Forums

class D amp

Started by John Larkin August 11, 2017
I want to build an AC source to test synchros/resolvers/RVDTs. The
target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate
aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz.

I'm thinking of using half of a TPA3251 audio amp and a step-up
transformer. Like this maybe:

https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1

The transformer DCR is low, so a blocking cap might be prudent to
prevent DC current in the primary. I have a cute little 1000 uF 2.5V
polymer cap in stock, so I could use a few of those in parallel. The
diodes keep the cap from exploding if the h-bridge ever rails.

How does that look? Any advice?

I've heard horror stories about "the devil's staircase" progressive
saturation, which I don't really understand. The transformer is a
toroid, low resistance and pretty dramatic saturation behavior.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On 08/11/2017 04:27 PM, John Larkin wrote:
> > I want to build an AC source to test synchros/resolvers/RVDTs. The > target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate > aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. > > I'm thinking of using half of a TPA3251 audio amp and a step-up > transformer. Like this maybe: > > https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 > > The transformer DCR is low, so a blocking cap might be prudent to > prevent DC current in the primary. I have a cute little 1000 uF 2.5V > polymer cap in stock, so I could use a few of those in parallel. The > diodes keep the cap from exploding if the h-bridge ever rails. > > How does that look? Any advice? > > I've heard horror stories about "the devil's staircase" progressive > saturation, which I don't really understand. The transformer is a > toroid, low resistance and pretty dramatic saturation behavior. > >
The caps will force the volt-seconds to average to zero, but not necessarily within a cycle. There may be some DC on the caps because once the average current in the core gets off zero, it'll walk in the direction of increasing magnetization. It does have to stop on account of the capacitor, but maybe there could be a bit of distortion as a result. Seems like you might get some subharmonics too. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message 
news:dj3sochc560sctdiur9h9vnm5jb8o5lnkv@4ax.com...
> > I want to build an AC source to test synchros/resolvers/RVDTs. The > target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate > aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. > > I'm thinking of using half of a TPA3251 audio amp and a step-up > transformer. Like this maybe: > > https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 > > The transformer DCR is low, so a blocking cap might be prudent to > prevent DC current in the primary. I have a cute little 1000 uF 2.5V > polymer cap in stock, so I could use a few of those in parallel. The > diodes keep the cap from exploding if the h-bridge ever rails. > > How does that look? Any advice? > > I've heard horror stories about "the devil's staircase" progressive > saturation, which I don't really understand. The transformer is a > toroid, low resistance and pretty dramatic saturation behavior. > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com >
How about using a VFD? Like this : http://www.ebay.com/itm/vfd-drive-vfd-110v-1-5kw-3hp-vfd-inverter-variable-frequency-drive-inverter-for/152622480735?_trkparms=aid%3D555014%26algo%3DPL.DEFAULT%26ao%3D1%26asc%3D41375%26meid%3D991b3a671908401c886fe47877d97a01%26pid%3D100506%26rk%3D1%26rkt%3D1%26&_trksid=p2045573.c100506.m3226 !20 VAC in, variable volts and frequency out and three phase also.
On Friday, August 11, 2017 at 2:00:36 PM UTC-7, tom wrote:
> "John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message > news:dj3sochc560sctdiur9h9vnm5jb8o5lnkv@4ax.com... > > > > I want to build an AC source to test synchros/resolvers/RVDTs. The > > target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate > > aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. > > > > I'm thinking of using half of a TPA3251 audio amp and a step-up > > transformer. Like this maybe: > > > > https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 > > > > The transformer DCR is low, so a blocking cap might be prudent to > > prevent DC current in the primary. I have a cute little 1000 uF 2.5V > > polymer cap in stock, so I could use a few of those in parallel. The > > diodes keep the cap from exploding if the h-bridge ever rails. > > > > How does that look? Any advice? > > > > I've heard horror stories about "the devil's staircase" progressive > > saturation, which I don't really understand. The transformer is a > > toroid, low resistance and pretty dramatic saturation behavior. > > > > > > -- > > > > John Larkin Highland Technology, Inc > > picosecond timing precision measurement > > > > jlarkin att highlandtechnology dott com > > http://www.highlandtechnology.com > > > > How about using a VFD? Like this : > > http://www.ebay.com/itm/vfd-drive-vfd-110v-1-5kw-3hp-vfd-inverter-variable-frequency-drive-inverter-for/152622480735?_trkparms=aid%3D555014%26algo%3DPL.DEFAULT%26ao%3D1%26asc%3D41375%26meid%3D991b3a671908401c886fe47877d97a01%26pid%3D100506%26rk%3D1%26rkt%3D1%26&_trksid=p2045573.c100506.m3226 > > !20 VAC in, variable volts and frequency out and three phase also.
Cute idea; an idling motor (or better, motor with a flywheel) would be a really fine low-Z filter, too. The old days (1930-ish) before 'electronics' got well established, saw motor/disk/generator audio generators as standard off-the-shelf items. You'd tune 'em by moving the driven wheel from rim to center of the drive disk.
On Fri, 11 Aug 2017 16:54:45 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 08/11/2017 04:27 PM, John Larkin wrote: >> >> I want to build an AC source to test synchros/resolvers/RVDTs. The >> target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate >> aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. >> >> I'm thinking of using half of a TPA3251 audio amp and a step-up >> transformer. Like this maybe: >> >> https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 >> >> The transformer DCR is low, so a blocking cap might be prudent to >> prevent DC current in the primary. I have a cute little 1000 uF 2.5V >> polymer cap in stock, so I could use a few of those in parallel. The >> diodes keep the cap from exploding if the h-bridge ever rails. >> >> How does that look? Any advice? >> >> I've heard horror stories about "the devil's staircase" progressive >> saturation, which I don't really understand. The transformer is a >> toroid, low resistance and pretty dramatic saturation behavior. >> >> > >The caps will force the volt-seconds to average to zero, but not >necessarily within a cycle. There may be some DC on the caps because >once the average current in the core gets off zero, it'll walk in the >direction of increasing magnetization. It does have to stop on account >of the capacitor, but maybe there could be a bit of distortion as a result. > >Seems like you might get some subharmonics too. > >Cheers > >Phil Hobbs
I wouldn't normally expect enough voltage drop across the caps to turn the diodes on, unless something goes wrong. I could use a shorted bridge rectifier and get two junction drops in each direction. That TI part has a max output DC offset of 60 mV, which is pretty good, but that's still about 1 amp into my toroid. It also has a shutdown if it sees DC load current, 1.5 amps, which is pretty bad. The data sheet is medium terrible. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Den l&oslash;rdag den 12. august 2017 kl. 00.12.59 UTC+2 skrev John Larkin:
