ON/OFF RELAY CIRCUIT WITH 30v SUPPLYand 5V supply and two picaxe.

JPU

Senior Member
Hi All

Sorry I have re enlisted the help of this thread again. I bet everyone thought this had been put to bed. However, having built the circuit and added it to my existing circuit, I am still having problems due to the load sinking the "switch circuit" when it is supposed to be off. The sad thing is the device works brilliantly and powers up and down without fail. I've tested it all week and no fires or odd smells! But I have a 0.15 milliamp power flow from the battery when the unit is powered down and I have traced that to a pin on the load PCB that I do need to keep (the pin that is)!

So, I am going back to the drawing board (all was not lost as I leaned a lot about PNP, NPN transistors and FETS.) and I am going to make use of Hippies suggestion of using a relay.

I have built the circuit shown here and it works! Please can someone tell me if there are any "pitfalls" to using a circuit like this and since my load is a brushless motor controller, what should I use and where should I put it to prevent any reverse spikes from the motor damaging my "picaxe- relay" circuit.

(Id like to use a picaxe as I want to integrate "auto off" )

Thanks
JPUrelay control circuit.png
 

hippy

Technical Support
Staff member
I'm a bit confused about needing a pin to the load but that isn't shown in your relay circuit.

Are you sure the relay can be driven directly by an I/O pin, and where does the other end go ? Are you sourcing it high or sinking it low ?

Also you'll want to do something about that short directly across the load, which I guess is only in the circuit diagram or your update would contain tales of woe.

It's hard to say what else needs adding without knowing exactly what the load is.
 

rq3

Senior Member
Hi All

Sorry I have re enlisted the help of this thread again. I bet everyone thought this had been put to bed. However, having built the circuit and added it to my existing circuit, I am still having problems due to the load sinking the "switch circuit" when it is supposed to be off. The sad thing is the device works brilliantly and powers up and down without fail. I've tested it all week and no fires or odd smells! But I have a 0.15 milliamp power flow from the battery when the unit is powered down and I have traced that to a pin on the load PCB that I do need to keep (the pin that is)!

So, I am going back to the drawing board (all was not lost as I leaned a lot about PNP, NPN transistors and FETS.) and I am going to make use of Hippies suggestion of using a relay.

I have built the circuit shown here and it works! Please can someone tell me if there are any "pitfalls" to using a circuit like this and since my load is a brushless motor controller, what should I use and where should I put it to prevent any reverse spikes from the motor damaging my "picaxe- relay" circuit.

(Id like to use a picaxe as I want to integrate "auto off" )

Thanks
JPUView attachment 21113
If your circuit is working, then it's not built per your schematic. As drawn, when the relay closes, the 30 volt supply is shorted directly through the vertical line directly to the left of the load. It also applies 30 volts to the input of the 7805, providing an alternate input to the switched input to the same regulator. You may have intended that for "auto-shut-down", or maybe not.

It's considered good practice to place a "snubber" or "steering" diode across the relay coil. The coil will generate a huge (sometimes hundreds of volts) surge when it is turned off. A few relays have the diode built in, but not many.

Also, I hope you have generous quantities of by-pass capacitors liberally sprinkled around. They are particularly critical in a high power circuit like this, in which the logic supply is derived from the "high voltage, high current" source.

We'd need to see the details of the motor controller to make any recommendations for suppression or bypassing. Again, liberal by-pass capacitors are called for. Probably, at least several hundred microfarads of tantalum on the 30 volt supply, paralleled with a 0.1 uF ceramic and a 0.001 uF ceramic. My rule of thumb for low level logic like the Picaxe chip, is at least 1 uF of Tantalum, and the same ceramics as close as possible to the Picaxe V+ pin (within an inch).

This starts to get into weird things like protecting the regulator from the big tantalum cap if you should happen to short the power supply, which is why it's important to show ALL of the components on a schematic (usually). The devil is in the details!

