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How to use a boost cap to kickstart a relay with low voltage

The DC voltage on many (all?) ham radio power supplies tops out at about 14VDC. So what do you do when you find the perfect relay, but it only comes in the 24VDC or 28VDC pull-in flavor? "Using Direct-Current Relays at Lower Coil Voltages" (KE0FF), in the current issue of _QEX_, talks about how to use a boost capacitor to kick start a relay using a voltage lower than the pull-in voltage, Vpi. The circuit exploits the difference between Vpi and the typically much smaller holding voltage, Vh. KE0FF's _QEX_ article builds on "How to Operate 24V relays on 12V" (KO4BB) [1]. The operation of KO4BB's circuit (shown at the link) seems simple enough. Be that as it may, allow me to "walk through" it for the sake of thoroughness. Any constructive criticism or elaboration on your part is encouraged. Next to the coil is a snubber diode. When the switch is open, the transistor's shut off and both relay poles are at a Vdd - 0.6V = V1 potential. The capacitor also charges to a V1 potential when the switch is open. When the switch is closed the transistor turns on and grounds the lower pole of the coil. The top diode turns off while the bottom diode free-wheels the cap and the top pole of the coil up to Vdd + V1. At this point a Vdd + V1 potential is impressed against the relay's coil, which causes the relay to close. The whole trick is to keep enough potential impressed against the coil long enough for Vh to come into play. At that point, as long as Vdd is greater than Vh the relay will stay closed. The cap must be sized to accommodate the timing constant from the coil's internal resistance.

Note.

  1. formatting link

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz
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Cute, thanks. (I hardly use any relays.)

GH

Reply to
George Herold

Might be better to use a switch symbol rather than the push button to avoid confusion.

Reply to
Rheilly Phoull

Yes, a switch works better. The circuit shown in the schematic's more conceptual than practical. A push button's used to it simple for the sake of discussion. The _QEX_ article uses a DPDT switch to embellish the concept. It then replaces the conceptual switch with MOSFETs in a practical circuit, which offers digital control. My mind got so preoccupied with the walk-through that it forgot to ask about the picture of Tek scope that appears near the bottom of the link. Does anyone recognize the model of the Tek scope from the picture of its display?

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz

Even if you don't have a voltage mismatch, it might be possible to use this method to reduce holding current flowing through the relay coil, and thus reduce power consumption. You could add a resistor in series with the upper diode such that there would barely be enough current flowing through the resistor and diode to maintain the relay in its holding position, and then depend on the capacitor to pull it in intially. If the relay is ON most of the time, and power consumption is material, it might be worth doing.

It's a clever circuit. Kind of a one-stage charge pump. Thanks for the heads up.

Reply to
Peabody

No need for diodes when V matches. Just put the power to the relay through a big cap & R in parallel.

When V doesn't match you can use huge voltage boost hits to get things going faster or protect against sticking mechanisms.

NT

Reply to
tabbypurr

I have a relay that has a specified must operate voltage of 9V (12V relay, so 85% of nominal)

The holding voltage is lower than the must operate, since the must operate is for pulling in the armature. When the armature is pulled in, the magneti c path is lower, and thus needs less current to have the same force on the armature to withstand vibration etc

I do not have a holding spec, but I need it and would like to deduce it fro m the must operate voltage

So I was thinking about opening up the relay, and mouting it downwards, so i could place a weight on the armature, to measure precisely the force need ed to keep the equilibrium state (no movement of the armature), both for th e pulled-in case, and also for the released state, so I can calculate the n eeded holding voltage

(and yes, I have contacted Omron, but they can sofar not present Holding vo ltage specs)

I could also measure the inductance of the coil, in both pulled-in state an d the released state, which also should give me the different in current to obtain the same magnetic field (magnetic path in pulled-in state is shorte r)

Anyone got ideas?

Cheers

Klaus

Reply to
Klaus Kragelund

SNIP

Drive it with a variable voltage PSU ???? Start with 9V to operate, then reduce the voltage until the relay drops out... then add some margin % ? Seems simple to me.

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Reply to
TTman

Your procedure gives the undisturbed value of Vh. If you lack the equipment to introduce temperature and vibration stimuli, the margin % must be estimated. The QEX author, KE0FF, believes that if the undisturbed Vh is lower than 75% of the rated coil voltage value, then the relay ought to reliably hold in the engaged position.

