Regulating Ignition Coil HV
Sulaiman, Sun Oct 01 2006, 11:43PMMy thoughts have been going around in circles lately - I need help.
I have a simple Ignition Coil HV power supply
(ac mains transformer, bridge rectifier and capacitor for 33V @ 2.5 A dc)
(555 driving 30N60 igbt with various clamps for Gate and Collector voltage)
(30kV @ 30 mA rectifier and MMC hv capacitor)
operating fixed-frequency variable-duty for 0 to MAX POWER.
My problem is that control is open-loop setting of power
(as are most flyback systems)
Anyone have a SIMPLE method of output voltage regulation?
Re: Regulating Ignition Coil HV
Wilson, Mon Oct 02 2006, 02:18AM
Resistive divider with capacitative compensation?
Wilson, Mon Oct 02 2006, 02:18AM
Resistive divider with capacitative compensation?
Re: Regulating Ignition Coil HV
Steve Conner, Mon Oct 02 2006, 10:01AM
Take whatever means you use to vary the duty (the control voltage pin on the 555 or whatever) and connect it to an op-amp wired as an integrator. Now hook this up so that it integrates the difference between a divided-down sample of the output, and a variable reference voltage (say from a voltage regulator and pot)
The duty cycle will now be adjusted by negative feedback until the divided-down sample of the output is equal to the reference voltage you set. Either that or it'll go unstable and oscillate like crazy. You may need to play with the integral time constant, or add lead and/or lag resistors, to make the feedback loop stable. Or it'll stick either completely off or completely on, in which case the feedback is probably positive rather than negative.
If you change your 555 to a TL494, it already includes all the op-amps and stuff needed for the feedback loop, and the datasheet gives example values of compensation components.
How much power do you get out of this supply btw?
Steve Conner, Mon Oct 02 2006, 10:01AM
Take whatever means you use to vary the duty (the control voltage pin on the 555 or whatever) and connect it to an op-amp wired as an integrator. Now hook this up so that it integrates the difference between a divided-down sample of the output, and a variable reference voltage (say from a voltage regulator and pot)
The duty cycle will now be adjusted by negative feedback until the divided-down sample of the output is equal to the reference voltage you set. Either that or it'll go unstable and oscillate like crazy. You may need to play with the integral time constant, or add lead and/or lag resistors, to make the feedback loop stable. Or it'll stick either completely off or completely on, in which case the feedback is probably positive rather than negative.
If you change your 555 to a TL494, it already includes all the op-amps and stuff needed for the feedback loop, and the datasheet gives example values of compensation components.
How much power do you get out of this supply btw?
Re: Regulating Ignition Coil HV
Marko, Mon Oct 02 2006, 11:00AM
For very simple regulation I would sometimes just use optocoupler in series with zener, and hooked it's output transistor just in place of regulating resistor on NE555 (or even transistor astable).
It didn't oscillate but for low ouput voltages regulation wasn't good..
SG3535 or TL seem to be the most economic solution..
Marko, Mon Oct 02 2006, 11:00AM
For very simple regulation I would sometimes just use optocoupler in series with zener, and hooked it's output transistor just in place of regulating resistor on NE555 (or even transistor astable).
It didn't oscillate but for low ouput voltages regulation wasn't good..
SG3535 or TL seem to be the most economic solution..
Re: Regulating Ignition Coil HV
Sulaiman, Mon Oct 02 2006, 02:15PM
I think I'll have to change to a 494
been trying to avoid 'learning' a new IC.
So far I've had 50W (10kV @ 5 mA) out,
I think it will supply up to 100W but not yet tried due to lack of suitable load.
I'm in the process of building a HV test load/ballast,
so maybe in a couple of days I can answer that.
Thanks for the advice guys, but looks like I'll keep it open-loop for a while yet.
Sulaiman, Mon Oct 02 2006, 02:15PM
I think I'll have to change to a 494
been trying to avoid 'learning' a new IC.
So far I've had 50W (10kV @ 5 mA) out,
I think it will supply up to 100W but not yet tried due to lack of suitable load.
I'm in the process of building a HV test load/ballast,
so maybe in a couple of days I can answer that.
Thanks for the advice guys, but looks like I'll keep it open-loop for a while yet.
Re: Regulating Ignition Coil HV
Steve Conner, Mon Oct 02 2006, 10:11PM
Wow, 10kV @ 5mA is quite impressive for something you can get from a car junkyard. I always wondered if it was possible to make a useful DC power supply with an ignition coil, and now I suppose it is
What frequency are you driving it at?
Steve Conner, Mon Oct 02 2006, 10:11PM
Wow, 10kV @ 5mA is quite impressive for something you can get from a car junkyard. I always wondered if it was possible to make a useful DC power supply with an ignition coil, and now I suppose it is
What frequency are you driving it at?
Re: Regulating Ignition Coil HV
Marko, Mon Oct 02 2006, 10:19PM
After I saw how iggies take some heavy abuse and live I tought ''wouldn't it be cool tu use them as a compact power supply for TC's and other stuff''.
Blackplasma used to pump over 500W in two ignition coils, resulting with some killer arcs.
(50W isn't all that much).
When driven with some kind of bridge output voltage of iggie's is limited ('kicks' are rectified back to supply) but a lot more current can be pushed for same power in that case.
Since core is iron rod it will hardly saturate but it can't go for too high frequency.
If high enough power can be held for longer period of time such supply may become a cheap substitute for smaller NST's and obt's.
Enough running offtopic altohugh
Marko, Mon Oct 02 2006, 10:19PM
After I saw how iggies take some heavy abuse and live I tought ''wouldn't it be cool tu use them as a compact power supply for TC's and other stuff''.
Blackplasma used to pump over 500W in two ignition coils, resulting with some killer arcs.
(50W isn't all that much).
When driven with some kind of bridge output voltage of iggie's is limited ('kicks' are rectified back to supply) but a lot more current can be pushed for same power in that case.
Since core is iron rod it will hardly saturate but it can't go for too high frequency.
If high enough power can be held for longer period of time such supply may become a cheap substitute for smaller NST's and obt's.
Enough running offtopic altohugh
Re: Regulating Ignition Coil HV
Sulaiman, Tue Oct 03 2006, 02:23AM
At 50W the coil runs very slightly above ambient temperature,
box ventilation is by a 24V computer-type fan which is also part of the 'dropper' for the low voltage supply (two 5v6 5W zeners in series), the front panel filament bulb is also part of the 'dropper' and acts as a fuse too.
I'm sure I'll get more out of it, operating in flyback mode,
the biggest 'problem' is that due to primary inductance and low supply voltage
I'm operating just below 1 kHz - which is audible/annoying.
I quickly built the ht supply out of parts to hand, hence the ignition coil.
Within the next few weeks I hope to power a SGTC from this ht supply,
once I decide how to protect the 30kV diode.
Others have used Ignition Coils with air core
I wanted to try with a ferrite core but couldn't find a suitably sized core (in my collection)
The coupling between primary and secondary is much less than unity (not measured)
so it's less than ideal
but if you need a simple/cheap ht source it's ok.
It would be better if I'd incorporated output regulation though (staying on-topic!)
Sulaiman, Tue Oct 03 2006, 02:23AM
At 50W the coil runs very slightly above ambient temperature,
box ventilation is by a 24V computer-type fan which is also part of the 'dropper' for the low voltage supply (two 5v6 5W zeners in series), the front panel filament bulb is also part of the 'dropper' and acts as a fuse too.
I'm sure I'll get more out of it, operating in flyback mode,
the biggest 'problem' is that due to primary inductance and low supply voltage
I'm operating just below 1 kHz - which is audible/annoying.
I quickly built the ht supply out of parts to hand, hence the ignition coil.
Within the next few weeks I hope to power a SGTC from this ht supply,
once I decide how to protect the 30kV diode.
Others have used Ignition Coils with air core
I wanted to try with a ferrite core but couldn't find a suitably sized core (in my collection)
The coupling between primary and secondary is much less than unity (not measured)
so it's less than ideal
but if you need a simple/cheap ht source it's ok.
It would be better if I'd incorporated output regulation though (staying on-topic!)
Re: Regulating Ignition Coil HV
Simon, Tue Oct 03 2006, 03:40AM
In theory a spark gap might be able to do the job. Have a spark gap after your output resistance and put your load in parallel with the gap. Any of the more practical minded members have thoughts?
It wouldn't be efficient and it might not regulate all that well. Simple, though. :)
Simon, Tue Oct 03 2006, 03:40AM
In theory a spark gap might be able to do the job. Have a spark gap after your output resistance and put your load in parallel with the gap. Any of the more practical minded members have thoughts?
