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Author Topic: Peterborough timer  (Read 8704 times)
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Yak 52
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« Reply #50 on: November 25, 2011, 12:22:16 PM »

Hi all.

Just a wee question (hijack!): How easy is it to make a timer for twin motor aircraft? I'm a total EP noob but most of the models I'd like to do as EP FF are twins...we are talking middle-distant future models rather than iminent projects ....but just wondering.

Thanks
Jon
« Last Edit: November 25, 2011, 02:15:15 PM by Yak52 » Logged
slipstick
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« Reply #51 on: November 25, 2011, 12:43:26 PM »

Just a wee question (hijack!): How easy is it to make a timer for twin motor aircraft?
If they're brushed motors you don't have to do anything special. Just connect the two motors in parallel and make sure the timer can handle the combined current. Done it several times Wink.

OTOH brushless motors make it considerably more complex (and expensive).

Steve

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boffin99
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« Reply #52 on: November 25, 2011, 02:13:31 PM »

Guys,

Seems that more than a few of you are getting the hang of the Peterborough timer, so here is a link to some of the models that can be made from simple brushed motors and the Pboro timer. The link also shows the next step into lightweight brushless with KPaero, Atomic Workshop and Ztron digital timers.

http://www.peterboroughmfc.org/membersmodels/effmodelswithdiff.htm

Regards Ian Peterborough MFC

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Woodster
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« Reply #53 on: December 10, 2011, 04:00:16 PM »

Thanks for the link Ian, I've PDF'd that info for future reference!
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cheesehead
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« Reply #54 on: December 19, 2011, 06:52:59 PM »

I hate to play the devil's advocate, but it might actually be cheaper to implement an AR6400 receiver from Parkzone, especially if you want a brushless motor.  Just set the throttle and let it go.

They're also prone to jamming servos in a crash, so you can get 'em as motor controllers awfully cheaply. Or you could just connect the servos and fly 'em as R/C. Cheesy
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Sky9pilot
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« Reply #55 on: December 23, 2011, 04:53:07 AM »

boffin99

Didn't mean to ignore your response and offer....was in a building slump for a while and I'm now looking at relocating so modeling will be taking a back seat for a bit.  I may take you up on your offer sometime early next year.

Thanks again to all for the support and encouragement.

Tom
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« Reply #56 on: January 08, 2012, 02:23:07 AM »

Hi all,
well, I must be one of the few not able to make this work! I have been trying the version from the Peterborough Jan 05 pdf newsletter, using a buz71a, 100uF cap and 1m resistor. What happens is that I only get a few seconds run.

If I put my voltmeter across the gate and source I can see the voltage shoot up to 9 when I zap it, then disappear to 0 in a couple of seconds. Varying the zapper pot varies the voltage I see across G & S, so that part seems to be working. And at lower settigns it stops in a shorter time, eg 1 second instead of 2!!

I thought maybe the resistor was too small and letting the cap drain too quickly, so tried 10m and 20m to no notable difference. I also tried a different buz71a and cap, but got the same results. Checked the buz datasheet and my wiring...cap -ve to source, cap and resistor parallel between G & S....I dunno what I'm doing wrong.

Any tips?

thx
- Mike
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slipstick
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« Reply #57 on: January 08, 2012, 04:21:58 AM »

I normally use 50uF and 1M or 1.5M resistors and get around a minute or so maximum.

What type of capacitor are you using ? I suppose it might be very leaky but only a couple of seconds sounds like something very odd. How about a picture of your wired up system ?

I suppose it's not just that your motor is drawing too much current for the battery so dragging the battery voltage right down ? What motor/prop and battery are you using ?

Steve
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Dave Andreski
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« Reply #58 on: January 08, 2012, 09:17:56 AM »

Mike,
Sounds like the capacitor is bleeding off without any resistance. Check your resistor wiring making sure the wires are solidly connected to the mosfet legs.

BTW, I'm using 33uF caps.

Dave Andreski
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boffin99
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« Reply #59 on: January 09, 2012, 02:22:36 PM »

Mike,
I am  the guy who wrote the Jan 05 pdf and am sorry that you have had a problem.
Dave A is quite correct in his observation. If you are getting a second or so then the resistor is at fault and is shorting the pins allowing rapid discharge of the capacitor charge. Are you sure you are connecting the resistor between the correct pins? Sounds like the capacitor is seeing no resistance at all
Other observations are :-
You are using a 100Uf cap, 33UF is preferred. This will not matter one  jot....just 3 times the run time and a bit more weight.
You talk of 10m and 20m resistors. I assume this is 10Mohm and 20Mohm resistors i.e 10 and 20 megohm.
If you have gone from 1Mohm to 10 and 20 times the correct value, then the capacitor is most deffo not seeing the resistor.
If the resistor checks out on the DVM then you are not connecting it properly.
Other check is if you keep the Zapper on all the time, the motor should run continously. If this is the case then all is well except the resistor.


If all else fails contact me via PM and send me the offending article. I will rework it and return it with an explanation of what went wrong.

Regards Ian M Peterborough MFC
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mike_st
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« Reply #60 on: January 09, 2012, 02:47:45 PM »

I'll have another go tonight. For the life of me I can't see how I can muck up soldering a resistor and cap across 2 pins! Also I guess an internal short between G &S would also produce the same result? But I don't see how I would have fried that either. My mosfet says "buz71a", the datasheet says "buz71a" and the legs are GDS when looking at the front. Will try again.
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slipstick
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« Reply #61 on: January 09, 2012, 03:33:24 PM »

Worth checking you have the capacitor the right way round...an electrolytic connected with the wrong polarity may do something like you describe (and may eventually blow its guts out Sad).

Steve
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Daithi
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« Reply #62 on: January 09, 2012, 05:26:17 PM »

The Time Constant on any CR network is the time taken in seconds for the charge on the capacitor to drop to 36.8% of the initial voltage - it's simply the product of the capacitance (in Fahrads) and the resistance (in Ohms) but for any practical application the value of the capacitance in microfahrads and resistance in the megohm range work just as well.

For 33 μF and 10 MΩ, i'd expect the time to be 33 x 10 = 330 seconds
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mike_st
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« Reply #63 on: January 10, 2012, 04:03:17 AM »

well, what ever I'm doing wrong I'm doing it again and again! Bought a new buz71a, tried new cap and resistor, same result.
It does run continuously if I keep the zapper pressed - let go the zapper and stops in a couple of seconds.
A couple of pics attached in case anyone can spot the blooper....
NB maybe the cap isn't actually charging up at all?
Attached files Thumbnail(s):
Re: Peterborough timer
Re: Peterborough timer
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Daithi
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« Reply #64 on: January 10, 2012, 04:16:50 AM »

The capacitor and resistor should go to the gate and drain and the source is not connected.

In yours you have a lead to the source - I'm not sure what it's doing

Suggested values (with a 33 μF Capacitor) for the resistor

Resistor value       Rapid part no            Approx mo
(sec)
150k                          64-0052                        19
270k                          64-1012                        34
390k                          64-1014                        49
470k                          64-0142                        59
560k                          64-1016                        71
680k                          64-0068                       86
820k                          64-1018                        104
1000k                        64-0072                        127
Attached files Thumbnail(s):
Re: Peterborough timer
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slipstick
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« Reply #65 on: January 10, 2012, 04:59:16 AM »

The capacitor and resistor should go to the gate and drain and the source is not connected.
I'm confused now because in the timer version I always build (with similar N-channel FETs) the capacitor and resistor go across Gate and Source and that looks like where they are. The Drain is then the -ve to the motor and again that looks right. Perhaps the official "Peterborough timer" uses high-end switching rather than low-end in which case although the principle is the same the polarities of SOME things will change.

Also that resistor value looks like 200 Ohms to me, but that's probably just the slightly blurry picture colours Wink. I can't see the polarity of the cap but I assume you've checked that it's +ve to gate, -ve to source.

In which case nothing I can see wrong, sorry. Over to Ian Smiley.

Steve
« Last Edit: January 10, 2012, 05:16:05 AM by slipstick » Logged
Woodster
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« Reply #66 on: January 10, 2012, 05:51:46 AM »

I'm looking to make a couple of these but noticed the specified capacitor is no longer available from Rapid. Any suggested alternatives anyone?
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boffin99
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« Reply #67 on: January 10, 2012, 06:00:52 AM »

Woodster,

Order on Ebay, 33uF ought to be available. If this is your first timer attempt then get a capacitor with wires. These are easier to solder than the surface mount stuff I use. You can go to 47uf if 33uF is not available.......this will give a longer run time but is heavier.

Regards Ian
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Woodster
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« Reply #68 on: January 10, 2012, 06:08:54 AM »

Thanks Ian,

I notice Farnell keep this type

http://uk.farnell.com/jsp/search/browse.jsp?N=204560+110073904+110119850&No=0&getResults=true&appliedparametrics=true&locale=en_UK&divisionLocale=en_UK&catalogId=&skipManufacturer=false&skipParametricAttributeId=&prevNValues=204560&mm=1000002||,1001880||,&filtersHidden=false&appliedHidden=false&autoApply=false&originalQueryURL=%2Fjsp%2Fsearch%2Fbrowse.jsp%3FN%3D204560%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Den_UK%26divisionLocale%3Den_UK%26catalogId%3D%26skipManufacturer%3Dfalse%26skipParametricAttributeId%3D%26prevNValues%3D204560

And we have an account at work for Farnell, but which one?!  Smiley
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boffin99
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« Reply #69 on: January 10, 2012, 06:20:51 AM »

Mike,

I used to use BUZ71 many years ago but moved to a much smaller surface mount FET. The graphic shows a IRFZ24N which from memory has the same pin outs as the BUZ71. Counting the pins 1,2,3 from left to right, the cap and resistor go across 1and 3 with 1 being positive. The motor goes across 2,3.
Your hook up appears to be the same but the pic is a bit blurry.
Does this help?

Regards Ian PMFC
Attached files Thumbnail(s):
Re: Peterborough timer
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boffin99
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« Reply #70 on: January 10, 2012, 06:28:29 AM »

Woodster,

The cap is not a critical component. Make sure it is tantalum  33uF to 47uF with wires.
You choose, you are the guy who has to work with your choice.

Regards Ian
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slipstick
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« Reply #71 on: January 10, 2012, 08:29:51 AM »

I used to use BUZ71 many years ago but moved to a much smaller surface mount FET. The graphic shows a IRFZ24N which from memory has the same pin outs as the BUZ71. Counting the pins 1,2,3 from left to right, the cap and resistor go across 1and 3 with 1 being positive. The motor goes across 2,3.
Standard To220 pinout for FETs is GDS from L to R. So that's cap/res across Gate and Source with motor connected to Drain (which is how I've been making mine for many years). But the motor can't be across 2 (D) and 3 (S) because those are the two pins that are effectively shorted when the gate is high. Unless I'm missing something silly you need a direct connection from one side of the battery to the motor with the other side of the motor connected to the FET drain.

But then I've always used low-side switching so it's battery +ve to motor then motor to FET drain, FET source to battery -ve. Perhaps you're switching +ve in which case the motor wiring will be slightly different.
 
Steve
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boffin99
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« Reply #72 on: January 10, 2012, 10:46:43 AM »

Slipstick,
Dead right, apologies for my confusing reply.
Regards Ian
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mike_st
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« Reply #73 on: January 10, 2012, 02:32:00 PM »

@everyone, thanks for your input.

The capacitor and resistor should go to the gate and drain and the source is not connected.
In yours you have a lead to the source - I'm not sure what it's doing
? thats not how I interpret the pdf?
I'll keep fiddling ......I'm gonna try a different FET
- Mike
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Woodster
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« Reply #74 on: January 10, 2012, 05:05:02 PM »

It says in the PDF: Solder the capacitor and resistor to the (G) and (D) pins of the FET. It then says solder a red wire to the (S) pin on the back of the FET and a black wire to the (D) pin. There is then a small diagram showing how to wire it up to the motor/battery. The only slightly unclear part (for me!) are the "Zapper" connections - I'm assuming +9V to (G) and -9V to (D).
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