Regulating Ignition Coil HV / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Regulating Ignition Coil HV

1 2 next

Move Thread

LAN_403

Sulaiman

Sun Oct 01 2006, 11:43PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

My 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?

Wilson

Mon Oct 02 2006, 02:18AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

Resistive divider with capacitative compensation?

Steve Conner

Mon Oct 02 2006, 10:01AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

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?

Marko

Mon Oct 02 2006, 11:00AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

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..

Sulaiman

Mon Oct 02 2006, 02:15PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

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.

Steve Conner

Mon Oct 02 2006, 10:11PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

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?

Marko

Mon Oct 02 2006, 10:19PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

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

Sulaiman

Tue Oct 03 2006, 02:23AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

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!)

Simon

Tue Oct 03 2006, 03:40AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

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. :)

Sulaiman

Tue Oct 03 2006, 04:29AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

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.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob