RC Series Circuit
RC Series Circuit
(OP)
Now this should be elementary, but I have an operational question regarding a series RC circuit. I have a small 12V Potter/Brumfield relay that I would like to make slow release by about 5 seconds. I figured I could use a simple RC circuit across the coils. I chose a 1Mb resistor and a 5uF capacitor (electrolytic), and am using a simple bench power supply for the 12 volts. However, when I denergized the coil it immediately drops out. I replaced the cap with a 10uF and a 100uF with no difference.
Am I missing something here? Is it because I am using an electrolytic?
Am I missing something here? Is it because I am using an electrolytic?
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James W. Hudgins, III
Union Switch & Signal





RE: RC Series Circuit
RE: RC Series Circuit
Dan - Owner

http://www.Hi-TecDesigns.com
RE: RC Series Circuit
Ed
RE: RC Series Circuit
0.2A*5s/12V = 83 mF, that's MILLI, not MICRO. And since you can't actually allow the voltage to drop to zero since there's resistance in the relay, you actually need something more like 830 mF, so a 1F capacitor might do the job pretty well.
TTFN
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RE: RC Series Circuit
RE: RC Series Circuit
RE: RC Series Circuit
Relay: http:
Potter/Brumfield KHAU-17D12-12
12Vdc, 160ohm, 0.9W
(no diodes)
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Addressing IRStuff:
Question...Since "holding current" is not listed in the spec. sheet, is it safe to assume that the holding current can be estimated at 20-50% of pick-up current? For now I will assume 50% for now.
Calculating pick-up current at 75mA with an estimated holding current at 38ma (~50%). Therefore, using your formula for capacitance (0.038A*5s/12V = 15 mF)
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Addressing OperaHouse:
Ok, so using my 160ohm coil as my R value and a desired time constant of 5 seconds, I get the following result: 5s/160ohm = 31.25mF (not micro). So using three 10000uF caps in parallel in conjunction with my 15ohm resistor would get it close. Am I following you right?
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Keep in mind that I don't need it to slop pick, just slow drop. I really appreciate all the feedback. I will play with this tonight and post all results.
-j
RE: RC Series Circuit
If you have power available use a FET or a 339 op amp to drive the relay and then you can get the performance you want without a monster cap.
RE: RC Series Circuit
Thanks again for your input.
RE: RC Series Circuit
See macgyvers2000 (Electrical) first response. The 1 megohm resistor is the same as a circuit interruption. Use a much smaller current limiting resistor in series with the capacitor. A few ohms are adequate for 12 V coils.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: RC Series Circuit
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It's worth pointing out the obvious:
If the power-off transition completely removes all power sources from the circuit, then an active circuit is going to do nothing in terms of holding the coil on for an additional five seconds in the absence of power.
If you want to coil to remain on for five seconds, then you need to supply current to the coil for those five seconds. A huge value capacitor, a battery, an alternate power supply, a flywheel motor-generator, a trained mouse...
RE: RC Series Circuit
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: RC Series Circuit
My example was simply to show that your initial capacitor choices were off by several orders of magnitude.
TTFN
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RE: RC Series Circuit
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: RC Series Circuit
I wonder what would happen if the drop out voltage of a particular relay was higher than 2.34 volts?
RE: RC Series Circuit
Typical drop out voltages for relays with DC coils is in the 10 - 25 % of rated voltage. So any precision cannot be had with a simple parallel capacitor. Also, the tolerance of an electrolytic capacitor can be from -50 % to +100 %. If all possible variations like drop out voltage, capacitance tolerance and vibration are taken into account, you will see variations in drop out time that are really 'huge'
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...