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Date: Mon, 10 Jan 2011 10:17:18 -0500
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From: Warren Ziegler <wd2xgj@gmail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Capacitor warning and First 500kHz Class E breadboard
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Andy,

    hams around here use multi-layer ceramic caps for high power
class-E service, specifically the ATC series caps from American
Technical Ceramics:
http://www.atceramics.com/products/multilayer_capacitors.asp

Very stable, high current (single ATC100C will handle 12 amps), and
small in size!
--=20
73 Warren K2ORS
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 WD2XGJ
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 WD2XSH/23
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 WE2XEB/2
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 WE2XGR/1



On Mon, Jan 10, 2011 at 9:22 AM, Andy Talbot <andy.g4jnt@gmail.com> wrote:
> A warning to anyone contemplating making power amps - especially class=
 E
> types.
> Don't even think about using the small high voltage ceramic caps availal=
ble
> at low cost from the suppliers - and sold as "suitable for SMPSUs etc"=
=A0=A0in
> resonant circuits
> They are appalling, terrible, revolting things.=A0 You only have to brea=
th on
> them and they change their value.wildly
>
> These=A0things=A0 =A0 http://www.g4jnt.com/Download/UselessCaps.jpg
>
> I've just =A0made up a first version of a breadboard for a 500kHz Class=
 E
> amplifier, (target design=A0500W from 50 V supply),=A0 calculated the L/=
C values
> as per Alan's spreadsheet,=A0combined with an L matched output for 50 oh=
m.
> For the capacitors I aimed to get within 5% of the calculated values usi=
ng
> many of these small ceramic objects=A0in parallel - lots in || should sh=
are
> the current, shouldn't they, which would be good?
>
> So I soldered them up - used the G4HUP L/C meter to measure the values=
 -=A0at
> which point I=A0should have been a bit suspicious as the measured C was
> sometimes a fair bit lower than it should have been.=A0 At the time I ju=
st put
> it down to "finger trouble" - one does tend to ignore little things like
> that sometimes.
> Anyway, made up the amp, used a 12V supply rail and monitored the switch=
ing
> waveforms, expecting to see something vaguely approaching the half sine=
 on
> drains etc.
>
> Results, terrible, nowhere near what they should have been - clearly sum=
mat
> was very wrong=A0 I remeasured the || combination of the caps - after
> unsoldering=A0they were sometimes as much as half the value they should=
 have
> been=A0 - until they cooled right down and approached room temp. Occasio=
nally
> a cap value approaching correct would show up on the meter once in a
> while=A0-=A0but that was=A0was probably just coincidence :-)
> And yes, I did double, triple and quadruple check the meter against good
> caps - it was OK every time.
>
> These little blue things appear to have a huge temperature coefficent -=
 are
> completely useless, designed to do nothing of value, and all have now go=
ne
> into the waste bin.
> In fact, I was wondering if they were random-capacitors.
>
> I did have a=A0just two=A0trusted 22nF caps in the junk box that were wi=
thin 20
> - 30% of the right values for the amp and an awful lot of 3.3nF 1700V th=
ings
> of a more respectable heritage, so just put these into the circuit, test=
ed
> at 12V and got waveforms almost correct.=A0=A0 Increased drive freq to=
 515kHz
> (no real scope for changing cap values at this stage, so freq change wou=
ld
> be easier to try with) where the waveform looked best, and=A0increased=
 supply
> volts.=A0=A0=A0=A0=A0 Got to 30V and it was delivering 150 Watts; 40V an=
d was getting
> around 350 Watts at which point=A0I=A0assumed it was going to work,=A0so=
 switched
> off for a break.=A0=A0=A0Wasn't going to tempt fate by gong to the full=
 50V having
> not even bothered to tune things up properly
> A picture of that breadboard can be seen at
> http://www.g4jnt.com/Download/ClassE_BrdBd01.jpg
>
> The yellow cap shunting the device was running=A0quite warm - which is=
 hardly
> surprising just a single poly[whatever] carrying all those amps - and th=
e
> one in series with the tank was slightly warm.=A0=A0=A0 So, now need to=
 browse the
> Farnell catalogue (the cubic printed paper=A0version is easier) for some=
 caps
> that will do the job properly - like more of those yellow ones.=A0 Actua=
lly,
> may have enough of the 3.3nF ones, but that would end up with lots in
> parallel
>
> So, for anyone who is contemplating a class E amplifier design:
>
> Get close to the calculated component values and it will probably
> mostly-work immediately.
> Play with the drive frequency while monitoring waveforms =A0to see which=
 way
> tuning needs to go
> Don't use cheap high voltage ceramic capacitors
> Don't use cheap high voltage ceramic capacitors
>
> They would work well in a random number generator tho.....
>
> Andy
> www.g4jnt.com
>
>
>