> On Fri, 11 Aug 2017 16:54:45 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >On 08/11/2017 04:27 PM, John Larkin wrote: > >> > >> I want to build an AC source to test synchros/resolvers/RVDTs. The > >> target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate > >> aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. > >> > >> I'm thinking of using half of a TPA3251 audio amp and a step-up > >> transformer. Like this maybe: > >> > >> https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 > >> > >> The transformer DCR is low, so a blocking cap might be prudent to > >> prevent DC current in the primary. I have a cute little 1000 uF 2.5V > >> polymer cap in stock, so I could use a few of those in parallel. The > >> diodes keep the cap from exploding if the h-bridge ever rails. > >> > >> How does that look? Any advice? > >> > >> I've heard horror stories about "the devil's staircase" progressive > >> saturation, which I don't really understand. The transformer is a > >> toroid, low resistance and pretty dramatic saturation behavior. > >> > >> > > > >The caps will force the volt-seconds to average to zero, but not > >necessarily within a cycle. There may be some DC on the caps because > >once the average current in the core gets off zero, it'll walk in the > >direction of increasing magnetization. It does have to stop on account > >of the capacitor, but maybe there could be a bit of distortion as a result. > > > >Seems like you might get some subharmonics too. > > > >Cheers > > > >Phil Hobbs > > I wouldn't normally expect enough voltage drop across the caps to turn > the diodes on, unless something goes wrong. I could use a shorted > bridge rectifier and get two junction drops in each direction. > > That TI part has a max output DC offset of 60 mV, which is pretty > good, but that's still about 1 amp into my toroid. It also has a > shutdown if it sees DC load current, 1.5 amps, which is pretty bad.
the filter inductors must also add some DCR how about using a stepper motor driver?
On 08/11/17 20:27, John Larkin wrote:
> > I want to build an AC source to test synchros/resolvers/RVDTs. The > target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate > aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. > > I'm thinking of using half of a TPA3251 audio amp and a step-up > transformer. Like this maybe: > > https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 > > The transformer DCR is low, so a blocking cap might be prudent to > prevent DC current in the primary. I have a cute little 1000 uF 2.5V > polymer cap in stock, so I could use a few of those in parallel. The > diodes keep the cap from exploding if the h-bridge ever rails. > > How does that look? Any advice? > > I've heard horror stories about "the devil's staircase" progressive > saturation, which I don't really understand. The transformer is a > toroid, low resistance and pretty dramatic saturation behavior. > >
Keep it simple. For that sort of voltage output, any audio amplifier with suitable output swing should get the job done, and distortion will be insignificant at that sort of frequency. Either that, or find one of the variable output lab ac supplies from Ebay or similar. Do you want to design it, or use it :-)... Chris
On Saturday, August 12, 2017 at 6:27:25 AM UTC+10, John Larkin wrote:
> I want to build an AC source to test synchros/resolvers/RVDTs. The > target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate > aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. > > I'm thinking of using half of a TPA3251 audio amp and a step-up > transformer. Like this maybe: > > https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 > > The transformer DCR is low, so a blocking cap might be prudent to > prevent DC current in the primary. I have a cute little 1000 uF 2.5V > polymer cap in stock, so I could use a few of those in parallel. The > diodes keep the cap from exploding if the h-bridge ever rails. > > How does that look? Any advice? > > I've heard horror stories about "the devil's staircase" progressive > saturation, which I don't really understand. The transformer is a > toroid, low resistance and pretty dramatic saturation behavior.
250mH would resonate with 0.63uf at 400Hz, 1.62uF at 250Hz. The transformer shown isn't centre-tapped so it doesn't lend itself to the usual Baxandall oscillator scheme, or my low distortion variant of it, but the voltage-switching variant - Figure 8 in the original paper - might work http://sophia-elektronica.com/0344_001_Baxandal.pdf That sort of scheme isn't susceptible to progressive saturation. Because the core permeability does vary with flux, you can get significant harmonic content from that alone, which would be less of a problem if you were using the transformer purely to step up a lower voltage output. I did put together a current-switched low-distortion quasi-Baxandall driver for LVDT in the - extremely precise weighing head of the Metals Research GaAs crystal puller back in 1987-88. It worked fine. -- Bill Sloman, Sydney
On Fri, 11 Aug 2017 15:33:02 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>Den l&#2013266168;rdag den 12. august 2017 kl. 00.12.59 UTC+2 skrev John Larkin: >> On Fri, 11 Aug 2017 16:54:45 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >On 08/11/2017 04:27 PM, John Larkin wrote: >> >> >> >> I want to build an AC source to test synchros/resolvers/RVDTs. The >> >> target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate >> >> aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. >> >> >> >> I'm thinking of using half of a TPA3251 audio amp and a step-up >> >> transformer. Like this maybe: >> >> >> >> https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 >> >> >> >> The transformer DCR is low, so a blocking cap might be prudent to >> >> prevent DC current in the primary. I have a cute little 1000 uF 2.5V >> >> polymer cap in stock, so I could use a few of those in parallel. The >> >> diodes keep the cap from exploding if the h-bridge ever rails. >> >> >> >> How does that look? Any advice? >> >> >> >> I've heard horror stories about "the devil's staircase" progressive >> >> saturation, which I don't really understand. The transformer is a >> >> toroid, low resistance and pretty dramatic saturation behavior. >> >> >> >> >> > >> >The caps will force the volt-seconds to average to zero, but not >> >necessarily within a cycle. There may be some DC on the caps because >> >once the average current in the core gets off zero, it'll walk in the >> >direction of increasing magnetization. It does have to stop on account >> >of the capacitor, but maybe there could be a bit of distortion as a result. >> > >> >Seems like you might get some subharmonics too. >> > >> >Cheers >> > >> >Phil Hobbs >> >> I wouldn't normally expect enough voltage drop across the caps to turn >> the diodes on, unless something goes wrong. I could use a shorted >> bridge rectifier and get two junction drops in each direction. >> >> That TI part has a max output DC offset of 60 mV, which is pretty >> good, but that's still about 1 amp into my toroid. It also has a >> shutdown if it sees DC load current, 1.5 amps, which is pretty bad. > >the filter inductors must also add some DCR > >how about using a stepper motor driver? > > > >
Driving the toroid from a 50 ohm fungen, at 400 Hz, it only takes 10s of mA of DC to make serious distortion. It will be better driven from a low-Z source, but I'm thinking that DC is bad. https://www.dropbox.com/s/0bl6jdkw4ljx4e2/400_Hz_Toroid.JPG?raw=1 The TI part has a lot of nice features and protections. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Sat, 12 Aug 2017 00:01:11 +0000, Chris <xxx.syseng.yyy@gfsys.co.uk>
wrote:

