PWM Circuit for multiple heaters

You have bypass caps...right?

And, do you really have three individual RECOM thingers? Those aren't cheap.

You should use all the pins provided on the gate driver, even if they're the same function. The chip expects low resistance between equal pins, and if it has to draw current through the die instead, bad things can happen.

High side? Where?

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

I'm trying to make 3 PWM circuits to control 3 different thermoelectric coolers (peltier cooler)

I'm aware that ideally you shouldn't run these with a PWM, that's fine, and I'm only using these in one direction, only as a heater.

So I made a simple PWM, with a Linear Tech. PWM part, a high side FET driver and a FET.

and the thing works ok for 1 heater, but once I plug three in... it goes bananas. Basically it looks like the PWM signal from one heater couples on to my rails somehow and then starts to impact the other heaters... I'm not exactly sure if that's what's happening, but they defeintely don't play nice together, once I plug two in the PWM goes all out of what, bascially becomes super sensitive and almost floats up to 100% duty cycle when I touch the potentiometer

My circuit is on the link below

My circuit is that image 3 times, all common grounds throughout

I've thought about

1. Increasing my gate resistor to the FET
2. Instead of taking my control signal from the rail take it from a voltage reference (TLV431)
3. Increase my input voltage control from 1V to 5V (it's an option on the linear chip)
4. I don't want to, but maybe add isolated supplies?
5. OR maybe I should buffer my control signal?

much thanks!

I'm trying to make 3 PWM circuits to control 3 different thermoelectric coolers (peltier cooler)

I'm aware that ideally you shouldn't run these with a PWM, that's fine, and I'm only using these in one direction, only as a heater.

So I made a simple PWM, with a Linear Tech. PWM part, a high side FET driver and a FET.

and the thing works ok for 1 heater, but once I plug three in... it goes bananas. Basically it looks like the PWM signal from one heater couples on to my rails somehow and then starts to impact the other heaters... I'm not exactly sure if that's what's happening, but they defeintely don't play nice together, once I plug two in the PWM goes all out of what, bascially becomes super sensitive and almost floats up to 100% duty cycle when I touch the potentiometer

My circuit is on the link below

My circuit is that image 3 times, all common grounds throughout

I've thought about

1. Increasing my gate resistor to the FET
2. Instead of taking my control signal from the rail take it from a voltage reference (TLV431)
3. Increase my input voltage control from 1V to 5V (it's an option on the linear chip)
4. I don't want to, but maybe add isolated supplies?
5. OR maybe I should buffer my control signal?

much thanks!

++++++++++++++++++++++++++++

Do you have a scope? have you looked at the rail(s)?

Is there only one bypass capacitor in the circuit? What is it?

Is the driver pulling down the low power regulator?

What are you using for a bread board? Take a picture of your setup and post it.

tm

Are you sure that you can leave the output power unconnected? What's powering the output of the gate driver?

I put 470uF across the rails, it didn't help. It was on a solderless breadboard, but I got frustrated and took it all apart, I'm going to redo it to try and locate the issue.

Yeah the gate driver is hooked up correctly, those pins are connected inside of it

The solution is simple. Take those cheapskate figners out of the pocket, pu lling out your wallet with the credit card. Call Digikey and buy some coils , caps and diodes. Problem solved. Hundred kHz ? Little low but you can pro bably get good coils for like two buck apiece. Caps, half a buck, diodes ev en less. We are talking a BOM here of about twenty bucks, and that's if you splurge.

Well actually at 1 kHz it might be mnore for the coils. Why not just switch them faster ? Any transistor on the market today that cannot effectively P WEWM at 10 kHz shouldn't be on the market today.

Maybe $25.

Three separate channels ? I had assumed that - am I wrong ?

Either way, it is most likely a ground loop problem.

And those chips, I guess they're OK but us oldtimers used to do shit ike that with three fifty cent trsansistors and a power transformer out of an old TV set.

