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Old 20th September 2012, 03:41 AM   #11
DUG is offline DUG  Canada
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"...thought that I should "multiply" the capacitor by using the transistor,..."

"...but I have not yet found a way to discharge the capacitor quickly..."

That would be the best idea...and put a reverse diode on the cap amplifying transistor's base-emitter junction. In the off state the cap will discharge through the diode and R3.

just to clarify, see jpg
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File Type: jpg hold-relay.JPG (16.4 KB, 51 views)
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Last edited by DUG; 20th September 2012 at 03:48 AM. Reason: added jpg
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Old 20th September 2012, 07:36 AM   #12
akis is offline akis  United Kingdom
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I have two projects where relays are used: a "slow-start" to power a big toroidal, which currently uses two relays at full power all the time (but it has worked fine for years), and a battery charger where the relay must disconnect the battery when a condition is met.

Latching relay requires a signal to switch it off, and if the power goes down I'd like the relay to disconnect too (battery charger).

I cannot use a solid state relay (but I would prefer it) because I will passing 12V/1A/2A, and have not found anything over 200-500mA (within reason).

Here's what I have come up with so far. It does use a few extra components than the basic resistor transistor relay driver: an extra two transistors, four resistors and one capacitor.

When the signal is "on" then:
Q1/R3 sets the holding current.
Q2 provides the initial energising current.
Q3 and the RC combination C1/R4/R6 provide a small delay after which Q3 shorts the base of Q2 and thus stops it from conducting. R4 provides the necessary base current to Q3.

When the signal goes "low" then R4 serves to discharge C1, so the system gets "reset" when the cap is more or less empty.

On the scope with 100ms divisions, you can see the initial energising voltage, and after about 50ms the steady holding voltage. When the signal is off, it takes about 40ms to drop to 0.

This is all done on the simulator, I have not built it yet.

I think the next step might be to try using a 555 or find the PWM relay driver chips that someone mentioned before.
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Old 20th September 2012, 08:04 AM   #13
Mooly is offline Mooly  United Kingdom
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So these two applications don't require a high repetition rate. One resistor, one cap and one transistor (the switching element) does it all.
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Old 20th September 2012, 08:06 AM   #14
AndrewT is offline AndrewT  Scotland
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emf drives a series combination of dropping resistor + relay coil + C to E of transistor switch.

If you have a 12V coil of 300r and adopt a 300r dropping resistor then in the long term with CE closed the relay coil sees half the emf.

Now add a capacitor from E to top of coil.

when emf is first applied and CE is open the capacitor charges up via r and reaches ~2/3rds of final voltage in 1 RC time period.

Close the CE switch and the capacitor partially discharges to half emf through the relay coil. This fires (triggers) the relay. and the remaining half emf holds the relay active.

I usually use 15V from a 7815 to drive a 12V relay. R= coil the relay fires on ~14V and holds on ~7.5V. Cool running, fast ON and fast OFF.

All it "costs" is one 33uF 25V electrolytic.
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Old 20th September 2012, 08:11 AM   #15
akis is offline akis  United Kingdom
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Quote:
Originally Posted by DUG View Post
"...thought that I should "multiply" the capacitor by using the transistor,..."

"...but I have not yet found a way to discharge the capacitor quickly..."

That would be the best idea...and put a reverse diode on the cap amplifying transistor's base-emitter junction. In the off state the cap will discharge through the diode and R3.

just to clarify, see jpg
Ahhh, that seems to work nicely. There is a problem reaching full energising voltage, and the drop out is very gradual rather than stepped, but it should work. Here it is.
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Old 20th September 2012, 08:36 AM   #16
akis is offline akis  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
emf drives a series combination of dropping resistor + relay coil + C to E of transistor switch.

If you have a 12V coil of 300r and adopt a 300r dropping resistor then in the long term with CE closed the relay coil sees half the emf.

Now add a capacitor from E to top of coil.

when emf is first applied and CE is open the capacitor charges up via r and reaches ~2/3rds of final voltage in 1 RC time period.