Is the 0.15 mA (150 microamps) critical? Where is it going? Do you care? Should you care!? A 1 amp-hour battery will supply that current for over 6600 HOURS (9 months) before it needs to be recharged.

Clean up and complete the schematic, and we'll have another look.

Nice job! It's been fun watching you get this going!
 
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JPU

Senior Member
Morning!

I guess that is why I should draw the schematic first and then bread board after! Especially as my computer is several miles from my "laboratory" :) . I did the schematic from memory and forgot about the transistor for the relay and the "load by pass wire" was another over site by me!

I cant really give much more information about the load other than it has a positive and negative connection block labelled 30V and when powered it starts up a brushless motor which draws around 3-4amps. The load also has a speed up and slow down buttons integrated into it and these "do what they say on the tin". The PCB of the load looks way above much league (those of you who just smiled then, stop it!) and I wouldn't know where to start drawing it up.

My concern was some sort of spike coming from the load and possibly blowing the PICAXE. (If you spot any other potential disaster in my schematic (other than me!) , please let me know). Should I add some zeners or TVS diode. I have reading about TVS diodes but I am not sure how it would be positioned and if its the tool for the job.

Thanks

relay control circuit.jpg

RELAY is : DE1A1B-L2-5V
 

AllyCat

Senior Member
Hi,,

The relay you've linked appears to be a twin-coil Latching type, so can't be driven by a single transistor (at least as shown). Also a 5-volt (125 ohm) coil will take about 40 mA, that's almost an extra (unnecessary) 1 watt dissipation in the 5 volt regulator, which may then need a heat sink. IMHO a 24 volt relay (coil) would be better. Also, put a diode across the coil (in a direction that is reverse-biassed when the relay is on) to protect the transistor.

There is no earth connection shown to the regulator. Is that a Schottky or Zener diode (neither type is necessary) from the pushbutton to the regulator? Personally I would add another diode in the path from the relay contact/load back to the regulator. That will prevent the 4 Amps (and perhaps a higher inrush) current for the load passing directly through the pushbutton!

Cheers, Alan.
 

JPU

Senior Member
Hi

Does this make a bit more sense. I used a 1n4001 diode as thats all I had in the shed. Its there so that the push button switch when pressed a second time will shut down the circuit. The diode will prevent the circuit from switching itself off when the relay activates and the push button released in the first instance. I forgot about the earth from the regulator but oddly I cant find regulators in diptrace, is there a reason for that?

Also, put a diode across the coil (in a direction that is reverse-biassed when the relay is on) to protect the transistor
Way over my head, please can you simplify eg cathode to ground or cathode to ? etc


Personally I would add another diode in the path from the relay contact/load back to the regulator. That will prevent the 4 Amps (and perhaps a higher inrush) current for the load passing directly through the pushbutton!
Would I need to use a zener for this?

Relay is now PHOENIX CONTACT 2961312 General Purpose Relay, REL-MR Series, Power, Non Latching, SPDT, 24 VDC, 16 A

Is this going to cause me issue, especially wasted power and heat?





Thanks
 

Attachments

AllyCat

Senior Member
Hi,

Yes that relay looks more suitable; less than 1 watt total dssipation (V^2/R = 30*30/1440 = 625 mW) and a higher current contact rating, which should be fine. The coil being connected to the regulator input is "OK", but I'd take it "directly" to the battery+ (which happens to be one of its own contacts); connect a diode across the coil with its anode to the NPN collector.

I definitely recommend the extra diode (not a zener) to the regulator because otherwise, after the second press, the PushButton will have to break the 4 Amps current flow. Small PBs don't like that. There are 7805 and 78L05 regulators in Myc's Diptrace library.

Cheers, Alan.
 

JPU

Senior Member
Hi Allan

Thanks, how is this now looking?

Ive Googled MYC and MYC's diptrace libraray but I cant find anything related. Can you please direct me.