>On 08/11/17 20:27, John Larkin wrote: >> >> I want to build an AC source to test synchros/resolvers/RVDTs. The >> target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate >> aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. >> >> I'm thinking of using half of a TPA3251 audio amp and a step-up >> transformer. Like this maybe: >> >> https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 >> >> The transformer DCR is low, so a blocking cap might be prudent to >> prevent DC current in the primary. I have a cute little 1000 uF 2.5V >> polymer cap in stock, so I could use a few of those in parallel. The >> diodes keep the cap from exploding if the h-bridge ever rails. >> >> How does that look? Any advice? >> >> I've heard horror stories about "the devil's staircase" progressive >> saturation, which I don't really understand. The transformer is a >> toroid, low resistance and pretty dramatic saturation behavior. >> >> > >Keep it simple. For that sort of voltage output, any audio amplifier >with suitable output swing should get the job done, and distortion >will be insignificant at that sort of frequency. Either that, or find >one of the variable output lab ac supplies from Ebay or similar. > >Do you want to design it, or use it :-)... > >Chris
Use it and maybe sell it if it works nice. It would complement our synchro/LVDT boards, which don't output much power. There aren't many low-power 400 Hz sources around. 1 KVA is small in this world. -- John Larkin Highland Technology, Inc lunatic fringe electronics