RC timer with PNP transistor  - Page 1
Forum section: electronics / beginners

author: abdullahseba
author_rank: Frequent Contributor
timestamp: August 06, 2017, 08:30:55 am
content: HiI have a circuit with a part that need to be powered after the rest. So I thought I'll use an RC circuit to delay power up by 2.2 seconds.I assume I have to use an NPN because I cant switch the negative side.Would this circuit work?Thanks in advance

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2017, 08:47:33 am
content: No, it won't work, because the emitter and collector terminals on the transistor are reversed.The component values are way off and won't give a delay anywhere near 2.2s.How much current does the circuit draw? The transistor needs sufficient base drive to turn on with a low enough collector-emitter voltage.The circuit is drawn in a really confusing manner. Flip the RC circuit and don't draw wires crossing one another or flip the earth symbol, when it's not necessary.

author: Ian.M
author_rank: Super Contributor
timestamp: August 06, 2017, 02:58:56 pm
content: Quote from: Hero999 on August 06, 2017, 08:47:33 am+1, good points.If the load draws more than a few mA I don't think its possible to build a single BJT power on delay circuit with reasonable timing circuit component values. You want to use a large resistor to avoid the need for a high value physically large capacitor, but that will starve the switching transistor of base current, which means it needs a very high gain. However you cant cheat*and use a single-package Darlington because Darlington's have much higher Vce_sat voltage drop when on.So whatever you do, you'll need an extra transistor to drive the base of the switching transistor, and it + a current limiting resistor (so you aren't relying on its HFEto set the base current) need to go between the base and ground. However the turn-on will still be pretty mushy as the voltage across the capacitor slowly increases through the region around 2x 0.65V Vbe where the transistors start to pass significant base current. See lower circuit.Add another transistor of the opposite type, connected b to c, c to b to effectively form aSCR(or even replace the driver transistor with a very small SCR or aProgrammable UJTif you can get one), and you get 'snap-action' switching. See upper circuit.There are a few more refinements needed:In both circuits, there is a reverse biassed diode across the timing resistor to discharge the timing capacitor quickly if the input power is removed - without it there is a risk of the circuit switching back on immediately if power is only briefly removed. The lower circuit uses a Schottky diode because it needs to discharge the timing capacitor more completely due to its lower threshold voltage..In the top circuit, to avoid the constructed SCR triggering due to transistor leakage currents or noise, there is a 1Meg resistor across each b-e junction to help keep the transistors off..It also has a 3.3V Zener in series with the base connected to the timing circuit to increase the threshold to around 3.15V(as the Zener current is *much* lower than the current required for its nominal voltage), which makes far better use of the timing capacitor. Without it, you'd need an order of magnitude bigger timing capacitor..Due to the snap-action switching, the switching transistor in the upper circuit has to pass a very high current surge at switch-on to quickly charge all the reservoir capacitors downstream of it. C3 connected b-e slows down its switch-on to limit the peak charging current..In both circuits, if there is any risk of the input being shorted to ground, and there is significant reservoir capacitance downstream of the switching transistor, and the supply voltage is greater than its reverse Vbe rating,(not a problem at 5V), you should add a beefy diode across the transistor, cathode towards the supply to divert the reservoir capacitor discharge current around the transistor, away from the b-e junction.Neither circuit is a good way to do this - you should be using a P channel MOSFET as MOSFETs draw negligible gate current so you can use high vaue timing resistors without any problems, and their Vds_on drop can be far far lower than a similar current rating BJT.LTspice sim with approx 200mA load current attached. Vce_sat for both sims at under 0.1V.* Cheating: a single-package Darlington actually contains two transistors.

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2017, 02:42:35 pm
content: Why not just use a 555 timer?

author: Ian.M
author_rank: Super Contributor
timestamp: August 07, 2017, 03:37:01 pm
content: Yes, I said above the BJT delay circuits are far from ideal. A 555 will do nicely for this sort of delay timing application but lets loose the BJT high side switch and make Q1 a P channel MOSFET, reducing R2 to something like 22R Also I'd probably use a CMOS 555 e.g. ST's TS555. It has picoamp level input leakage current so you can ditch the electrolytic and use a 1uF cap and a 2Meg resistor for the timing components. It also would reduce the On-state quiescent current to under 1/4mA.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: August 07, 2017, 05:38:33 pm
content: Quote from: Hero999 on August 07, 2017, 02:42:35 pmI dunno I feel a 555 is a bit of an over kill. Its a complex amplifier.

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2017, 08:43:43 pm
content: Quote from: abdullahseba on August 07, 2017, 05:38:33 pmThe 555 timer is not an amplifier.Yes, the circuit is slightly more complex, than just using a single transistor with an RC circuit, but delay is much more predictable. If the load current is small and you're not bothered about 1.4V or so of voltage drop, then connect the load directly between the 555's output and 0V, reverse the positions of R1, D2 and C1, so C1 goes to +V and D1 & R1 go to 0V and remain in parallel.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: August 07, 2017, 09:25:33 pm
content: Quote from: Hero999 on August 07, 2017, 08:43:43 pmLoooooooooool xDI meant mycircuitis a complex amplifier not the timer. Should have phrased it better.