Thanks again
 

Attachments

AllyCat

Senior Member
Hi,

Yes that schematic looks electrically correct. It could be a little neater/clearer and the junction of the four wires at the regulator input could be misread (as a crossover). Personally, I never arrange four wires to meet at a point (even with a dot) to reduce the risk of mistakes. But IMHO one of the (few) poor features of Diptrace is the way that it handles the "elastic banding" of connections/wires when components or tracks are moved around.

The Myc library was created by a previous forum member, Mycroft (RIP); I don't recall how or where I obtained my copy but probably from this thread. Together with WA55's library it's virtually all that's needed with Diptrace. ;)

Cheers, Alan.
 

AllyCat

Senior Member
Hi,

Theoretically yes, hippy. But there is also the risk of some leakage current across the pushbutton or relay contacts, or between tracks on the PCB (which was the topic of a thread quite recently). Not many of the components have been identified yet, but to put it into perspective, if we assume that it is a 10 Ahr battery (to deliver 4+ Amps) and it will fully "self-discharge" in 5 years (optimistic for most rechargeables, perhaps pessimistic for Alkaline/Lithium types) then the effective internal leakage current is about 2000 mAhr/yr = 228 uA.

If we assume a humble BC547 transistor, its maximum collector-base leakage current is 15 nA (but probably ten times smaller) at 25 degrees C. The leakage might double for each 7 degrees rise, but I'd be surprised to see any more than 100 nA under "normal" operating conditions.

That collector-base leakage may be amplified by the current gain of the transistor, which I doubt would be more than 100 at a nA base current, so maybe a total collector leakage current of 10 uA, or less than 5% of the (hypothesised) battery leakage. However, if the transistor leakage current is considered to be excessive, then add a resistor (say 10k) across the base-emitter junction (which I must admit I normally do recommmend) so that the only current which flows is the c-b leakage.

I'm not convinced that Diptrace always puts dots at junctions, but my comment above was more based on Murphy's Law, particularly when viewing the schematic on a small, low resolution, Netbook or phone screen. I must admit that I missed the much more obvious error of the reversed electrolytics (which were correct in previous versions of the diagram) and probably many others did too.

Cheers, Alan.
 
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pxgator

Senior Member
When the relay turns off the magnetic field in the relay coil collapses and creates a very high reverse voltage.
This 'energy spike' can easily damage the transistor. The diode essentially shorts out the spike and protects the transistor.
Because of the high voltage potential of this spike a diode with a high reverse voltage rating such as a 1N4007
should be used. Your project looks promising....please keep us posted on your progress.

Cheers to All
 
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JPU

Senior Member
Hi Guys

Its hard to believe its been nearly 8 months since I last acted on this thread.

I built the PCB detailed above and have had several made for testing (20 units in fact). The whole thing has worked well (perfectly until now) but this week I have had a couple of test units fail. On inspection it appears that the transistor which pxgator mentioned has been damaged on each occasion. Once the the transistor is replaced the unit functions perfectly again. This has only occurred in 2 out of 18 trials units.

I was stumped so I thought I would get onto the forum for help and was amazed to read the last post by pxgator where he highlighted the fact that the transistor might get damaged if the diode D2 was not present. In the case of the the two failures the diode was present yet the transistor still got burnt out. I have checked voltages into and around the transistor and they all appear normal. This means the transistor is get damaged by some occurrence I have yet to establish.

I do very much appreciate that this is now a very old thread but I would be very grateful if anyone who can remember this may have an idea why the transistor is getting burnt out.

Thank you

JPU
 

premelec

Senior Member
Is the circuit identical to post 54 ? what transistor is being used and how does it fail? [shorted, open etc.]. Also what is the relay coil resistance?
 
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JPU

Senior Member
Hi

The transistor is a BC817-25LT3G and it goes short. The diode after the momentary switch is a 1N4448W-7-F and the relay being used is SRM-1C-SL-24VDC and has a 1.6kohm resistence.
There is also a TVS diode now shown SMBJ54A and this component was added as it was recommended to be part of any circuit in which the "load" was being used. The load is a brush less motor controller.

