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Starter SERVO amperage


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My other project has a Renault UN1 transaxle, with a cable-operated gear change.    There is an interlock on reverse, that was originally operated by another cable, but I've adapted the servo from a starter motor to pull on it.    But what amperage will that draw? I presume that it will be less than a starter motor itself, which could be 300A or more when churning.   I have ruined a multimeter by trying to measure it (!) so it's more than 10A!    Any way to measure it cheaply?   I see 'clip-on' ammeters up to 75A at less than £30, but would they be enough?

Otherwise, I'll wire it like a starter motor, with a solenoid switch in the cable.

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Thanks Phil, but Ouch!    Affordable for MS Megatanker, maybe, but not for me.

I have an old dashboard ammeter that I think reads +/- 30A.    I'll try that momentarily!

JOhn

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I wouldn't have thought it would need much more than 10A John, though the initial in-rush current could be a fair bit higher (3 - 4 times maybe?)  Are you sure you haven't just blown the internal fuse in the meter?  Most have them for just this reason!

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Yes, I have, Nick!  But the fuse is labelled 315mA, which is ridiculously low, surely.     I rang my favourite supplier (Vehicle Wiring Products) and they sucked their teeth and said, Oh,Yer can't get them fer luv nor money these days, Guv.      But I've just bought one off the bay, fingers crossed.

Mike, that clamp meter very tempting!    Esp. if I do need a new multimeter!

 

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7 hours ago, JohnD said:

Mike, that clamp meter very tempting!    Esp. if I do need a new multimeter!

 

Yeah, I was going to post a link last night but my internet died. The fluke option is the expensive end, you can pick up basic models for £20-30.

Multimeter - most likely the fuse has blown. It is surprisingly difficult to actually destroy a multimeter.

 

Phil

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11 hours ago, JohnD said:

Yes, I have, Nick!  But the fuse is labelled 315mA, which is ridiculously low, surely.     I rang my favourite supplier (Vehicle Wiring Products) and they sucked their teeth and said, Oh,Yer can't get them fer luv nor money these days, Guv.      But I've just bought one off the bay, fingers crossed.

Mike, that clamp meter very tempting!    Esp. if I do need a new multimeter!

 

that sounds like the low current range fuse. If you try to measure high currents you usually have to use the high current sockets  on the meter.
Normally there is a 10A (or bigger) fuse for the high current range.

mike

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Thanks, Phil & Mike!

Yes, I carefully used the Amps sockets on my MM, but it does say "not for more than 10A".

On the fuse itself, I've opened the case, and the 315mA fuse is prominent, with no sign of another.    It is - was - a 'Gunson' MM, would there be another fuse if I delved a little deeper?

John

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Good news and ? News;     I put the 315mA fuse in the multimeter and it works again!  Alleliujah!

Also, assisted by No.1 grandson, and an old dash ammeter, we tested the current on, first a 12V bulb, and then the solenoid.   The bulb draws 1 or 2 A, the needle just jumps a bit.   The solenoid jams the needle right over to full scale deflection, labelled as 20A.     

So it draws at least that much. Is there a way of adding a resistance in series, so that the current is less than 10A, and then using the value of the resistance to calculate what the current would be without the resistance?  Perhaps also measuring the resistance of the solenoid??

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Those current transformers/Rogowski coils only work on AC and transient currents not DC.

The easiest way is to get a low value resistance and put it in series with the load. Measure the volts drop on the resistor.  

The problem is in finding something with low enough resistance - 1.5mm^2 mains cable has a resistance of 14.5milliohms/meter so if you can get 0.69 metres of that you have 10 milliOhms.   30 Amps will drop 300mV on the wire. 

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Thanks, Spider, "shunt resistor" was what I must have had in my unconcious memory.  Great!   Then I read the paragraph on Wiki.  Complete gobbledegook, clear to a electrical engineer but mud to me.

