help with heater circuit schmatics - Page 1
Forum section: electronics / beginners

author: AndyNZ
author_rank: Contributor
timestamp: August 23, 2021, 12:55:38 am
content: Hi all,I'm an electronics noob, my background is chemistry, so please be kind :-).I've got a heater circuit with the following schematics:It's on a thick film PCB attached to an aluminium block and it's used to heat the block to a temperature in the 30 to 60 degC range quite precisely (tolerance better 0.05 degC).It is driven by around 25 V DC supply voltage and some control signals from an external instrument, and I was hoping I can use the hardware to build a heating block to calibrate thermo-couple thermometers with the help of a reference thermometer. The hardware has an NTC thermistor attached as temperature readout (not shown in schematic), and I'd like to control it all from an arduino nano using the PID library. For the temperature readout I'll probably need an external ADC, as I gather the built in 10 bit arduino ADC is not precise enough.I'd like to use the existing heater if possible, but don't have access to the control signals. From what I read in the schematics, V2 is the positive supply voltage, V3 ground, and V4 and V5 are the control signals.My questions now is: what kind of control signals is it expecting? The use of BC transistors suggests to me that it may require an analogue signal rather than PWM, but I don't understand why two inputs seem to be necessary.Any help is much appreciated.Cheers,Andreas

author: mc172
author_rank: Frequent Contributor
timestamp: August 23, 2021, 01:30:42 am
content: V4 is current feedback, used to measure the total current through Q1-Q5. This current is not measured directly but is measured as a voltage across a known resistance, in this case 1 Ohm: R11. The fact that it's 1 Ohm makes the maths easy: all you have to do is measure the voltage across this resistor and you get the total heater current (plus a little bit from the driver (Q2)). The scaling is 1 volt per Amp because of Ohms Law: I = V/R. If R = 1, you can hopefully see where this is going.Therefore V4 is not an input, it's an output. You can use it to measure current with a 1:1 scale, V:I. If you don't use it, nothing adverse happens, you just don't know how much current is flowing. That might be OK, you might not care.I'm not sure what you mean about using "BC transistors" and "analogue". NPN transistors are NPN transistors. Grounding V5 turns Q2-Q5 off, thus turning the heater off. Letting it float (in other words, failing to ground V5) turns them on.

author: AndyNZ
author_rank: Contributor
timestamp: August 23, 2021, 02:17:20 am
content: Excellent, thanks so much for your help.Quote from: mc172 on August 23, 2021, 01:30:42 amI thought that it may regulate the heater current / power according to the input voltage applied, not like an on/off regulation. To me it's unexpected that the heater doesn't seem to require an input signal to turn on, as this doesn't seem to be fail safe, but with the 20W power or so I suppose it's inherently safe anyway.Thanks again for your help.

author: bob91343
author_rank: Super Contributor
timestamp: August 23, 2021, 02:30:26 am
content: The control signal can be a square wave for digital operation or it can be an analog signal. Current limiting is provided by the diode connected transistor. The total current will be less than 1 Ampere. The 3 Ohm resistors help balance the currents of the main transistors. The control circuit is not shown.

author: AndyNZ
author_rank: Contributor
timestamp: August 23, 2021, 02:48:15 am
content: Thanks for your reply. So I should be able to use a PWM signal from the arduino to control the heater power? Will there be a problem if I connect V5 directly to a PWM output on the arduino? I was hoping to use an old laptop power supply (19.5V DC) for the heater and a 7805 regulator to provide 5V to the arduino.

author: WattsThat
author_rank: Frequent Contributor
timestamp: August 23, 2021, 03:34:48 am
content: An Arduino Nano would be fine, the circuit only requires about 5 ma drive and an atmega328 output can provide 20 ma without breaking a sweat.As for the 10 bit analog resolution, yes that will be a problem for 0.05 degrees. Assuming a range of 0-100 degrees, at least 14 bits of resolution would be required. The external 16 bit ADS1115 breakout boards might be a good starting point, depending on the sensor and input circuitry.

author: AndyNZ
author_rank: Contributor
timestamp: August 23, 2021, 04:01:54 am
content: Thanks again. I've been trying to read up on arduino output pins, and it looks like they don't like over-voltage above 5.5V, but I guess it depends on the current going through it? If V5 is tied to +20V via the 12K pull-up resistor R1, does this mean I'd kill the output pin if I connect it directly? The max current would be 1.7 mA, so should be safe, correct?

author: WattsThat
author_rank: Frequent Contributor
timestamp: August 23, 2021, 04:40:21 am
content: Oops, my bad. R1 is a problem. Not sure now if you can drive that with an AVR output pin directly. I didn’t spend enough time looking at that circuit, not what I originally thought. Do you have any idea what the original PWM frequency is? Or is it in fact analog controlled?Every pin on the processor (with perhaps the exception of the reset pin) has two diodes connected to them, one to VCC, one to gnd, for ESD protection. That’s why you see the absolute voltage limits listed for the pins. Raise the pin voltage more than 0.3-0.4 volts above the supply voltage (or less than ground) and those diodes begin to conduct, passing current into the supplies.Connecting an output pin to V5 will cause about 2ma to flow from the +24 volt supply through the ESD diode on the AVR output pin which is at least double what Atmel says is acceptable, IIRC.

author: AndyNZ
author_rank: Contributor
timestamp: August 23, 2021, 07:01:16 am
content: Quote from: WattsThat on August 23, 2021, 04:40:21 amSorry, I don't know how it was driven originally, and don't have access to the control signal to look.I've tried 5V on V5 from my lab power supply, and that didn't reduce the heater current, it's about 0.47A at 20V supply voltage. If I tie V5 to ground the heater current drops to pretty much zero.Now I just have to figure out a safe way to drive this from the arduino. Would dropping the voltage between V5 and the arduino PWM pin by means of a voltage divider work?

author: AndyNZ
author_rank: Contributor
timestamp: August 24, 2021, 10:52:42 pm
content: So the voltage divider didn't work, when the arduino output pin is at ground level the was heater is still on. I've now used a BC547 transistor with a 2K base current resistor, which seems to do the job. So far the arduino hasn't gone up in smoke, I hope it stays that way.Again thanks for all who helped, much appreciated.