Thank you for helping.
 

Attachments

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AllyCat

Senior Member
Hi,

To be honest, I can't see anything obviously "wrong" with the circuit diagram, so the failure of the transistors does seem "unexpected", although the voltage and power (dissipation) ratings are not as generous as they might be. Is 30V the maximum battery voltage, or just the nominal value (which might be exceeded immediately after charging, for example)? But, rather clutching at straws, I can suggest three "possible" causes of the failures:

1. The transistors are "Fake", i.e. they have poorer voltage, power or gain ratings than the official data sheet.

2. The 10 Amps and high switching speed (relay contacts) can produce large voltage spikes across quite a small inductance, for example the "stray" inductance of just some inches of connecting wire. So you/we may need to look at the exact physical layout to see if there is any "common impedance" between the 10A circuit and the relay/transistor. Ideally, the 10A and PICaxe circuits will have separate connections right back to "star points" directly at the battery terminals. IMHO this is the most likely cause, so a "layout" diagram/photo might be helpful.

3. I see the pin which drives the transistor is labelled as SerOut/InfraOut, so there is a small risk that the transistor might be driven by a rapid pulse train (due to a software bug or a system glitch). When the transistor switches off the relay inductance causes the collector to rise to the battery voltage (plus the diode forward drop) and the current (~20 mA) continues to flow in the diode for some time. If the transistor then immediately switches back on again, the transient power is 30 v * 20 mA = 600 mW. That's above the continuous rating of the transistor, but it shouldn't last long enough to cause any damage (as I said, clutching at straws).

Cheers, Alan.
 

techElder

Well-known member
I always use a 2N7000 mosfet to interface with a PICAXE output. 200mA and 60v. Replace that BC817-25LT3G with the 2N7000. Just guessing.

If the BC817-25LT3G is "shorting out", then is it getting hot/really warm during normal operation? If not getting warm, then its probably getting too much voltage.

You are kind of running that 7805 near its maximum input of around 35 volts (depending on several factors.) Just guessing.
 

newplumber

Senior Member
Allycat said:
2. The 10 Amps and high switching speed (relay contacts) can produce large voltage spikes across quite a small inductance, for example the "stray" inductance of just some inches of connecting wire. So you/we may need to look at the exact physical layout to see if there is any "common impedance" between the 10A circuit and the relay/transistor. Ideally, the 10A and PICaxe circuits will have separate connections right back to "star points" directly at the battery terminals. IMHO this is the most likely cause, so a "layout" diagram/photo might be helpful.
I'm learning something new everyday good info alan

I would do the same with Texasclod ...use a mosfet....then its bullet proof ....but not water proof (tried that)
 

premelec

Senior Member
@JPU - I'd go with Tex's suggestion of heaftier transistor and perhaps TVS diode across the transistor - looking at the tiny transistor's Safe Operating Area you could be getting close if it isn't turning on fully and fast...
 

tmfkam

Senior Member
My thoughts would be to add two further diodes around the transistor's Collector.
One, Cathode to Collector, Anode to the junction of the existing flyback protection diode and the "Collector" end of the relay coil.
Another, Anode to Emitter, Cathode to the junction of the existing flyback protection diode and the "Collector" end of the relay coil.

My thinking being that if the circuit is present when a charger is connected and or disconnected while the charger is powered, voltage spikes could attempt to pass in the "reverse" direction from emitter to collector. The first diode would prevent this from being possible as it would be reverse biased in this condition, the second diode would shunt this voltage into the relay coil thus limiting it to a (hopefully) safe level. Not only that, but any other voltage jumps from the relay or load being turned off would be limited in a similar way.

Both would have minimal effect during normal operation, are inexpensive and would be easy to add post manufacture.

relay circuit for forum.jpg

Apologies for the jagged lines on the circuit, I had to copy and paste the parts from the original.
 