Detracted, that also was how I was thinking, but surely, to get the Amps that are to be measured, with a meter that blows over 10A, when the unmodified current is 20A++, could be several times that, a large reistance wiil be needed to bring the current down?

But hang on!  A=V x R  I can measure the resistance of the solenoid, and the voltage in my test battery.     Will that calculation give a valid value for A, the curent though the solenoid at that potential?

The solenoid resistance is 0.6 Ohms, and the Battery volts 12.84, so current should be approx 8 Amps, when as I said the ammeter (of no known accuracy) showed Full Scale Deflection  and at least 20A.  Shome mishtake Shurely, Shirley?

John

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Hello John,

 

yes that will give an approximation of the instantaneous current draw, (measurement of low resistance is relatively inaccurate by multi meter).

By coincidence I have been measuring the resistance of Triumph 2.5 starter motors this after noon with a reading of about 0.6 ohms giving an energising current of 20 amps, in theory.

When the coil is energised it creates a back emf so that the constant (working) current will be much less much as a motor current behaves. I presume that you had the armature in during your tests, as otherwise the current draw will be much higher.

I don't really understand what you intend this solenoid to do but, bear in mind it is unlikely to be rated for continuous use.

 

Alec

 

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0.6 Ohms, and 20A gives 16V .   Surely not???    

"Armature in"?   In place, certainly, although it will only move if I apply voltage.

This is to disengage the reverse gear interlock.   Many cars have some means to prevent reverse engagement without a posirivevactipn from the driver.  Triumphs, you press down on thw gear lever.   Moderns often have a collar on the column to be lifted.   But this is a transaxle, remote from the gear stick.    Cables allow gear changing, possible to do the same, but I fancied a switch on the stick!    For how long could such a solenoid function reliably?

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2 hours ago, JohnD said:

Detracted, that also was how I was thinking, but surely, to get the Amps that are to be measured, with a meter that blows over 10A, when the unmodified current is 20A++, could be several times that, a large reistance wiil be needed to bring the current down?

I think you have misunderstood John.   Just put the resistance in series with the solenoid. Measure the voltage across the resistor when you apply power - no need to use the amps range on the meter, use it on voltage.   By Ohms law I = V/R  so with R= 0.01 if you read 300mV across it the current is 30 Amps. 

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Thanks, DeTRracted, I'll do that as a check on the Volts times resistance calc!

I just want to klnow what wring to use.    A relay next to the solenoid with a live power cable that will supply the amps needed would seen best.

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Hello John,

 

yes, in place that's all that is necessary.

How long, I really have no clue. If you energise it and check how hot it gets, if it is untouchable after two or three minutes I would think that it's life would not be long. Could you have a spring engage for the interlock and power disengage?

 

Alec

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Alec,

Thats what it's doing!     The spring loaded defualt, internal to the gearbox, is 'reverse locked out'.      This solenoid pulls the interlock open.

There is also the Q of the moving and holding coils.     I know that in the Laycock de Normanville overdrive, the solenoid has a coil to engage overdrive, which draws a large current, and a holding coil that keeps it there and draws much less.      Some descriptions of starter solenoids mention those, which will be deisgned to make prolonged use safe.

John

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Indeed!   After writing my previous post, I thought Doh!  Chose the wrong assembly to steal from!

And a quick search of eBay shows a selection of O/d solenoids, at not unreasonable prices.  

BUT, they are all about twice as long as the starter solenoid, that may not fit in the space I have, and they push, not pull.   Not unsurmountable, except it's taken me a four days to build the bracket and lever to mount the starter solenoid1

Hmmmmmmmmm.     As usual, the Common Room of Sideways U. provides ideas, not answers.   Which is as it should be!

Thanks all!

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18 hours ago, DeTRacted said:

I think you have misunderstood John.   Just put the resistance in series with the solenoid. Measure the voltage across the resistor when you apply power - no need to use the amps range on the meter, use it on voltage.   By Ohms law I = V/R  so with R= 0.01 if you read 300mV across it the current is 30 Amps. 

