the main window and I just see white space when I'm looking for more search parameters!

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Rick
Reply to
rickman

No, You run it upside down so V out is -12V wrt ground. (ground the Vout pin and take the voltage from the ground pin.) And the 15 volt one I have here runs "upside down" to ~5 V in.. I think the spec is 7V. You can't get as much current out when it's upside down....

(It might be a silly idea.) George H.

Reply to
George Herold

First, I'm not getting your circuit. You are suggesting that the input voltage be applied across the regulator input and output and ground will be generated at -15 volts? Ok, if you say so.

Second, you would be depending on unspecified operation... I think. Can you get CUI to say this is a valid way of running it? Or is this a perfectly reasonable way of looking at the circuit that I'm just not familiar with?

What's wrong with using a 6 volt relay and a 6 volt converter in the way it is specified?

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Rick
Reply to
rickman

Cute. Sort of an absolute-value buck-boost converter. The relay doesn't care about polarity.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Actually if I understand your circuit, it would exceed the max Vin rating on the device. 30 volts from in to out and 12 volts from out to ground is 42 volts from in to ground, or 10 more than rating.

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Rick
Reply to
rickman

Hmm.. yeah I think you are right... Here's a spec sheet for a similar CUI product, but with more detail when running it negative.

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I ran a 15 V one from +24 and got -15 out.. (no smoke)

George H.

Reply to
George Herold

Yeah, I see the info in both data sheets on the negative output. I wouldn't have expect that, but I suppose it works ok. The 7805 data sheet actually lists max Vin for -12 volts out and it is just 20 volts. Other than that, this would be a good circuit.

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Rick
Reply to
rickman

It's kinda good, actually; at low currents, you can check it with an 'octopus tester' rig, just put a load resistor from output to GND, and treat the input/GND terminals like a diode. The (low-current) curve looks resistor-like until it gets past 12V, then it's flat (same current, independent of Vin). Different manufacturers may have different behavior (I'm recalling this from years ago, sorting through some surplus house-numbered regulators).

Oh, yeah: you'd want the HV versions for 30V input. LM341 goes to 35V. Might be better to just use resistor and zener to clamp an NPN pass transistor base, emitter to coil and collector to Vinput.

Reply to
whit3rd

sure, use a low drop out 12 volt regulator for the relay coil... most should accept up to 30 volts with no issues and most relays I've seen rated for 12 volt coil will pull in just fine at 10 volts.

Jamie

Reply to
M Philbrook

2 to 3V Digikey says 7V is enough for that relay.

that comes out about 2W, which is borderline for needing a heatsink on TO220.

Or put a 220 ohm 2W resistor parallel with the 7812

.-[220/2W]-. | | -+---7812---+- in | out | Gnd that'll burn less energy in the silicon. However a buck converter might be the best answer, shouldn't cost more than 70c.

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Reply to
Jasen Betts

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