JPU

Senior Member
HI Guys

Thank you all for your valued input. Thanks for taking the time to read though the thread again, as I`m sure that as it was an old thread it must have been long gone from your minds.

Ill try to get a pic up of the PCB so that you can see it in a couple of days. I think I will try the mosfet suggestion as most of all it appears to be the least invasive solution. Its going to take me time to get to the bottom of this as the problem is so intermittent!

I originally hand soldered these transistors onto the PCB and My soldering iron is set to max, 400C. Could I have damaged the transistor as the only problem boards are the ones I created. I have also had the PCBs professionally made and had no problems with those but they are still young in comparison to the initial hand soldered boards?

Premelec suggested I also add a TVS diode across the transistor, please can someone put this into laymans. Anode to what? cathode to what?

Thanks as ever, this forum is an awesome "life saver" on the web!

JPU
 

AllyCat

Senior Member
Hi,

... looking at the tiny transistor's Safe Operating Area you could be getting close if it isn't turning on fully and fast...
Hmm, I looked in the data sheet for a SOAR (Safe Operating ARea) graph but couldn't find one.

"In the old days", SOAR was quite a hot topic for some medium power transistors, but I don't know if it's still even relevant. Basically, it said that if a (bipolar) transistor was rated at (say) 45 volts and 1 amp (which I believe is the case for the BC817) it could NOT handle both together for ANY period of time (not even ns). IIRC, it wasn't (isn't?) a "thermal" (power dissipation) issue but an instantaneous, catastrophic failure mode. However, your BC817 shouldn't be seeing anything like 1 amp (only around 20 mA through the relay coil) and if the maximum battery/applied voltage is only 30 volts then I'd still be looking for "spikes" due to some "unexpected" electrical/magnetic coupling. Or of course Fakes/mishandling.

Cheers, Alan.
 

premelec

Senior Member
@JPU TVS from collector to emitter could kill a spike that might cause the transistor to short... I mentioned SOA because it is shown in the data sheet [at bottom of curves in sheet I downloaded] - innocently assuming if it was there it was relevant ;-0
 

lbenson

Senior Member
Whatever... and look at the actual specs for 2N7002 or 2N7000 ;-0
I've successfully used the 2n7000 as a low-side switch with a 5V-powered load and a 5V-powered gate, but without truly understanding the characteristics as given in the datasheet.

In the attached image I've put together four pieces of the datasheet that seem to me pertinent and wonder if someone with a firmer understanding can enlighten me as to what they mean in practice.
2n7000.jpg

It seems to me that item 1 says that the 2n7000 (numbers in the first column) can pass 200mA but constrained by a maximum of 400mW at 25C. So does this mean that at 5V you have a maximum continuous current of 80mA?

Item 2 (RDSon): at 4.5V with a current of 75mA the maximum RDS(ON) is 5.3 ohms. Does this speak to the size of the gate resistor needed? What does it imply is required?

Item 3--Drain-source on voltage: I guess I don't understand the relevance of this at all.

Item 4--On-region characteristics: This seems to me to be saying that at a gate-to-source 5 volts and a drain-to-source 5 volts, the drain-to-source current is about .8A, which would be 4W, far in excess of the 400mW maximum, so what is this actually saying? Is it that with a 5V VDS and a 5V VGS, with the proper gate resistor you can draw any desired current up to the maximum 400mW at 25C?

I'd be obliged if someone can comment, and also if other relevant characteristics can be pointed out.

(Hope this isn't too much of a hijack.)
 

premelec

Senior Member
This is a bit complicated but Ohm's law and power equations still can clarify - think of the 2n700 as a variable resistor [Rx]- in series with a load resistor [R] - the current in series circuit is I = V / (Rx+R). The Power in each resistance is I*I*R. or for the transistor is I*I*Rx.