Just a matter of selecting thos varibales.

... You have bypass caps...right?"

You cannot just do that without sticking some inductance in there.

It has not to do with the Peltier, it has to do with the silivcon that must PWM that power.

inductors, caps, diodes? for what? filter out the PWM? do some kind of buck converter thing?

Tim, just 1 recom, I mis-spoke, no high side FET here... i'll externally connect the internally connected stuff

You still want to connect all of the pins, *particularly* on a solderless breadboard (such a kludge may never work properly). They're there for a reason.

Here's my entry to the guess the circuit problem game.

Adding extra heaters increased current through your ground setup. Ground floating higher messed up programming of fancy PWM chip. Ground floating higher messed up min output voltage of fancy FET driver chip and low output fails to go below transistor Vth.

To test the idea, beef up the ground.

I would have put in bypass caps too, but someone else already spoke for that entry.

Why you need to switch so fast for heater control?

ChesterW

"Fibo"

My circuit is on the link below

My circuit is that image 3 times, all common grounds throughout

I've thought about

1. Increasing my gate resistor to the FET

** Try making it 10kohms -

the FET make get a bit warmer, but switching transients & noise will be below the AM radio band.

.... Phil

"Neon John"

** What do you think the impedance of that electro is at say 100kHz & 1MHz ? ** Plus a few electros too.

.... Phil

you might try wiring each PWM circuit AND its associated heater with a single point ground and then tie the individual grounds together.

ground current interactions probably your problem

Mark

Three problems are visible. First, you can use a triangle generator and three comparators to perform this function: your hardware is needlessly expensive and single-sourced. Second, 'high side FET driver' is unnecessary: a biased transistor, or just a wire, would work fine. Third, your modulated current is drawn from a "+12V" supply at high frequency, with no clear current limiting. Are you sure your supply is handling it?

** Or just an NE555.

** Huh ? It's 1kHz.

... Phil

My thought was directed at three independently adjustable units, so 3 NE555 or one LM339...

No current limit, and hard-switching the MOSFET means all the dI/dt of the sum of harmonics ( 1 kHz being the lowest). If this is a lead/acid battery, it'll probably hold up. If it's a linear regulator it probably won't. Loss of +12 power resets the +5 logic...

** Wrong thought - fool. ** Gobbledegook.

Put an 1000uF electro across the PSU and you have it.

.... Phil

and three comparators to perform this function:

frequency,

The idea was not clearly expressed, but a 1kHz PWM waveform could be a 1kHz string of narrow spikes, whose Fourier transform includes all the harmonic s of 1kHz up to a limit set by the rise- and fall-times of the spikes.

If the OP had enough sense to put a bit of inductance in series with his Pe ltier elements (which is a very good idea) there wouldn't be as much high-f requency current circulating in his power leads to radiate noise to screw u p the other bits of PWM-generating circuitry

It would help, but it isn't always the whole solution.

Sloman A.W., Buggs P., Molloy J., and Stewart D. "A microcontroller-based d river to stabilise the temperature of an optical stage to 1mK in the range

4C to 38C, using a Peltier heat pump and a thermistor sensor" Measurement S cience and Technology, 7 1653-64 (1996).

does talk about the problems involved and describes an over-kill solution.

E-mail me at snipped-for-privacy@ieee.org if you want a reprint.

--
Bill Sloman, Sydney

coolers (peltier cooler)

and I'm only using these in one direction, only as a heater.

driver and a FET.

bananas. Basically it looks like the PWM signal from one heater couples on to my rails somehow and then starts to impact the other heaters... I'm not exactly sure if that's what's happening, but they defeintely don't play nice together, once I plug two in the PWM goes all out of what, bascially becomes super sensitive and almost floats up to 100% duty cycle when I touch the potentiometer

voltage reference (TLV431)

the linear chip)

Since i have made this kind of error i must ask. Have you hooked up one of your Peltiers backwards?

?-/