A sort of silly circuit

Yep. But at some risk making sure you always have enough hold-in current over varying supply and temperature, etc. ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

It's easier to use one driver and PWM.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Looks like a good job for a beta-graded transistor with a series-parallel RC speedup network.

+5 | R R R *----RRRRR--------------* | | C C | |/ 0--*----C C----*---RRRRR---*-----| C C | |V --- \ A GND | GND

Which is a lot cheaper than 63 cents.

You can do something similar with a single 2N7002 plus two resistors, a diode, and a capacitor, but the hold current depends too much on the threshold voltage of the FET.

With a dual transistor, e.g. a BC817DS, you can do the following. It's a couple more parts, but more predictable than the single-transistor version, and a lot cheaper than 63 cents.

Cheers

Phil Hobbs

Version 4 SHEET 1 1092 680 WIRE 496 0 368 0 WIRE 368 16 368 0 WIRE 496 48 496 0 WIRE 96 64 48 64 WIRE 144 64 96 64 WIRE 96 80 96 64 WIRE 144 80 144 64 WIRE 368 128 368 96 WIRE 368 128 208 128 WIRE 368 144 368 128 WIRE 496 160 496 128 WIRE -16 192 -16 64 WIRE -16 192 -64 192 WIRE 224 192 -16 192 WIRE 208 224 208 128 WIRE -64 240 -64 192 WIRE 96 272 96 160 WIRE 144 272 144 144 WIRE 144 272 96 272 WIRE 96 288 96 272 WIRE 368 304 368 240 WIRE -64 336 -64 320 WIRE 96 400 96 352 WIRE 208 400 208 320 WIRE 368 400 368 384 FLAG 368 400 0 FLAG 496 160 0 FLAG -64 336 0 FLAG 96 400 0 FLAG 208 400 0 SYMBOL res 352 0 R0 SYMATTR InstName R1 SYMATTR Value 167R SYMBOL voltage 496 32 R0 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL voltage -64 224 R0 WINDOW 3 -277 153 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(0 3.3 30m 100n 100n 150m) SYMBOL npn 304 144 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL res 352 288 R0 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL npn 144 224 R0 SYMATTR InstName Q2 SYMATTR Value 2N4401 SYMBOL res 112 64 M0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL cap 48 80 M270 WINDOW 0 32 32 VTop 2 WINDOW 3 41 96 VBottom 2 SYMATTR InstName C1

SYMBOL res 320 176 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 10k SYMBOL schottky 112 352 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 128 144 M180 WINDOW 0 24 64 Left 2 WINDOW 3 43 99 Left 2 SYMATTR InstName D2 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode TEXT 432 280 Left 2 !.tran 200m

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

25 cents for 20 cent microcontroller, 5 cent FET. Add features of intelligent turn on, error detection, etc

Cheers

Klaus

If you want the relay manufacturer to stand behind the design, let 'em put some break-after-make contacts in, and have those contacts (NC, of course) shunting a current limit resistor. Or, put a PTC resistor in series with the coil (gonna have to have a minimum OFF time, though, for that one).

Neither would take an extra 63 cents per unit...

You can minimize beta dependence like this...

+5 --- ---o |__ ^--o _ )|| C1 ^ )|| .---||---. |__)|| | | |/

--+--[R1]--+----| | |>. [R2] | | [R3] === | ===

(Your C1 recharge diode omitted for clarity)

Personal preference: 20% duty PWM would give a safer pull-in/hold ratio than the EDN circuit, and save more power.

Version 4 SHEET 1 1092 680 WIRE 368 0 304 0 WIRE 496 0 368 0 WIRE 368 16 368 0 WIRE 304 32 304 0 WIRE 496 48 496 0 WIRE 304 128 304 96 WIRE 368 128 368 96 WIRE 368 128 304 128 WIRE 496 160 496 128 WIRE 256 208 160 208 WIRE 160 240 160 208 WIRE 160 336 160 320 WIRE 304 336 304 224 FLAG 496 160 0 FLAG 160 336 0 FLAG 304 336 0 SYMBOL voltage 496 32 R0 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL voltage 160 224 R0 WINDOW 3 -308 150 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value PULSE(3.3 0 20m 100n 100n 40uS 50uS) SYMATTR InstName V2 SYMBOL ind 352 0 R0 SYMATTR InstName L1 SYMATTR Value 1H SYMATTR SpiceLine Rser=167 SYMBOL schottky 320 96 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value 1N5817 SYMATTR Description Diode SYMATTR Type diode SYMBOL nmos 256 128 R0 SYMATTR InstName M1 SYMATTR Value NDC7002N TEXT 432 280 Left 2 !.tran 100m TEXT -184 80 Left 2 ;PULL-IN:\n Current:29mA, dissipation:150mW\n HOLD:\n Current:5mA, dissipation:

Some example data--

+5 --- ---o |__ ^--o _ )|| ^ )|| 176 ohms |__)|| 1H ||--' ||

At the cost of power dissipation in R3 and reduced pull-in current. I still like the PIC/PWM solution.

Solenoids sometimes do that, actually by shorting out a tap on the coil.

A parallel RC, in series with the coil, will work as long as you don't flip things too often.

For energy efficiency, use PWM, and not a resistor or other dissipative device.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

For energy efficiency,don't use a relay. The crummy reduced hold current method leaves no immunity against vibration.

A 'commercial' part should (at least optionally) PWM the output after the initial delay.

--sp

--
Best regards,  
Spehro Pefhany 
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8 
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48

Predicable though- just make the holding current increase with ambient

for constant current drive.

--sp

--
Best regards,  
Spehro Pefhany 
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8 
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48

I would think shock might be more of an issue. Have you analyzed this in depth?

--sp

--
Best regards,  
Spehro Pefhany 
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8 
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48

Good point! ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

method leaves no immunity against vibration.

I wouldn't know where to begin an analysis or if it's even possible. Like m ost characterizations of relay performance, it probably has to be derived b y testing- a shake table setup maybe. You could take a wild guess that the specified holding current at worst case operating conditions is twice as mu ch as needed for the spec'd g rating and from that keep your PWM average at no less than 70%.

a

s

Yep.

Yep. It feels like there's a 555 inside this problem somewhere, waiting to come out. 20mS initial pulse, then 20kHZ at the duty-cycle of your choice. ;)

Cheers, James Arthur

Or using a latching relay. Their gain is, essentially, infinite.

Cheers, James Arthur

Relay dropout currents tend to be really low, like 10% of rated voltage. So one can easily hold in a relay at 10% of nominal coil power, open-loop PWM.

I sometimes reduce coil power to reduce contact thermocouple voltages.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Most DC relays, once seated, stick closed like glue. Which is why they have such high ratios of nominal current/hold current.

At modest current turndown ratios, like 3:1 maybe, things are perfectly safe.

AC relays are a little different, because coil current drops automatically when the armature is seated.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics