On board glow driver

[Peter Beeney]

Ernie, - You could in fact take the mains supply, at 50 Hz, convert it to DC, at around 300 volts, feed it into a an AC inverter running at 20 kilohertz, apply this to the primary winding of a transformer at 5 amps and extract around 35 volts AC from the secondary, and then convert it back to DC to give you 50 volts at 25 amps to supply to the load. This will all go in a portable box which you can lift with one hand, about 13 kg; because of the small size of the transformer. The equivalent circuit running at 50 Hz straight off the mains would fit in a container the size of a fridge and would take about 4 men just to hump it around; because of the large size, (and weight!!) of the transformer. This is one of the advantages of switched mode power supplies.

We use much smaller versions to compete with the linear regulator, because they are more versatile than the standard BEC. Do you think it might be worthwhile striking some very simple drawings to try and explain the workings?

PB

[Ernie]

Thanks Peter, Thats pretty well what I thought. I've a couple of reservations about the AC invertor running at 20 kilohertz, I'll probably try an upright invertor at around 45 kilohertz, I'll let you know how I get on. Do you want a circuit diagram?

ernie

[Peter Beeney]

Ernie, - Thank you very much very much for the offer, but I’m afraid I’m long out of circuit diagrams. By about 20 years now. I don’t think I’m going to be into AC inverters anytime soon! The 20 kHz figure was a bit of a random hit, I think these things now run up to a MHz or more, maybe, but as I remember 25 to 40 kHz was popular. You can’t hear 25 kHz, at least I can’t, (couldn't!) and that makes them nice and quiet! A bit eerie at times, you could be surrounded by mega-amps and still hear a pin drop. Not so with 50 hertz, though, where everything shakes, rattles and rolls.

All I had in mind, if anyone’s interested, was a simple diagram of a BEC regulator, explaining the principles, and then comparing with a switched mode UBEC. The UBEC employs an inductor, which doesn’t use an inverter, it’s straightforward DC. Which makes it slightly more complicated. (Than the BEC!).

With regard to the actual glowplugs, I’ve invariably found that I prefer a nice bright glowing orange element to ensure a good start. So to that end for years I’ve used an 8Ah Cyclon cylindrical 2 volt lead acid; prime the motor, give it a few good flicks to circulate the prime, throttle a couple of notches open, clip on, show it the starter and we’re away. Often just one flick with a finger is sufficient, too. I’m sure that it’s possible that prolonged use of the battery may cause the plug to oxidise, but in the event that the on-board electrically energised plug keeps the motor going and stops the model spinning in on a landing, for instance, would be my choice every time. But this, of course, is something that we might find difficult to establish if it would ever happen.

From the point of view of the on board glow, certainly the temperature controlled version would be very useful from the point of saving on battery power. So that’s an excellent idea, and I believe there are commercial units available.
The methanol has a catalytic action on the platinum wire element, I believe, heating it up to some extent, this apparently is what helps to keep the element glowing whilst the engine is running. I’ve used many inverted engines and I’ve never had any problems with the engine cutting due to the plug going cold. So I’ve never used an on-board glow. But on a prolonged run with the throttle closed I’m sure it would only need a tiny smidgen of unburnt oil to put the fire out. In this case it seems the temperature sense would be ideal. I can certainly remember one gentleman with his engine, a OS Max40, that after starting for a session the first time, would not need the glow start again. Just a flick would see it running, even if it had been standing for a while.

Good luck with the inverter circuit, if it works ok do a thread on it.

PB

[Idris Francis]

First, to Stephen Jones - I stll don't know what the flying widgets item you mentioned actually does!

To Steve Olsen - many thanks for the detailed explanation.

