Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Closed Thread
Thread Tools Search this Thread
Old 15th December 2004, 01:03 PM   #1
diyAudio Member
Workhorse's Avatar
Join Date: Dec 2004

Hi everybody,
Recently I came across bassbins fitted 18" Peavey Lowrider speakers along with Passive 24dB/octave linkwitz - Riley passive crossovers.
Our further experiments shows that there is lot of Reactive energy [due to passive crossover]fedback to amplifier, thus turning the overcurrent protection ON without any coniderable power output to load.The normal trip level of protection sensing was 28A peak, but the measurement shows 40A peak of destructive current was present without the protection circuit involved.
This event triggers us to think a new way of activation of shortcircuit protection involved than using traditional approach of sensing overcurrent through a series resistor.

We want to propose a new idea of protection which includes :

1 Sensing the gate to source voltage overdrive during overcurrent conditions, then limit the gate drive or if necessity arises even mutes/limits the input which also avoid harsh clip conditions.

2. A comparator scheme which involves:Sensing the input voltage at the input of amp, sensing the corresponding voltage at output of amp. If the input voltage is present and output voltage is not present, which clearly indicates the conditions of short-circuited output, the the comparator latches or activates the mute circuitry, thus protects the amp.

Kindly , give your valuable suggestions so that we can further continue the discussions involved and a new project will be started soon involving various parameters to be catered along.

Old 15th December 2004, 01:28 PM   #2
diyAudio Member
Workhorse's Avatar
Join Date: Dec 2004
Arrow Proposed Idea

Hi Everyone

Here are the simple image illustrations involved.

Attached Images
File Type: jpg combo.jpg (32.4 KB, 1554 views)
Old 15th December 2004, 02:35 PM   #3
diyAudio Member
sivan_and's Avatar
Join Date: Dec 2004
Location: B'lore/ Calicut India
Send a message via Yahoo to sivan_and
Limiting MOSFET GATE :
I WOULD suggest a low current,high voltage mosfet as the sense element fed thru a zener, which will conduct as soon as the drive level exceeds a pre set level,The Mosfet should switch an optocoupler for isolation,the output of Optocoupler should turn on a small signal FET connected across the input of amplifier.Of course the amplifier input should have a resister in series with the signal sourse.This resistor and FET will form an attenuator or even mute.
Old 15th December 2004, 03:14 PM   #4
diyAudio Member
sivan_and's Avatar
Join Date: Dec 2004
Location: B'lore/ Calicut India
Send a message via Yahoo to sivan_and
See the circuit...The fet wrongly shown as P-channel>>N-channel.
It would have been easier if source resisters are present...Read else where that u dislike the idea of source resisters..
Attached Images
File Type: jpg over drive preventer.jpg (94.8 KB, 1346 views)
Old 15th December 2004, 03:48 PM   #5
diyAudio Member
Join Date: Aug 2003
Location: Prague,Czech Republic
Hi ampman, your contemplation is uncorrect : every amp have phase shift, which bring to the comparator circuit big fault. Also is uncorrect to evaluate input vs output voltage - how will be function of this circuit, when will be any signal on input ? In mosfet amp must be current limiting circuits quite the same as in bipolar amp, but must have still " zener " protection for overvoltage on gates, that's all
Old 15th December 2004, 04:24 PM   #6
gmarsh is offline gmarsh  Canada
diyAudio Member
gmarsh's Avatar
Join Date: Apr 2004
Location: Halifax, NS
For "better" current limit protection, i'd probably go with some sort of multistage approach:

- keep the existing super-fast current protection, which limits instantaneous peaks. Make it less sensitive than before - keep it as an "oh @#$^" circuit that will perform instantaneous protection from dead shorts across the output.

- add a second, similar circuit which has a lower current-limit threshold. But low-pass filter the current sample going into it, so that it doesn't act as fast. This protects from low-resistance loads that can damage the amp from sustained high current, but shouldn't clamp down on a quick current spike coming from a funny crossover reactance.

As for the implementation, that's an exercise left to the reader

I've designed RF amplifier protection circuits that work a similar fashion, and they work well.
Old 15th December 2004, 06:19 PM   #7
diyAudio Member
darkfenriz's Avatar
Join Date: Jun 2004
Location: Warsaw
hallo amp_man1
as Upupa Epops said amp has a phase shift, but there are also other problems with simple compairing: amplitude distortion('filtering'), overdriving (even slight), you also should know EXACTLY the voltage amplification factor.

for me it is much simpler (maybe I am missing something):
protection shuold turn on when
1.current via source resistor is to big
AND (even ttl or cmos and)
2. output voltage is zero (null, ground, gnd...)
Old 15th December 2004, 11:14 PM   #8
Eva is offline Eva  Spain
diyAudio Member
Eva's Avatar
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
This reactive overcurrent effect happens when passive crossovers :

- Are not designed properly
- Have capacitors failing in an intermittent short due to excessive voltage
- Have iron core coils saturating due to excessive current
- Have any component with bad solder joints losing contact intermintently
- Have any of their outputs not loaded with a suitable impedance or not loaded at all
- Include impedance compensation for a particular loudspeaker but this driver is not the one used
- Etc..

So I would rather not waste my time trying to make my amplifiers capable of driving a defective passive crossover. I see it pointless. When passive crossovers are working properly there are no reactive impedance drops

Also, loudspeakers only are reactive at frequencies where they show high impedances, so if the problem persists without the passive crossover then the loudspeaker is defective [probably shorted turns on the voice coil]

Your protection is already optimum enough. It trips just because the load is defective, so fix the load instead of the amplifier

Passive crossovers for subs are very critical, that's why almost nobody uses them
Old 16th December 2004, 02:07 PM   #9
diyAudio Member
Workhorse's Avatar
Join Date: Dec 2004
Arrow Reply

Hi everybody,

ThanX all of you for your valuable suggestions.

I myself feel that the comparator scheme is not good as there is some amount of phase shift still present at output.
Hafler also uses this comparator scheme but he senses the drive signal and output instead of input signal.

Now I will work on following aspects:

1 Gate overdrive voltage,
Overdrive on gate will be dealt with appropriate design.

Sivanand , your suggestion regarding optocoupler is good but it is not as fast because optocouplers are slow.

2 Traditional method of sensing current through resistor is still good but there must be some tradeoff value for peak current requirements for limiting.

your suggestion is absolutely right, but some customers will never want to put blame on their speakers rather they blame the amps for not driving them properly.

Any more suggestions are most welcomed

Old 16th December 2004, 02:28 PM   #10
diyAudio Member
sivan_and's Avatar
Join Date: Dec 2004
Location: B'lore/ Calicut India
Send a message via Yahoo to sivan_and
amp_man_1 !!

Why u need high speed !! Opto's are usable upto 50Khz.(Your not in the RF arena).I 've tried a current limit scheme in a UPS system having 25KVA it was a PWM sinewave output. With Mosfets, I noticed the system would trip whenever a small surge appears the output of UPS,then i deliberately slowed down the response.The Mosfets can and will sustain short duration overloads w/o damage.

Closed Thread

Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
short-circuit protection in mosfet's amp...?? csl113 Solid State 8 31st January 2005 06:10 PM

New To Site? Need Help?

All times are GMT. The time now is 11:36 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2