It wouldn't be efficient and it might not regulate all that well. Simple, though. :)
Re: Regulating Ignition Coil HV
Sulaiman, Tue Oct 03 2006, 04:29AM
I have a simple regulation method using a sample of the output voltage
to turn on an npn transistor which pulls the RESET pin low
but it's very crude causing the next ON priod to be longer than normal.
For SGTC open-loop will do so I'm abandoning output voltage control of a 555 invertor for now,
UNLESS someone has a 'cute' idea.
Sulaiman, Tue Oct 03 2006, 04:29AM
I have a simple regulation method using a sample of the output voltage
to turn on an npn transistor which pulls the RESET pin low
but it's very crude causing the next ON priod to be longer than normal.
For SGTC open-loop will do so I'm abandoning output voltage control of a 555 invertor for now,
UNLESS someone has a 'cute' idea.
Re: Regulating Ignition Coil HV
Marko, Tue Oct 03 2006, 01:28PM
SPark gap has a negative resistance and would likely form a relaxation oscillator with filter cap unless very big series resistor is used, and it would waste a lot of power. But would work too..
Measuring one typical iggy I have here, it has primary resistance of 3 ohms and secondary resistance of 7 kiloohms (wich is good considering 30kiloohms for 400VA obit I have here)
If driven properly those little thingies may make a nice high-power HVDC supply. Most worrying are actually core losses with iron. Could end on my projects list one nice day...
Marko, Tue Oct 03 2006, 01:28PM
Simon wrote ...
In theory a spark gap might be able to do the job. Have a spark gap after your output resistance and put your load in parallel with the gap. Any of the more practical minded members have thoughts?
It wouldn't be efficient and it might not regulate all that well. Simple, though. :)
In theory a spark gap might be able to do the job. Have a spark gap after your output resistance and put your load in parallel with the gap. Any of the more practical minded members have thoughts?
It wouldn't be efficient and it might not regulate all that well. Simple, though. :)
SPark gap has a negative resistance and would likely form a relaxation oscillator with filter cap unless very big series resistor is used, and it would waste a lot of power. But would work too..
Measuring one typical iggy I have here, it has primary resistance of 3 ohms and secondary resistance of 7 kiloohms (wich is good considering 30kiloohms for 400VA obit I have here)
If driven properly those little thingies may make a nice high-power HVDC supply. Most worrying are actually core losses with iron. Could end on my projects list one nice day...
Re: Regulating Ignition Coil HV
Sulaiman, Tue Oct 03 2006, 03:01PM
That 3 Ohm resistance is common for the lowest end of ignition coils;
better are the type that have an external resistor that's bypassed for cold-start
or ignition coils designed for electronic ignition.
My coil has a primary resistance of less than 1 Ohm when hot.
Sulaiman, Tue Oct 03 2006, 03:01PM
That 3 Ohm resistance is common for the lowest end of ignition coils;
better are the type that have an external resistor that's bypassed for cold-start
or ignition coils designed for electronic ignition.
My coil has a primary resistance of less than 1 Ohm when hot.
Re: Regulating Ignition Coil HV
Steve Conner, Tue Oct 03 2006, 03:33PM
You could try connecting that NPN transistor to the 555's control voltage pin (the one you're normally supposed to decouple to ground with a capacitor) instead of the reset pin.
I fancy trying this now since I've got all the parts except the ignition coil
It would make a great low-powered rugged HV supply for testing things with, and it would be easy to have switchable polarity for the output, by swapping the iggy primary terminals and turning the diode round.
Steve Conner, Tue Oct 03 2006, 03:33PM
You could try connecting that NPN transistor to the 555's control voltage pin (the one you're normally supposed to decouple to ground with a capacitor) instead of the reset pin.
I fancy trying this now since I've got all the parts except the ignition coil
It would make a great low-powered rugged HV supply for testing things with, and it would be easy to have switchable polarity for the output, by swapping the iggy primary terminals and turning the diode round.Re: Regulating Ignition Coil HV
Marko, Tue Oct 03 2006, 03:43PM
I already mentioned something similar few posts above, exscept I just directly used phototransistor from optocoupler, in place of one duty-cycle resistors.
Pin 5 could also work but it seems somehow limited in range.
Marko, Tue Oct 03 2006, 03:43PM
You could try connecting that NPN transistor to the 555's control voltage pin (the one you're normally supposed to decouple to ground with a capacitor) instead of the reset pin.
I already mentioned something similar few posts above, exscept I just directly used phototransistor from optocoupler, in place of one duty-cycle resistors.
Pin 5 could also work but it seems somehow limited in range.
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