My Series 1 Flight Link Tx design used a valve output stage (in those days transistor output was only just beginning to give adequate power at acceptable cost so I played safe). A 2 transistor inverter used a hand-wound toroildal ferrite core as the HT transformer, from a single 6 volt battery. The HT winding was 1,000 turns of bi-filar wire, wound using a long thin frame that would just pass through the centre even when fully wound. Also a separate winding with far fewer turns to give 1.5 volts or so for the valve filaments...........and also the feedback voltages to make the circuit oscillate, at medium audiio frequencies I think.. The core plus winding was about 12 diameter by 1/2" thick, from memory, the output a couple of Watts or so.

It was compact and efficient, and although not small by modern standards, a similar arrangement to step down 4.8 volts to 1.2 volts would be practical for say a 50" span aircraft upwards, being smaller and lighter than a 2000mA NiMh cell switch, harness etc.

That said, if I used the Rx battery to drive it (and/or retract U/C that could jam and absorb high stall currents) I would definitely use a throttle fail-safe module that sends the throttle to slow when Rx battery voltage becomes low - but still with enough capacity to land.

One of the problems in running a small electronics company, including not only design but manufacturing, sales, accounts and everything else is that there is no time to spare to keep up to date with electronic design, unless vital to the business to do so - which I would like to think (excuses, excuses) why I did not know in any detail, until now, how switched mode supplies work!

As I may have mentioned before, Mick Wilshere of World Engines told me that he does not like on-board glows for precisely the reason discussed here - that applying extra heat when already hot from high rpm is not a good for the element - it makes sense to me. Of course we all do, from time to time when starting up and adjusting, but that is rather different from having it come on in flight at significant power settings. But a circuit that measures the temperature eliminates these problems. And of course those systems that switch the power on only at lowish throttle also to, to an extent.

Even in the 1960s, it seemed to me, more models were lost due to engine failures at awkward moments, and given the massive increase in reliability of RC since then I suspecy it is still true now.

It certainly used to be for me - though part of that might of course have been due being less than expert in setting them up in the first place. It was for this reason that I decided several years ago to try on-board glows and chose the Intelligent Glow unit because the concept made sense to me. Having found that it did what it said on the tin, however, I decided to design the simplest possible circuit to do that same job, at what turned out to be quite trrivial cost. And I am glad I did because they work reliably, give a more reliable idle (also slower if needed) and because it is a pleasure not to have to connect and disconnect an external glow lead, an inch or two from the prop.

And as mentioned here, using battery power only when it is needed makes sense in several ways, includding tthose mentioned here.

I don't have the detailed knowledge to design a switched mode system to go with it, though I probably could an inverter using a toroidal ferrite core. If time permitted I would like to try, but as it does not, probably not.

I set the desired temperature, using a preset, first just to give a visible glow, and then tweaked so that it will just allow the engine to start - by definition, hot enough to keep the engine running. Rather to my surprise, when then checked out of the engine, it turns out not to be the nice bright - yellow approaching white - we normally expect, but simply a dull organge.

The LED indicator which glows when starting then flickers a little at idling rpm but does out altoghether at significantly higher rpm.

[Idris Francis]

First, to Stephen Jones - I stll don't know what the flying widgets item you mentioned actually does!

To Steve Olsen - many thanks for the detailed explanation.

My Series 1 Flight Link Tx design used a valve output stage (in those days transistor output was only just beginning to give adequate power at acceptable cost so I played safe). A 2 transistor inverter used a hand-wound toroidal ferrite core as the HT transformer, from a single 6 volt battery. The HT winding was 1,000 turns of bi-filar wire, wound using a long thin frame that would just pass through the centre even when fully wound. Also a separate winding with far fewer turns to give 1.5 volts or so for the valve filaments...........and also the feedback voltages to make the circuit oscillate, at medium audiio frequencies I think.. The core plus winding was about 1" diameter by 1/2" thick, from memory, the output a couple of Watts or so.

It was compact and efficient, and although not small by modern standards, a similar arrangement to step down 4.8 volts to 1.2 volts would be practical for say a 50" span aircraft upwards, being smaller and lighter than a 2000mA NiMh cell switch, harness etc.