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

so i could place a weight on the armature, to measure precisely the force needed to keep the equilibrium state (no movement of the armature), both fo r the pulled-in case, and also for the released state, so I can calculate t he needed holding voltage

g voltage specs)

e and the released state, which also should give me the different in curren t to obtain the same magnetic field (magnetic path in pulled-in state is sh orter)

I know how to do that, but I need the robustness margin for vibration etc t o be valid

Cheers

Klaus

Reply to
Klaus Kragelund

s, so i could place a weight on the armature, to measure precisely the forc e needed to keep the equilibrium state (no movement of the armature), both for the pulled-in case, and also for the released state, so I can calculate the needed holding voltage

ing voltage specs)

ate and the released state, which also should give me the different in curr ent to obtain the same magnetic field (magnetic path in pulled-in state is shorter)

n
?

to be valid

I like the idea of hanging it upsides down. I have no idea how to guesstimate the vibration? Say 10g.? (98 m/s^2) So hang a weight that is 10x the armature weight?

George H.

Reply to
George Herold

s, so i could place a weight on the armature, to measure precisely the forc e needed to keep the equilibrium state (no movement of the armature), both for the pulled-in case, and also for the released state, so I can calculate the needed holding voltage

ing voltage specs)

ate and the released state, which also should give me the different in curr ent to obtain the same magnetic field (magnetic path in pulled-in state is shorter)

n
?

to be valid

the only way to ensure it's valid is test it, using frequency swept vibrati on. Reducing coil power is nice but it does erode that margin.

NT

Reply to
tabbypurr

I don't want to start a flame-war with everyone on this thread, but circuits like the one described are exactly why I am proud to say I never wanted to get my Amateur Radio license!

It's not the worst hack I've seen (that honor might go to two back-to-back Motorola Mitrek mobiles to make a "repeater"?), but it's in the running.

Disclaimer: I'm presently hyper-sensitive to crappy shortcut circuits. Co-worker. Don't ask. :)

Reply to
mpm

Tek TDS 210 or 220.

Reply to
Perry

Ham radio power supplies typically output ~13VDC. Many coax (microwave) relays operate only at a higher voltages. That's why this circuit comes in handy for ham radio operators. It sounds like your work might suck a little. Maybe you ought to lighten up, get your ham license, and enjoy the on-the-air party.

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

------------------------------------------------------------------------ Surplus microwave relays are relatively widely available on eBay and other surplus stores but they usually have a 24 or 28V coil.

Most of the smaller ones I have (non-latching, SMA connectors) have a

200 ohm coil, and they turn on around 14-16V and turn off around 7-9V, which makes them unuseable directly from a 12V battery.

If your transverter uses a MA/COM brick or other similar oscillator, you already need a 21V nominal source, which can be used to drive the relay(s). However, if you do not already need a higher voltage, it is unnecessary to build a voltage converter just for the relays, since it is much simpler to use the circuit below.

------------------------------------------------------------------------

Thank you,

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Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

You're a good man. Here's the designer's rational for this circuit (from the link):

------------------------------------------------------------------------ Surplus microwave relays are relatively widely available on eBay and other surplus stores but they usually have a 24 or 28V coil.

Most of the smaller ones I have (non-latching, SMA connectors) have a

200 ohm coil, and they turn on around 14-16V and turn off around 7-9V, which makes them unuseable directly from a 12V battery.

If your transverter uses a MA/COM brick or other similar oscillator, you already need a 21V nominal source, which can be used to drive the relay(s). However, if you do not already need a higher voltage, it is unnecessary to build a voltage converter just for the relays, since it is much simpler to use the circuit below.

------------------------------------------------------------------------

Thank you,

--
Don Kuenz, KB7RPU
Reply to
Don Kuenz, KB7RPU

Operate voltage is usually 75% of rated, hold voltage usually 25% both are affected by vibration, gee forces (orientation, acceleration), vibration, aging etc.

To measure just hook it up to a variable supply turn the voltage up and down and shake it etc. Perhaps connect a lamp to the relay contacts so you can see the state. Some relays have a half-on state where both contacts are open.

Once you get below its operating voltage the relay will switch in response to external mechanical impulses. (Tap on the bottom/back and it turns off, tap pn the front/top and it turns on. etc...)

if it must be reliable stick to "must operate", or arrange a feedback loop in-case it drops out...

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Reply to
Jasen Betts

Drop it from some height? 10g is like one inch onto a wooden desktop (guesstimate).

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Reply to
Jasen Betts

Even then I think there is still the possibility of variations in the springs (when new and also perhaps aging). The manufacturers must also have to allow for this in the spec. So maybe determine what the pull-in margin actually is and then use the same margin for the hold?

Seems like some other workaround would be more practical here.

There are latched relays, maybe one of those would be better for the application?

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John Devereux
Reply to
John Devereux

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