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2017, 09:54:51 pm
content: Quote from: abdullahseba on August 07, 2017, 09:25:33 pmI still don't know what you mean. The original circuit is a timer. Do you mean that tThe transistor will at one point be in its linear region, at which point it would be acting as an amplifier?You still haven't said what circuit is attached to the timer or how much power it uses.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: August 07, 2017, 10:11:47 pm
content: Quote from: Hero999 on August 07, 2017, 09:54:51 pmQuoteThat circuit is an amplifier and it has a part that needs this timed power up.the current draw is about an amp

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2017, 10:45:37 pm
content: Quote from: abdullahseba on August 07, 2017, 10:11:47 pmThen you need plenty of base current to ensure the transistor goes into saturation. A tenth of the collector current is generally recommended but you'll probably be able to get away with less. If you use the 555 timer circuit then R2 can be 47R and the values of R1 &R2C1 as per the schematic.If you use the original circuit, in your first post, then R31 = 220R and C45 = 68,000µF.The transistor can be the BD132.http://pdf.datasheetcatalog.com/datasheet/philips/BD132.pdfYou could try higher values of R and lower values of C but there will be a huge voltage drop across the transistor.What is the amplifier? Are you using an IC or is it a module or a pre-built item? Some amplifier ICs have an enable pin, which you can add an RC circuit, to delay the start up.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: August 07, 2017, 11:45:53 pm
content: Quote from: Hero999 on August 07, 2017, 10:45:37 pmWell its not the amps them self's than need this but there is a separate circuit with an omega2. The control unit has a USB to audio Ic which needs to be turned on after the omega. There about three amps in this circuit. The Amplifier also acts as a power supply for the controller. It for an underground trainSo Ibuil the circuits for the control panels, built the controller now the apm.https://www.eevblog.com/forum/beginners/atmega2560-not-working-but-bootloader-burns/msg1266859/#msg1266859

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2017, 01:37:12 pm
content: It isn't clear on the schematic where you want the timer.If all you want to do is delay the turning on of the amplifier, the TDA7388, then just use RC circuits on the standby and mute pins which are shown on the data sheet.http://www.st.com/content/ccc/resource/technical/document/datasheet/3d/05/8a/ee/2f/3d/45/f7/CD00179160.pdf/files/CD00179160.pdf/jcr:content/translations/en.CD00179160.pdf

author: abdullahseba
author_rank: Frequent Contributor
timestamp: August 08, 2017, 05:23:19 pm
content: Quote from: Hero999 on August 08, 2017, 01:37:12 pmHave you read any of what I've said?You can see my original RC circuit with the pnp in the bottom right hand corner of the schematic.I have already explained that's its not for the amplifiers IC's

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2017, 06:19:08 pm
content: Quote from: abdullahseba on August 08, 2017, 05:23:19 pmI see now. The schematic is big and the dark background makes it difficult to decipher and the transistor is still connected incorrectly.Where does J32 and J33 go? The schematic doesn't show what the timer is powering, which was my question. How much current is flowing between J32 and J33?Can't you just control the PNP transistor with the ATMEGA328 using software?

author: abdullahseba
author_rank: Frequent Contributor
timestamp: August 08, 2017, 06:50:55 pm
content: Quote from: Hero999 on August 08, 2017, 06:19:08 pmJ32 and J33 power the control unit that has the Omega2 and the USB audio source. The Atmega might not start in time to do this because if the USB - audio IC starts before the omega the omega wont boot. At least it did that on my first PCB but the second seems fine. I still would like to do it like this though to be on the safe side even if it was probably a fault.

author: Ian.M
author_rank: Super Contributor
timestamp: August 08, 2017, 06:55:06 pm
content: Does the USB Audio chip have a reset pin you could use to hold it in reset until its released to enumerate and run by an I/O pin on either the Omega2 or the ATmega?

author: abdullahseba
author_rank: Frequent Contributor
timestamp: August 08, 2017, 06:57:59 pm
content: Quote from: Ian.M on August 08, 2017, 06:55:06 pmNo, Its the PCM2901http://www.ti.com/lit/ds/symlink/pcm2901.pdf

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2017, 10:16:12 pm
content: That IC only uses 70mA tops, so where did the 1A come from? Presumably you have an audio amplifier there too?The 555 approach is probably your best bet. If you want to save space, the 555 can be replaced a single gate Schmitt trigger IC, which will also draw less current. The only downside is accuracy: the high level threshold is less well controlled, than the 555 timer.

author: abdullahseba
author_rank: Frequent Contributor
timestamp: August 08, 2017, 10:25:15 pm
content: Quote from: Hero999 on August 08, 2017, 10:16:12 pmWhen the rest of the controller is powered with the Omega the 5v line becomes shared and with the rest of the circuit its under an amp. Idontthink it draws more than 500mA but I'm not sure.