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Old 19th September 2012, 12:07 PM   #1
akis is offline akis  United Kingdom
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Default Relay driver with holding current

Hello

I am looking for a clever/simple/few parts technique of activating a relay and then dropping off the current to a "holding" level to minimise the energy spent on the coil.

I am working with a 12V relay that activates on 7.5V and deactivatres on 3V.

I want to start it off at 12V, hold it there say for 0.5 seconds and then gradually drop it to 5V-8V which will be the "holding" state. If I switch it off it should clear itself immediately and forget it was in a holding state before.

I have experimented with adding an RC controlled bypass transistor but am not getting anywhere. Simple schematic attached. The idea is that you press the switch, the capacitor is short circuit and Q2 shorts the series resistance allowing full current to pass through. Once the cap is charged Q2 switches off and then the current though the coil has to go through an extra resistor. But when you try to switch off the relay and back on this circuit fails.


Thanks
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Old 19th September 2012, 01:33 PM   #2
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A capacitor appears as a short at power-on, so you could try a completely passive approach by connecting the resistor and capacitor in parallel and putting them in series with the relay coil. When switched, the cap will pass current until the resistive DC voltage divider takes over to set the holding current.
When switched off, the cap discharges through the resistor. If you need a faster reset, use a DPDT switch the quickly discharge the cap. You'll probably need something bigger than 10uF for an acceptable delay.
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Last edited by sofaspud; 19th September 2012 at 01:37 PM. Reason: typo
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Old 19th September 2012, 05:08 PM   #3
akis is offline akis  United Kingdom
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I tried exactly that approach to start with and needed a very big cap (coupled with a 300ohm resistor , or even less for some relays), so then I thought that I should "multiply" the capacitor by using the transistor, so that I can set an RC independent of the coil resistance. This is great to start with, and you can adjust the RC to suit, but I have not yet found a way to discharge the capacitor quickly. The schematic is very simplified, the control signal will be the output of an op-amp, which will be "high" for "relay on" and "low" for "relay off".
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Old 19th September 2012, 05:16 PM   #4
Mooly is offline Mooly  United Kingdom
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How quick does the repetition rate have to be ?
You could use a series resistor calculated to give say 4 volts across the relay coil. The coil is switched at the "earthy" end. An electroylitic cap is connected from the top of the coil to ground. When off the cap charges to 12 volts and this voltage is available to "kick" the relay smartly closed. Its a technique that works well.
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Old 19th September 2012, 05:35 PM   #5
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Hi,
Attached it is a schematic of a circuit that allow you to do what are you looking for. You can doing it using a voltage regulator as the LM317 with 2 resistors. What you do it is using two resistor to adjust the voltage output one for 5 volt and the second one for the 12 volt. To energize the relay you set the voltage to 12 volt and to drop to 5 volt you short the R3 to drop the voltage to 5 volt. You can use an LM555 to set the delay or use a resistor and cap for the delay. Sorry but I am not technical writer. Hoped you understand the principle.
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Old 19th September 2012, 07:59 PM   #6
WSJ is offline WSJ  United States
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A LM555 can drive a relay coil with 100 mA, use FET or bipolar if more current is needed. Use a power up RC delay or a push button to start with the PW at 100%, then set the pulse width to hold the relay on.

Here are some PWM relay drivers.
http://www.ichaus.de/upload/pdf/Je_f1e.pdf
http://www.ti.com/lit/ds/symlink/drv120.pdf

Replace the motor with the relay.
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Old 19th September 2012, 10:26 PM   #7
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You haven't given any performance reason for the long delay, and "very big cap" is relative, especially considering a half dozen other parts. If I were using an op amp control signal, I'd probably look to pair it with a solid-state relay. Or perhaps you could use a "logic-activated" FET, similar to that used in the power management section of the O2 HPA.
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Old 19th September 2012, 10:50 PM   #8
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A latching relay would serve your goal . Only takes energy on activating or deactivating .

Cheers , Rens
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Old 19th September 2012, 11:31 PM   #9
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This is a good idea I had not thought of.

I have a (battery) circuit that switches on a relay after a delay, using a PIC micro. I have a spare pin on the PIC. It's a 5V relay with a low enough current to run off a PIC pin.

I can turn on the relay with one pin, then hold it on with a second pin, dropping some voltage using a diode or two, maybe a LED, then turn off the first pin. This will save a bit of current.

A latching relay is no good to me because the relay must drop out when the PIC is powered off.

Thanks. I mean, thanks a lot.
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Old 19th September 2012, 11:53 PM   #10
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It's worth knowing that if you have a freewheeling diode across the relay (protecting a transistor or other driver) then it delays the relay dropout by some milliseconds. If you want the relay to drop out as quickly as possible, then include a resistor in series with the diode, of about the same value as the DCR of the relay coil. This will dissipate the energy in the coil quicker than the diode alone.
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