Close the CE switch and the capacitor partially discharges to half emf through the relay coil. This fires (triggers) the relay. and the remaining half emf holds the relay active.

I usually use 15V from a 7815 to drive a 12V relay. R= coil the relay fires on ~14V and holds on ~7.5V. Cool running, fast ON and fast OFF.

All it "costs" is one 33uF 25V electrolytic.
Hi,

Here is how I understand your suggestion with realistic values - looks nice, and will definitely go and try it because it is the simplest. Only thng I am not sure about is the energising "spike" which I'd much rather it were a "energising plateau". But I will build it and test it in a minute.
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Old 20th September 2012, 09:00 AM   #17
AndrewT is offline AndrewT  Scotland
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The spike you are seeing is the charge held on the capacitor.
It cannot be higher than the emf you decide to adopt.
15V if your use a 3pin 7815 or 12V or rectified Ac.

It's that current spike that pulls in the relay a little faster if emf is >coil voltage. But don't worry about it. It works.
It also works with emf = coil voltage. The cap helps meet current draw when the transistor first switches ON.

BTW,
I adopted this on the channel selector and the mute relay of the Mesmerize. The cap charges fast enough between the break of the break before make of the channel selector switch.
RC = 300ohms * 33uF = 10ms.
5*RC for >90% charging is just 50ms.

post16, r1 is too biug.
A transistor switch works by ensuring the transistor is saturated. That requires the base current to be ~ 10% of the collector current.

If you know the relay coil current then the base current target should be ~ 10% of this.
a coupe of k should be about right. You can go to as much as base current ~=20% of collector current, but not less than 5%.
Don't use hFE for this !!!!!
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Last edited by AndrewT; 20th September 2012 at 09:06 AM.
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Old 20th September 2012, 09:17 AM   #18
frank1 is offline frank1  United Kingdom
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Quote:
Originally Posted by akis View Post
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.
Wow..all these fantastic, re-inventions of the square wheel when the "round" version has been around for decades.
Have you ever heard of an "Economy resistor" invariably used with larger contactors?
Economy Resistor in DC Coils | Electrotechnik - The Website on Electrical Engineering

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Old 20th September 2012, 09:54 AM   #19
Elvee is offline Elvee  Belgium
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Here is a circuit that does the job without additional contact or transistor:
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File Type: png RelHold.png (99.5 KB, 34 views)
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Old 20th September 2012, 01:05 PM   #20
akis is offline akis  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
The spike you are seeing is the charge held on the capacitor.
It cannot be higher than the emf you decide to adopt.
15V if your use a 3pin 7815 or 12V or rectified Ac.

It's that current spike that pulls in the relay a little faster if emf is >coil voltage. But don't worry about it. It works.
It also works with emf = coil voltage. The cap helps meet current draw when the transistor first switches ON.

BTW,
I adopted this on the channel selector and the mute relay of the Mesmerize. The cap charges fast enough between the break of the break before make of the channel selector switch.
RC = 300ohms * 33uF = 10ms.
5*RC for >90% charging is just 50ms.

post16, r1 is too biug.
A transistor switch works by ensuring the transistor is saturated. That requires the base current to be ~ 10% of the collector current.

If you know the relay coil current then the base current target should be ~ 10% of this.
a coupe of k should be about right. You can go to as much as base current ~=20% of collector current, but not less than 5%.
Don't use hFE for this !!!!!
Hi

I have used a coil with R=300Ohm (approximately) and inserted a series resistor of 560R, and tried a few caps. Here are pics with 470uF and 100uF - oscilloscope probe on the coil (working off battery so not earthed). Resistor divider now feeds 1.6mA to base.

Edit: on initial power up, the relay remains in the off position because there is no time to charge up the cap and create the spike that is needed. That is a problem that needs to be solved.
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File Type: jpg IMG_20120920_111244s.jpg (291.5 KB, 19 views)
File Type: jpg IMG_20120920_111356s.jpg (316.2 KB, 12 views)

Last edited by akis; 20th September 2012 at 01:23 PM.
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