That said, if I used the Rx battery to drive it (and/or retract U/C that could jam and absorb high stall currents) I would definitely use a throttle fail-safe module that sends the throttle to slow when Rx battery voltage becomes low - but still with enough capacity to land.

One of the problems in running a small electronics company, including not only design but manufacturing, sales, accounts and everything else is that there is no time to spare to keep up to date with electronic design, unless vital to the business to do so - which I would like to think (excuses, excuses) why I did not know in any detail, until now, how switched mode supplies work!

As I may have mentioned before, Mick Wilshere of World Engines told me that he does not like on-board glows for precisely the reason discussed here - that applying extra heat when already hot from high rpm is not a good for the element - it makes sense to me. Of course we all do, from time to time when starting up and adjusting, but that is rather different from having it come on in flight at significant power settings. But a circuit that measures the temperature eliminates these problems. And of course those systems that switch the power on only at lowish throttle also to, to an extent.

Even in the 1960s, it seemed to me, more models were lost due to engine failures at awkward moments than due to radio failure and given the massive increase in reliability of RC since then I suspect it is still true now.

It certainly used to be for me (though part of that might of course have been due being less than expert in setting them up in the first place, as Mick is too polite to point out.). It was for this reason that I decided several years ago to try on-board glows and chose the Intelligent Glow unit because the concept made sense to me. Having found that it did what it said on the tin, however, I decided to design the simplest possible circuit to do that same job, at what turned out to be quite trrivial cost. And I am glad I did because they work reliably, give a more reliable idle (also slower if needed) and because it is a pleasure not to have to connect and disconnect an external glow lead, an inch or two from the prop.

I set the desired temperature, using a preset, first just to give a visible glow, and then tweaked so that it will just allow the engine to start - by definition, hot enough to keep the engine running. Rather to my surprise, when then checked out of the engine, it turns out not to be the nice bright - yellow approaching white - we normally expect, but simply a dull organge.

The LED indicator which glows when starting then flickers a little at idling rpm but does out altoghether at significantly higher rpm.

 

 

And as mentioned here, using battery power only when it is needed makes sense in several ways, includding tthose mentioned here.

I don't have the detailed knowledge to design a switched mode system to go with it, though I probably could an inverter using a toroidal ferrite core. If time permitted I would like to try, but as it does not, will probably not.

 

 

 

 

 

 

 

 

 

Edited By Idris Francis on 27/09/2012 22:18:09

[John Olsen 1]

Well Ernie, it is kind of tricky to explain complex things without either over simplifying, or else getting too verbose. If you really want to know how this stuff works, you will have to master differential equations. Actually the main one you need for this is not too bad....V = di/dt

Incidently a "Plan B" to consider would be to fit a spark plug and a cdi unit. I've done this to a Saito 62, and it runs very nicely, on (relatively!) cheap petrol too, and idles very slowly and reliably. The CDI unit and battery is probably not much heavier than a glow driver and battery would be. (I'm not enough of an optimist to try and run the receiver and CDI from the same battery!)

Actually a further advantage of the CDI approach is that you can have a very reliable cutoff. Mine has an ignition switch controlled from a spare channel, when you turn that switch off the engine stops just as soon as inertia and friction permit.

John

Edited By John Olsen 1 on 28/09/2012 08:40:40

[Sean Murray-Smith]

Hi,

My version of Boddo's "Old Bill" has an enclosed cowling so I use the RCD 3001 on board driver. This little unit has a button which will power the plug for 15 seconds. I run it via the UBEC which supplies power to my servos c/o a 2200mAh LiPo even though it will take 6-10 volts. Works a treat and costs only $AU20.00.

[fly-navy]

After seeing an article how to make one on the "Cub Den" site I made one for an inverted Saito on a Cub. Basically some wire, switch, SubC battery and small micro switch which is operated from the throttle servo arm at just under 1/4 throttle.

John

[Former Member]

[This posting has been removed]