You can put this on a spread sheet and vary the Rx with a particular load R...

The gate resistor is a very different matter since the Gate R is very high and usually you'd just want the transistor on or off. This implies that you need the gate voltage high enough to turn 'on' - or well over Vth [threshold gate voltage where it begins to conduct].

The 2n700 Rx varies with both the gate voltage and the current through the transistor - that's why they publish those fancy curves... ;-0

Does this help?
 

rq3

Senior Member
It may help to think of these Mosfets as voltage controlled resistors. The resistance (Rdson) between the drain and the source varies with the voltage on the gate. The gate itself is essentially a very small capacitor, and draws current only for the very small time it takes to charge to the gate voltage at hand. Generally, the gate should be driven ON and OFF as quickly as possible, so that the drain to source resistance doesn't linger in the nasty linear area where the device is neither fully on nor fully off, it's resistance is high, and the power dissipation is therefor also high.

Item one says that the maximum current for the device is 200 mA. This is a thermal bond wire limit, as you can see by the allowed pulse current. Pulsed current is very much larger, because the bond wires have a chance to cool off between pulses.

Item two says that at a gate drive of 4.5 volts (for example, driven by a Picaxe), the resistance between the drain and source wil be, at most, 5.3 ohms. This means that the device would conduct 5/5.3=0.943 Amps if shorted across a 5 volt supply and turned on.
Of course, this would destroy the Mosfet. It is up to you to ensure that the current through the device you are switching is no more than 200 mA due to the switched device's own reistance.

Item 3 is an encapsulation of the other two items above. It says that the voltage drop from drain to source, due to the Mosfet's Rdson, is gate voltage dependant. For 75 mA through the Mosfet, when the gate is driven to 4.5 volts, the resistance of the Mosfet will be such that the voltage drop across the Mosfet will be, worst case, 0.45 volts. From this, using Ohm's law, you can "back calculate" Rdson as: 0.45/0.075=6 ohms for that condition. The power dissipated by the Mosfet would be 0.45 x 0.45/6= 0.03375 Watts.

You have Item 4 exactly right, but the drain to source voltage would never be 5 volts, unless the Mosfet were shorted directly across the power supply. If it were, the on-region characteristics indeed say that the device would (attempt) to conduct 0.8 amps. It just wouldn't do it for very long.

Basically, you need to make sure that the LOAD you are controlling CANNOT conduct more current than the Mosfet is capable of handling. The only reason you might ever want to connect the drain and source directly across an unlimited power supply is if you wanted to use the Mosfet as a detonation squib for an explosive!
 

The bear

Senior Member
@Ibenson,
Plenty info. on search engines, http://https://electronics.stackexchange.com/questions/86688/understanding-mosfet-gate-characteristics[/URL]uestions/86688/understanding-mosfet-gate-characteristics

Regards, bear.

Watching TV..." Wild Canada"
 

lbenson

Senior Member
rq3--thanks very much for the detailed response to my questions. I appreciate your taking the time. Thanks also to premelec and bear (your link could use some editing).

On a practical note, Manual 3, page 8 shows a circuit for an IRL520 with a 10k pulldown for the gate, but no series resistor. Is this considered best practice, or should there be a series resistor to the right of the pulldown, and if so, is the typical "pin-protection" 330 ohms recommended, or is it dependent on the characteristics of the MOSFET and the load (assuming basic on/off operation, without PWM)?
 

premelec

Senior Member
A 'nasty' MOSFET failure possibility is gate shorting which can [rarely we hope] feed voltage back thru the gate; a series resistor reduces doing damage to the driving circuit - note that gate capacitance can be quite large in power units so there is also the consideration that you are connecting a 2nF capacitor directly to driving pin if no series resistor is in the circuit... that may or may not be trouble... in cases where you need very high speed gate drive use a special driver IC made for that purpose.
 