Done this, Alec, and get an unhelpful answer, I'm afraid.  Using this circuit:

image.png.44706d1bbbe50a7e38d741d8e5d95e48.png

 

I measured the voltage drop across the resistance for two different resistors, both the large ceramic type you use with an FIA cut-off switch. to dump residual charge from the alternator.   This is what I got, with a test battery (Vb) delivering 12.8V

Resistance      Volts drop   Calculated current

3.3 Ohms        10.9V           3.3A

1 Ohm              8.4V             8.4A

In words, the volts drop, and current, depends on the added resistance.  Tells nothing about the current without the added Ohms.  Or am I doing it wrongly?

The solenoid resistance was 0.6 Ohms, so at 12.8 V the current should be 21.3A (A=V/R)  I got this simple calc wrong before and got a much lower figure.   That agrees with the 20A+ that my old dash ammeter reads.  Does that sound reasonable?     Wiring that will take 20A is not hard to find - my Pi fuel pump has 30A wire via the relay.

John

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Afternoon @JohnD,

I was going to chime in and say you'd got Ohm's law wrong but maybe that was a typo earlier on, as your latest post has it correct (I=V/R).

Two things on your latest measurements:

1. Multimeters are often unreliable when measuring low resistance. Depending how good your meter is, I'd treat anything below 10 ohms as 'approximate', and would apply at least 100% tolerance onto your 0.6 ohm measurement. You need a Megger test set or similar, which is specialist equipment.

2. Notwithstanding point 1, the resistors you've added in series are significantly larger than the test specimen, and consequently throw your results off. You want something an order of magnitude smaller - a length of cable can be used if you know what cross-section it is (as @DeTRacted suggests).

Multimeters are much better at measuring small voltages, so if you get a known small resistance (10 milli-ohm, for example) you can measure the small volt-drop across it when the current flows, and get a useful result.

You asked about dividing the current, and so measuring a smaller portion of it.  That's possible but you will only get a useful answer if you have an accurate measurement of the servo resistance. So I wouldn't pursue that option myself. 

 

Now just a question about why you're doing this! What exactly is this thing you call a "starter servo"?? 

Pete

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John - as you say if the series resistance you are using is equal to or greater than the resistance of the solenoid, it is that resistor which is mostly determining the current so the reading isn't much use.  To do this you need to use a much lower resistance ideally less than a tenth of the solenoid resistance which is why I suggested using a length of mains cable in my original post, since that will only be about 0.01 Ohms and will not influence the result to any degree.  You do need a meter which will read down to millivolts though. 

If you are certain about the DC resistance of the solenoid I think you are safe to use Ohms law anyway and your 21.3 Amps is correct. The resistance of copper increases as it gets hot so the current can only be lower than that. 

The solenoid is an inductor so opposes applied voltage. That means the current rises from zero to 21.3A as the back-emf dies, so there is no inrush current peak to be worried about either.

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My feeling is that a starter solenoid will be designed for seconds to (single digit) minutes operating time and will fry itself if left energised longer.

However, this might not be a show stopper.

-Scenario 1. You might take the view that reverse is only likely to be engaged for short enough periods to be acceptable. You might be right for 99% of the time but I’d be a bit anxious about the design safety.

-Scenario 2. Your linkage design might be such that you only need a momentary lift to allow you into the reverse gate. That being so you could make the button momentary or use a relay from the reversing light switch to cut power once reverse is engaged.

-Scenario 3. Assuming a constant pull is required, but the holding pull may be less than the initial pull, it might be possible to use the reverse light switch and relay to bring in a series resistor (a hefty one mind) to drop the voltage to the solenoid once the gear is engaged.

I’m liking 2 best at this point if the mechanics allow. Failing that I suggest seeing what is available in the 12v solenoid market. Central locking solenoids are cheap and come in many forms......

 

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