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AllyCat

Senior Member
Hi,

I didn't want to take this thread OT (or OTT) earlier, but this might be of interest now:

(post #59) 2. The 10 Amps and high switching speed (relay contacts) can produce large voltage spikes across quite a small inductance,, for example the "stray" inductance of just some inches of connecting wire.
I'm learning something new everyday
The magic formula for inductors is E = -L di/dt. ;)

"E" is the voltage generated by (or applied to) the inductance (usually a coil, but it could be just "a piece of wire") in Volts. E because it was originally called the "electro-motive force" (emf).

"L" is the inductance in Henries. A Henry is quite a large value but you might encounter it occasionally. Less unusual than a Farad which is really big (Supercaps are rather exceptional components). My guess is that the 24 volt relay coil might be about 1 Henry, but I couldn't find it specified in any data sheets, probably because the inductance increases somewhat when the relay operates (as the air gap in its magnetic circuit reduces).

"di/dt" is the "rate of change of current with time", in Amps per Second. Here we might have 10 Amps but a time in us or even ns. So di/dt also can be a big number (millions or even billions). This is the principle used by automobile ignition coils and Tesla high voltage generators, etc..

The "-" sign (often overlooked) indicates that the inductor always tries to resist any change in the current flowing (just as a capacitor tries to resist a change in voltage across its terminals). i.e. if you try to reduce the current very rapidly, the inductor will generate a large (reverse) voltage in protest (just as a capacitor may deliver a very large current to prevent the voltage changing). ;)

So, what is the inductance of "a piece of wire"? Many years ago I was impressed by an advert for an inductance meter that claimed to "measure the inductance of a paper clip". I don't know what that value is, and Google failed me, but it's perhaps around 10 nH. However, I did find an inductance calculator which is totally OTT for this thread and probably the forum, but perhaps worth bookmarking. It appears to accept quite unusual coil dimensions, for example a single turn helix 1 mm in diameter and 10 cms long (i.e. an almost straight piece of wire) is calculated as 0.31 uH. Usefully, it also gives the impedance (reactance) as 6 ohms at 3 MHz, whilst the (dc) resistance is only a few milliohms.

The reason for choosing 3 MHz is that it is approximately equivalent to a rise or fall-time of 100 ns (0.1 us), so if the current in that 10 cms piece of wire reduces from 10 A to zero in 100 ns, then a voltage spike of around 60 volts (i.e. 10 amps * 6 ohms) would be generated across its ends. Thus if the wire formed a common impedance between the "10 Amps" and "PICaxe" parts of the circuit (e.g. a shared wire to a battery terminal 4 inches away) then 60 volts is "available" to destroy any of the semiconductors, such as the relay driver transistor. I can't say if these values are realistic for this, or any other, circuit but the calculation does give an indication of the kind of parameters which might lead to problems.

Cheers, Alan.
 

darb1972

Senior Member
Hi Gents

Some very interesting coversations going on here, even if sometimes slightly OT.

@ Alan, thanks for that information. As always, very interesting and useful information. I'd like to keep you in my workshop for when I have questions!!! You're a very knowledgeable fellow Alan. Thank you again.

@lbensen. Years ago I did FETs at college, but, like most of us, I require a brain "refresh" ever so often to bring the information back to the forefront of the mind. I found this series very interesting and informative. It's in four parts. There are plenty of good tutorials on this site. Good magazine too. I'm sure you'll find these tutorials really useful.

http://www.nutsvolts.com/magazine/article/fet_principles_and_circuits_part_1

Regards
Brad
 

newplumber

Senior Member
I second darb's quote on Alan and all of you smart people

Whats funny is I can get confused by reading the word confused :) but with " E = -L di/dt " is over my head (for now) tho its been explained in simplest terms
E = volts generated (why not Vg?)
L = inductance Henry? (why not Ih?) ...I'm guessing someone like me invented this abrev?
SLOW = Mark (newplumber for short)
over all tho I'm slowly learning which is fun...love the info thanks
 
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